April 30. Not with a bang

It’s the end of the Silurian. You’re probably expecting a big extinction event like the ones we had at the end of the Cambrian and Ordovician – but I have to disappoint you. The last of the relatively small extinction events we discussed the other day, the Lau event, was about 8 million years before the end of the Silurian, and there’s really nothing notable to mark the end of the Period. 

So how do we pick it? All these time periods – eras, periods, epochs and so on – are of course arbitrary constructions by humans to make it easier to refer to periods of geologic time. It took an international committee to define the boundary between the Silurian and the Devonian Periods, and their proposal was adopted in 1977. It’s defined by the occurrence of some particular fossils – index fossils, which are characteristic of a particular, narrow time interval. For this boundary, the base of the Devonian is defined as the first appearance of one particular type of graptolite species. Graptolites (see March 8) were floating colonial animals that created a wide variety of distinctive fossils, so they are very useful in relative age dating. This position, the base of the Devonian, is strengthened by the presence of some particular trilobites and conodonts too. It’s such a characteristic and specific assemblage of fossils it’s called a “golden spike,” and although it’s used to define the base of the Devonian, that also defines the top, the end, of the Silurian Period. On the whole, it appears that there was pretty much uninterrupted and continuous deposition of limestones and shales from the Silurian into the Devonian, so there really isn’t anything dramatic to mark the boundary.

Silurian-Devonian boundary monument at Klonk

By international agreement, there’s a particular place – called the type section – which is the official “best” example or at least the official reference example, of the rocks that define the boundary. It’s at Klonk, a village in the Czech Republic about 35 km from Prague. It’s called the global stratotype for the Silurian-Devonian boundary, and there’s a monument there to commemorate it.

So with that, at 416 million years ago, we’re done with the Silurian, and the last of the Silurian invertebrates. Mark Twain cited a Mississippi river boatman who had a library consisting of one book, Charles Lyell’s Principles of Geology, which he used as a source for what he perceived as profanity, to use against his roustabouts to stir them into action…. Twain reports the man would rail at the roustabouts and charge them “with being Old Silurian Invertebrates out of the Incandescent Anisodactylous Post-Pliocene Period and damn the whole gang in a body to perdition.” (From Twain’s Autobiography.)

So no more Silurian invertebrates for us. The Devonian starts tomorrow.

—Richard I. Gibson

Photo by Huhulenik via Wikipedia under Creative Commons license.

April 29. Carlin gold

The Roberts Mountains Formation in Eureka County, Nevada, is mostly carbonate, limestone and dolomite that was laid down in a shallow Silurian sea. Interesting rocks, certainly, but geologists and prospectors alike walked over those rocks for decades without realizing it was the host to one of the largest gold deposits on earth. 

They missed the gold because it’s in the rock as tiny tiny grains – often smaller than a micron. A micron is one-one thousandth of a millimeter. Pretty small. Gold was discovered in 1961, and the first mine began production in 1965, near Carlin, Nevada, which gives its name to these “Carlin-type” gold deposits. 

Goldstrike Mine, Carlin Trend, Nevada

What happened is that the limestones and dolomites, which are soluble in even slightly acidic water, were pretty much turned into a really finely porous sponge – many tiny holes were dissolved in the rock. And the water that dissolved the holes – or maybe later water – was mineral rich, and carried gold in solution that precipitated into those little holes. That happened long after the Silurian rocks were laid down around 417 million years ago. The gold mineralization of these rocks probably happened more like 40 million years ago, when Nevada was beginning to be pulled apart and big normal faults were starting to form. Those faults probably served as conduits for the hot, acidic, mineral-rich waters to percolate through the Silurian strata.

Exactly where the mineral-rich waters came from is controversial – did they pick up gold as they passed through older rocks, leaching out the gold and then redepositing it here? Or did the water come out of magma, molten rock, deeper in the earth? There’s geochemical and geophysical evidence to support the magmatic idea, that the stuff came from molten rock deep down in the earth’s crust, but I don’t think this question is fully resolved.

The gold ore at Carlin is typically only 1 to 10 grams of gold per ton of rock. A gram is about the mass of a paper clip, so you can see how the gold must have been really thinly scattered through the tons of rock – but there was a lot of it. The Carlin Trend in Nevada has produced way more gold than the Mother Lode in California, and today The Silver State – Nevada – produces about 80% of all the gold mined in the United States, and more than 10% of world gold production. The Carlin mines passed the 50-million-ounce mark in 2002 and 70 million ounces by 2008, and they’re still going strong. New mines continue to be opened along the 5-by-40-mile zone, whose production is more than $85 billion at 2010 gold prices.

Much of the gold at Carlin is produced from open-pit mines. It’s the second-largest gold district in the world, second to the Witwatersrand in South Africa, where the gold comes from underground mines almost 2½ miles deep, the deepest on earth. With Carlin approaching 100 million ounces of gold, it’s a distant second to Witwatersrand, which has produced about 1.5 billion ounces of gold since it was discovered in 1886. That’s about half the gold ever mined on the planet. The origin of the gold there is about as different as possible from Carlin. The South African gold is related to a huge meteorite impact more than two billion years ago, back in the Precambrian.

The main use – 38% – of gold in the United States is in electronic components, because gold is an excellent non-reactive conductor. Things like computers are among the main consumers. Jewelry takes another third, coins amount to about 19% of consumption, and all the gold in all the new gold crowns and other dental uses adds up to about 5% of US gold each year.

Thanks to Carlin, Nevada, the US is a net exporter of gold, but China produces twice as much. Australia is #2 in world gold production, and the US and Russia are usually about tied for third place. 

—Richard I. Gibson 

Photo of Goldstrike Mine, Carlin Trend, from USGS 

Links:

http://en.wikipedia.org/wiki/Carlin%E2%80%93type_gold_deposit
http://www.geotimes.org/apr06/feature_GoldOrigins.html
http://minerals.usgs.gov/west/projects/nngd.htm
http://goldinvestingnews.com/6070/the-geology-of-the-carlin-trend.html

April 28. Fault types

There are three basic types of faults, which you probably know are breaks in rocks in the earth – breaks with offset and movement along them. The three types represent the three ways earth materials can move relative to each other. They can pull apart, they can collide, or they can slide past each other. 

When rocks in the earth on scales of miles or many miles are pulled apart, eventually something has to give. If the rocks were plastic, like silly putty or caramel candy, they might stretch and thin, but in the upper crust of the earth, most rocks are not plastic. They’re brittle, and when you pull them apart, eventually they will snap. One piece will probably drop down – or appear to have dropped down –  relative to the other along a break, a fault. The part that slides down is above the other part, and above the inclined fault between them. Faults like that, where one block slides down relative to the other, are called normal faults.
 
Normal faults aren’t really any more “normal” than other kinds of faults, but the name comes from the way miners referred to fault blocks. In a mine, when an ore body was cut by a fault, miners called the two blocks the footwall – the block below the fault, which they were standing on – and the hanging wall, above the fault and hanging over their heads. That was the “normal” way of things, with the footwall down and the hanging wall up. When the miners encountered faults with the opposite sense of relative offset, with the footwall up and the hanging wall down, they called them reverse faults.

So, normal faults usually come from extension, pulling apart. A good example in the United States is the Wasatch Fault in Utah at Salt Lake City. The mountains, to the east, are uplifted relative to the Salt Lake Basin to the west, which has been faulted down. Most of the western half of Utah and most of Nevada are undergoing extension, which has caused many blocks to go up and down relative to each other.

When we talked about the Caledonian Orogeny a few days ago, we talked about colliding continents and other terranes, and it’s easy to see how collisions would push a mountain range up. It’s like two big pieces of furniture on a carpet and you’re pushing the furniture – the continents – together. The carpet between them is likely to be squeezed from a flat carpet to a rippled surface, and eventually the carpet ripples, which are like folds in rocks, might even get pushed up and over each other. If the carpet was something more brittle, it would break.

The faults that result from compression, squeezing the earth between two blocks, are usually the up-and-over type, which the miners called reverse faults. They could be at any angle, and today geologists have two general terms for them. High angle reverse faults are just reverse faults, but if one package of rocks was pushed up and over older rocks at a low angle, we usually call those thrust faults. It’s a little subjective as to when you say reverse fault or thrust fault, but they are basically the same thing, caused by the same mechanism – compression.

The third kind of fault occurs where two separate blocks of the earth slide past each other, and we call that a strike-slip fault. Strike is the orientation of anything in the earth with respect to the geography of the surface, so you might have a bed or a fault or something else that is oriented north-south, or east-west, or any other angle relative to geographic coordinates. And the two blocks slide along the strike of the fault.

Probably the most famous strike-slip fault in the United States is the San Andreas fault, which moved to cause the great earthquake we mentioned on its anniversary on April 18. But there are many strike-slip faults around the world.

That’s your lesson in geological jargon for today.

* * *

Today is Eugene Shoemaker’s birthday. He was born in 1928 in Los Angeles, and he pioneered the field of planetary geology. He was involved in many American space missions, and co-discovered the comet Shoemaker-Levy in 1993, which gave scientists the first look at a planetary impact when the comet collided with Jupiter in 1994.

—Richard I. Gibson

Drawings by Richard Gibson
Fault types from USGS
Animated normal and reverse faults (USGS) 

April 27. Extinction events

Yesterday I mentioned in passing some relatively small extinction events during the Silurian. There were at least three, which more or less mark the boundaries between the four epochs of the Silurian. The first one, at the boundary between the Llanderovy and Wenlock epochs, was pretty intense at some locations – more than half the trilobite species disappeared at a location in Sweden, and worldwide, 80% of conodont species died off. Conodonts are generally deeper-water animals, as are graptolites, which also suffered a lot in that 200,000-year event. Shallow water life, like corals, had little impact. Why is that? I don’t know. 

The Mulde Event, about 427 million years ago, between the Wenlock and Ludlow Epochs, coincided with a global drop in sea level. What caused the sea-level drop is not known. The last extinction event also did a number on conodonts but had little effect on graptolites. Go figure. But the graptolites were decimated shortly after the sea-level change that marks this extinction, at a time when sea-water isotopes changed significantly. I hope it is evident that these things are complicated – most extinctions are way more complex, and less well understood, than the end of the dinosaurs.

One interesting aspect of these little extinctions is that we actually see an increase in stromatolites during the extinction periods. You remember stromatolites – buildups of calcareous material by algal mats. The theory seems to be that when animals that ate algae were decimated, it allowed for an expansion of the algal mats. But when life rebounded after the extinctions, the stromatolites declined.

—Richard I. Gibson

April 26. Silurian deserts?

We know from all that salt in Michigan that the climate during part of the Silurian was hot enough in places to evaporate small seas. Were there deserts?  We don’t really know. There’s a lot of work on Silurian sea-level changes and geochemistry that attempts to unravel the nature of planet earth during the Silurian – and there is still quite a bit of controversy, as far as I can tell.

Scientists studying oxygen and carbon isotopes find that variations in them during the Silurian coincide with some relatively small extinction events. If nothing else, it’s beginning to give a picture of the Silurian as a time with repeated climate changes, in contrast to what I was taught 45 years ago that the Silurian was stable, uniform, and hot. Changes in temperature affect the uptake of different isotopes of carbon and oxygen in rocks and organisms such as the shells of animals – that’s how we use isotope ratios to infer changes in climate. But what is clearly not clear is what was causing those climate changes.

Most of what I have read suggests that indeed, there must have been periods of arid conditions and high temperatures, which fits with those vast salt deposits. So it’s reasonable to suppose that there were deserts somewhere during at least parts of the Silurian Period. But so far as I can tell, there’s little other direct evidence for such deserts – no vast deposits of wind-blown sand, for example.

As usual, research continues.

* * *  

Today, April 26, is the birth date of Leopold von Buch, in 1774, at the castle of Stolpe, Brandenburg, Germany. You probably haven’t heard of von Buch, but he was a prominent and important geologist in the early 19th Century. He studied the volcano Vesuvius and recognized the volcanic origin of basalt – at the time, many scientists thought basalt, and virtually everything else, crystallized from water. And von Buch defined the subdivisions of the Jurassic Period of the Mesozoic Era, which we’ll get to in October.

It’s also the birthday of a geophysicist you probably have heard of – Charles Richter, born this day in 1900, in Ohio. He invented the Richter scale, the first real quantitative way of estimating the magnitude of earthquakes. Seismologists use the moment magnitude scale today, but the Richter scale was the principal way of evaluating earthquakes from 1935 into the 1980s.

—Richard I. Gibson

References
Paper by Munnecke et al. 2010
Silurian climate

April 25. Cooksonia

We talked about early plants on April 11, and last month I mentioned that there are some fossil spores from the Ordovician that indicate there were some primitive plants on land that long ago. But really, even though there are some early to middle Silurian land plants known, there still wasn’t much, even towards the end of the Silurian, coming up at about 416 million years ago. 

Most of the land surface was bare rock or the kind of soil that forms from the physical weathering of rock. Plants seem to have mostly lived mostly along sea coasts, in wetlands, and it’s reasonable to suppose they might have grown along some rivers. But even with that limited range, they were still pretty much worldwide – from what is now Greenland to Siberia to Australia – all areas that were within the temperate or tropical zones during the Silurian.

Cooksonia

Cooksonia was the first plant on land that had an upright stalk, rather than mossy ground cover. It was a vascular plant, meaning it had a system for delivering nutrients and water around its body. It wasn’t very tall, though – just a few inches at most. It seems that the stalk was mostly there to help disperse the plant’s spores, which were in clusters on the tips of the branched stalks. Cooksonia was discovered in 1937 in very late Silurian rocks in England and Wales, but it’s since been found all over the world.

Cooksonia didn’t have leaves, or flowers, or a root system. William Lang, who described the first specimens, named it for Isabel Cookson, an Australian botanist and paleobotanist who worked with him in Britain. Her work on Silurian and Devonian plants was important in establishing theories of the evolution of land plants.

So, we’re getting there – the land isn’t just bare rock any more. Short stalked plants, mosses, and a millipede here and there to eat the dead plant detritus, and that’s about it. But it was a start.

—Richard I. Gibson

Drawing by Smith609 via Wikipedia, under Creative Commons license

April 24. The Silurian-Devonian aquifer

Yesterday we talked about the porosity, or open space, you need to make an oil reservoir. It’s the same concept as you have for an aquifer, a subsurface reservoir for water. Across most of Iowa and parts of Wisconsin and Michigan, the Silurian and some Devonian rocks in the subsurface form an important groundwater aquifer. They are mostly porous carbonates – dolomites and limestones.

Some of the water wells in the Silurian aquifer in Iowa have yielded more than a million gallons per day for 40 years, but flows are pretty variable and a few hundred thousand gallons per day per well is probably more common. In the 1980s in Iowa, especially northeastern Iowa, two-thirds of the water from this aquifer was used for domestic and commercial water supplies, and about a quarter went to agricultural use including irrigation. The total was about 130 million gallons a day in 1985.

—Richard I. Gibson

Reference:
USGS report

April 23. Pinnacle Reefs

Coral reefs developed around the margins of the Michigan Basin during the Silurian, but even in deeper water some corals thrived. They grew upward in cylindrical, column-like structures, keeping pace with the rate of rising sea level, like modern atolls. Some reached heights of a few hundred feet, and because of their shape they’re called pinnacle reefs. 

Pinnacle reefs are especially common in a northeast-southwest trending zone across northern Lower Michigan. A belt that’s about 150 miles long and 40 miles wide. The reefs probably grew on a linear break in the sea floor – there’s some evidence in geophysical data for such a break, which would have been the easiest place for corals to get started because it was somewhat shallower than the deepest part of the basin, to the south.

When the tall cylindrical reefs were eventually buried beneath later sediments including salt as the water evaporated, the porous limestone structures built by corals became excellent traps for oil and gas. The surrounding sediment is the seal, the stuff that keeps the oil and gas from escaping from the reservoir, which is that tall little reef. They are usually no more than a half-mile across, but there are more than 700 of them across northern Lower Michigan that have been found to contain oil and gas. A typical oil field there contains around a half-million barrels of oil, which is maybe 45 minutes worth of oil consumption in the whole United States today. To have a good oil reservoir you need good porosity – that’s the holes in the rock where the oil is found – and good permeability, which is how interconnected the pore spaces are. These little reefs have both – they average about 6% pore space, but some are as much as 37% - that’s really huge, more than a third of the rock. The permeability can be as much as 500 times that of a “good” oil reservoir.

Most of those little fields were discovered after 1970, which was the peak of oil production in the United States. In 1970, the U.S. produced around 10 million barrels of oil per day. Today, even with the dramatic additions in production from North Dakota, the U.S. only produces about 7.9 million barrels per day – but that’s significantly up from the 5 million that was usual through most of the 1990s and 2000s.

There are Silurian pinnacle reefs in Illinois, Indiana, Ohio, and Ontario as well, and some contain oil, but the reefs in Michigan are the richest.

* * *

Today’s birthday is William Dean Thornbury, born April 23, 1900, in English, Indiana. He became a professor of Geomorphology at Indiana University and wrote the textbook that was used in geomorphology classes across the United States for decades. Geomorphology is the study of landforms and the processes that create them.

—Richard I. Gibson

Links:
Pinnacle reefs 
Porosity and permeability 
Map from Michigan DEQ

April 22. Fish fins get strong

 


First off, I’m pleased to announce that you can now subscribe or listen to our podcasts in Stitcher. Stitcher is a free, mobile-optimized app that you can download, so if you prefer to listen that way, it’s now available. Thanks to listener Max for pointing me to Stitcher.

* * *

A couple days ago we talked about fish getting jaws. They were also getting other things during the late Silurian, including stronger fins. The crossopterygians – that means fringe-finned – and sarcopterygians – that one means fleshy fin – are most closely related to today’s lungfish and the famous “living fossil,” the coelacanth, as well as the tetrapods – four-limbed animals including amphibians, reptiles, birds, and mammals. “Living fossil” isn’t really a very useful term, but it’s out there – it mostly means that whatever we have today isn’t too much different from fossil versions.

Reconstruction of Guiyu oneiros, late Silurian of China

The earliest known sarcopterygians are from the very late Silurian, about 418 million years ago. The Silurian ended about 416 million years ago, and it’ll be next month, the Devonian, when the fish really took off and diversified.

The modern coelacanth, which was thought to have been extinct since the Cretaceous Period, was first caught off the coast of South Africa in 1938. Its four strong fins are the main connection to the Silurian and Devonian forms that also gave rise to tetrapods, land-walking animals. And obviously, to live on land animals had to be able to breathe air directly rather than taking it from water, and the closely related lungfish, which also probably originated in the very late Silurian but survived to the present, are probably examples of early stages of that evolution.

A related footnote – Sharks are in the news, fossil sharks. A report in the journal Nature last week described a fossil shark with significant differences from modern sharks – in the jaws and gill structures. It’s common to say that sharks haven’t really evolved all that much for millions of years, but this discovery reminds us that there has indeed been considerable change even in things that seem to remain the same for long periods of time. Sharks have evolved, too.

—Richard I. Gibson

Image by ArthurWeasley via Wikipedia under Creative Commons license.

April 21. Terminology -- a short rant

Today is a break from the Silurian, and a bit of a rant on my part but one that I hope is informative for you. When we recently talked about the Caledonian Orogeny, I mentioned that it is sometimes called the Acadian Orogeny in the United States. That’s just one source of potential confusion caused by geologists using different names for the same thing. I know I’ve been guilty of it too – it’s hard to put hard and fast names to things that happen over millions of years, and over somewhat different millions of years in one place versus another. Some geologists would call the Taconic Orogeny one of the early phases of the Caledonian. And the Caledonian itself might be considered to be the start of an even grander development that could be called the Appalachian Orogeny – but if we do, then the Appalachian Orogeny lasted at least 150 or 200 million years. To have reasonable conversations, you have to subdivide that into discrete parts – for the sake of discussion, if nothing else.

Still, it can be frustrating to me – and certainly to you – if I use one word for a geological event here and a different word there. Some geologists will use different names to refer to a location – Caledonian in Europe, Acadian in North America, when the cause was the same geologic process. And other geologists might use the term Caledonian to refer to something going on in China at the same time, Silurian, as the events in Europe and America, when there’s no connection to speak of in terms of events.

Sometimes there’s a style to the mountain building events – it might be one involving intense brittle deformation, breaking the continent into big blocks, or it might be more of a matter of pushing some rocks up and over other rocks. Sometimes geologists will call one style one name and the other another – and those names might be the same as the names others use for a particular place or a particular time. It can most definitely get confusing.

My goal with these podcasts is to provide technical information about earth history in an interesting and understandable way, so I will be trying diligently to avoid these sorts of pitfalls. But be aware that they are out there, part of the nature of science, and I might do it sometimes too. Please don’t hesitate to inquire, by way of a comment on the blog, or send me an email at rigibson@earthlink.net. I’ll try to clear up any confusion that I cause.

—Richard I. Gibson

April 20. Niagara Falls

Niagara Falls drops over a ledge of Middle Silurian Lockport Dolomite, a relatively resistant rock unit about 100 feet thick. The dolomite is underlain by easily-eroded shales of the Medina and Clinton Groups, both Silurian, and the Upper Ordovician Queenston Shale. Near the whirlpool below the falls is a thin beach deposit called the Whirlpool Sandstone, which marks the base of the Silurian. The ledge, the Lockport Dolomite, was laid down in part of that shallow marine shelf we talked about the other day – the low-lying platforms that surrounded the deep pool in the Michigan Basin. 

Long after the Silurian Period, when the entire region of the Midwest and Ontario was lifted above sea level, erosion left the most resistant layers including the Silurian Lockport Dolomite standing as ridges and ledges. Niagara Falls is pretty much a legacy of the last Ice Age, and the falls have developed pretty much in the past 10,000 years because they serve as the outlet for one of the largest volumes of fresh water anywhere on earth – the Great Lakes. That water has to cut through or go around resistant materials, and in this case it’s cutting through the Silurian escarpment that forms the neck of land between Lakes Erie and Ontario.


The technical term for a long, low ridge of resistant rock is a cuesta. It’s a Spanish word meaning the slope of a hill. The Silurian Cuesta, which holds up the ledge over which Niagara Falls flows, can be seen easily on a map of the Great Lakes. Many of the peninsulas around Lakes Michigan and Huron are formed from the resistant Silurian rocks that were laid down around the Michigan Basin. Wisconsin’s Door Peninsula and Michigan’s Garden Peninsula in Lake Michigan, and Drummond Island, Manitoulin Island, and the Bruce Peninsula in Lake Huron are all above lake level because they are underlain by resistant Silurian rocks.

Because of flow controls the rate that Niagara Falls is cutting back through the rock has decreased from something like 7 or more feet per year to three or four feet per year – but eventually, as long as water flows, Niagara Falls will disappear. No need to rush to see it though – we’re talking many more thousands of years before Niagara Falls disappears.

* * *

April 20 is the birthday of a couple of prominent geologists. William Edward Logan was born this day in 1798 at Montreal, Quebec. He established the Geological Survey of Canada in 1842 and had a prestigious career. The highest mountain in Canada (and second highest in North America), Mt. Logan in southwestern Yukon Territory, was named for him.

Also on this day, April 20, 1824, Jules Marcou was born at Salins, France. He worked extensively in Europe but is probably best known for his 1853 Geological Map of the United States, and the British Provinces of North America, probably the first good comprehensive geologic map of the US as it was then known. He also helped establish The Louis Agassiz Museum of Comparative Zoology at Harvard.

—Richard I. Gibson 

References 
Good field trip report
Facts and figures 

Geology

Drawing after G.K. Gilbert (USGS)

April 19. Fish get jaws

Last month we talked about some early animals that you’d call fish if you saw them: the ostracoderms, a general name for several groups of armored, jawless critters. The Silurian Period was a time of considerable diversification among the fishes. Some of the structures called gill arches evolved into the lower jaw, a movable structure. It’s easy to imagine the advantages of a jaw that could open and close as compared to a relatively solid opening that could take in food but relied on whatever happened to be in front of the animal. Now, fish could bite.

Placoderm

Some of the earliest Silurian fish to develop jaws were primitive sharks, which are cartilaginous animals rather than bony. Their skeletons are made of tough fibers – cartilage – rather than solid bones. That model works, since we have sharks and their relatives today. Another group that developed jaws during the Silurian were the placoderms – they looked a lot like ostracoderms and their name means plate-like skin, but they also had jaws. Placoderms appeared in the late Silurian, and they are extinct today, dieing off at about the end of the Devonian. You really wouldn’t think twice about calling a placoderm a fish – possibly a somewhat weird-looking fish, but certainly a fish. Some of them, especially the group called arthrodires, grew to huge sizes during the Devonian, reaching 10 meters or more in length , more than 30 feet. And they were predators.

Bony fish

The very first bony fish – the group that includes most modern fish, with 28,000 modern species, got started in the late Silurian, about 420 million years ago. In addition to jaws, bony fish have swim bladders, organs they use to maintain their position in the water.  

Fish, of course, have fins. The evolution of fins into legs suitable for walking on land is a huge event in the story of life, which we’ll get into a bit more, but in the meantime I want to recommend, yet again, Your Inner Fish by Neil Shubin – it’s a book and a three-part program on PBS that tells not only this story, but connects that evolution to modern vertebrates including humans.

—Richard I. Gibson

http://en.wikipedia.org/wiki/Evolution_of_fish

Bony fish image by ArthurWeasley via Wikipedia underCreative Commons license 

Placoderm by Nobu Tamura under Creative Commons license 

 

April 18. Saudi Arabian Oil

April 18, 1906. At 5:12 in the morning San Francisco was rocked by a powerful earthquake. The magnitude was 7.8 or 7.9 and the death toll is estimated today at about 3,000 people. 

* * *

Now back to the Silurian. Oil is another resource associated with Silurian rocks.  Worldwide, something like 9% or so of all the oil comes from Silurian source rocks – and most of that is in Saudi Arabia. The Silurian Qalibah Formation is a thick shale and sandstone package. The lowest part is a black shale rich in organic material. It seems to have been deposited relatively early in the Silurian, when the land was being flooded as the glacial period at the end of the Ordovician Period ended. It appears that this shale was deposited during a sea-level drop, an anomaly during a time of generally rising sea levels as the ice melted. The receding sea might have stranded piles of plants and other shallow-sea life in shallow pools on land, where enough organic material was caught in the mud to make these black shales. It may be that there was a proliferation of life associated with the end of the glaciation – there was such a proliferation, but how fast and extensive it was can be debated. But with the glaciers’ withdrawal, there was more land to erode and to provide nutrients into the sea for life to take advantage of in the relatively suddenly warmer waters. The exact origin of these rocks remains somewhat contentious.

They call these source beds “hot shales” because they also tend to have concentrations of radioactive elements, which make them easy to recognize on well logs, which are basically measurements of information taken down a drill hole. One tool measures gamma-ray intensity, which is greater in radioactive materials. Similar rocks of similar age are found in North Africa as well – Algeria, Libya, and Egypt have such shales and oil explorationists are really only just beginning to understand them.

The known oil from Silurian source rocks in Arabia and Algeria amounts to probably about 95% of the Silurian-sourced oil in the world, but there’s some oil in Silurian rocks in the U.S. as well. We’ll get to that in a few days. 

—Richard I. Gibson

References:
Hot Shale 
Role of glaciation
North Africa

April 17. Salina salt sea

Back on April 7 when I first mentioned coral reefs, I also mentioned that you could find them around the edges of the Michigan and Illinois Basins. Today, let’s get into that a bit more. 

I’ve also mentioned that North America lay along the equator during part of the Silurian. By mid- to late Silurian time, it had moved south of the equator to southern latitudes, the tropical zone where deserts like the Sahara form. So it was hot.

The Michigan Basin is that bull’s-eye geologic depression centered on the lower peninsula of Michigan. It was a bowl-like depression in the Silurian too, and around the shallow rim of the basin, on the lip of the deeper central part, coral reefs grew. The shallow platform behind the reefs was a fairly typical Paleozoic shallow water sea for some time, with the standard cast of characters such as crinoids and brachiopods.

With gentle uplift, the shallow platforms may have become dry land. The deeper sea, in the middle of the basin, was largely encircled by the edges of the platforms, and even more restricted by the reefs growing there. The result was a nearly circular sea – really a large lake – that was cut off from oceanic circulation. So combined with a really hot climate to evaporate more water than came in from rivers, the sea became saltier and saltier. It must have been much like the Caspian Sea today, but with at least some breaks in the encircling reef that periodically allowed more sea water into the basin where it could evaporate.

What do you get when you evaporate sea water? You get salt. The mineral halite, sodium chloride. In many separate beds, reflecting that episodic influx of sea water and subsequent evaporation, there’s something like 2,000 feet of Silurian salt under the lower peninsula of Michigan. One bed alone is 500 feet thick. It’s an important economic resource, and in the same strata just across Lake Huron at Goderich, Ontario, we have one of the largest underground salt mines in the world. The mine is actually about 1,500 feet down and lies beneath Lake Huron. I went there on one of my first geologic field trips, when I was at Flint Junior College in Michigan. Apart from the coolness of going into an underground salt mine – most geologists crave going underground – the thing I recall the most from that 1967 excursion was the 1500-foot descent in the mine cage. For reasons I don’t recall there were 6 or 8 nuns with us on the trip. In full habit. We all had to wear hard hats to go into the mine… and so did the nuns. I’ll never forget being crowded into a mine elevator with a clutch of nuns in regalia and hard hats… and praying loudly and fervently all the way to the bottom.

Sea water has more than salt dissolved in it, of course. So in addition to halite, you can get a whole suite of other minerals, called evaporites because they crystallize from evaporating water. Gypsum, calcium sulfate with water, is a common one – it’s the stuff most building wallboard or drywall is made from. Anhydrite, which means without water, is another common evaporite mineral. It’s gypsum, calcium sulfate, minus the water in the crystal structure. All this stuff precipitates out because the chemicals – sodium, calcium, sulfur – become unnaturally concentrated by the evaporation of water in a restricted sea, where fresh influxes don’t keep the water at a typical concentration of those elements. It’s because the evaporites are so thick that we know there was some influx – bringing in more water to evaporate to deposit more and more salt and gypsum and anhydrite.

How fast does this happen? It’s possible for halite to build up at a rate of a half a foot a year under proper conditions of chemistry and evaporation. It’s not clear how much time was involved between periods when virtually all of the water might have evaporated and the next influx of water came in, but it’s possible that the entire accumulation was deposited over a period as short as a few thousand or a few tens of thousands of years – almost instantaneously, geologically speaking.

The United States consumes about 50 million tons of salt every year. Close to half of it goes to make chemicals – sodium and chlorine are in all sorts of things. The chlorine in polyvinyl chloride (PVC) for example, comes mostly from salt that is mined or extracted from salty brines. Highway deicing is the second largest consumer of salt. All the salt on home and restaurant tables and all the food additive salt in the United States only adds up to about 5% of the total. Even with all the salt that there is in Michigan and elsewhere, the U.S. still imports 22% of the salt we need, mostly from Canada and Mexico. Salt isn’t valuable enough to ship it great distances… but the price of salt has almost doubled from 1991 to 2013, from $19 per ton to $37 per ton. In the same time frame, salt imports doubled, from 11% to 22%.

—Richard I. Gibson

Map based on Briggs (1958)
Good Reference: The Geology of Michigan, by Dorr and Eschman (U. of Michigan Press, 1970)

April 16. The Missing Silurian

We’re in the Silurian, but today we’re going to talk about the absence of Silurian rocks. Where I live here in western Montana, and across much of the west, there are no Silurian rocks present. In fact, in much of the area, there are no Ordovician rocks either. You probably recall that when there is a break in the rock record, we call that an unconformity, and unconformities can form in one of two common ways: either the rocks were never laid down, or they were laid down but were eroded away later. Sometimes that “later” is after a lot of tectonic activity, so the rocks below the unconformity might be tilted, making an angular unconformity with the younger deposits above.  

Out here in Montana, there doesn’t seem to be an angular relationship between the older Cambrian rocks and the younger Devonian rocks that are on top of them. The two packages of rock are parallel to each other, just as if sedimentation had proceeded continuously from the Pilgrim formation, near the top of the Cambrian, into the Jefferson formation in the Devonian. But there is still a gap there, a gap that spans the entire Ordovician and Silurian Periods, a gap that represents at least 75 million years – more than the time span since the dinosaurs died off. What happened?

Well, again, there are two choices. Nothing was deposited, or it was deposited but eroded away later. How can we tell which it was?

If you go to the last places where Silurian or Ordovician rocks do crop out, you can look at the nature of those rocks. If they were at the edge of the basin in which they were deposited, it’s pretty likely that we’d see changes in the rocks to tell us that we reached the edge of deposition. For example, you might see a gradual transition, over many miles, from the kinds of sediments and fossils that are found in deep sea, progressing to a shallow sea environment, to increasing amounts of silt and sand as we get closer and closer to shore, to a good beach deposit at the former water’s edge, and maybe even really coarse pebbles in a rock we’d call conglomerate, close to the uplifted source area, dumped there because they couldn’t be carried far offshore. You might also see changes in the types of fossils, reflecting the changes in the environment that would mean we were approaching the edge of deposition.

On the other hand, if we have a wide expanse of sediments and fossils that indicate a huge area of deposition, something grand in scale like the entire Florida shelf, extending for hundreds of miles, and it comes to an abrupt end without any of those things that might indicate an expectable transition to land where there was either no deposition, or deposition of a very different kind, then we might conclude that the rock had gone on much further at one time but had since been eroded.

When we look at the edges of Ordovician and Silurian rocks in Montana, Wyoming, and Nevada, the whole northern Rocky Mountain region, it does seem like the Ordovician and Silurian rocks must have once been much more extensive than they are today – meaning they were laid down and then eroded.

To erode vast areas of rocks, you pretty much have to raise the rocks up above sea level. So a big chunk of land, two-thirds of Montana and beyond, was gently uplifted while holding the older geologic layers perfectly horizontal, eroded for 75 million years down to one precise layer in the Cambrian, then gently lowered back down so that the surface was still perfectly horizontal, so that the Devonian sediments could be laid down essentially perfectly parallel to the Cambrian rocks that were not eroded away.

Sound good? Well, I don’t buy it. There are way too may “perfectlys” in there. It’s not impossible for something like that to happen, but geologic events are typically a lot more chaotic than they are “perfect.” So let’s look more closely – or in this case, more broadly. If you look at the rocks that are under the Devonian, under that unconformity, across a really wide area, 500 miles or more, we find that they change. Here in western Montana, it’s Cambrian rocks under the Devonian. In central Montana and northern Wyoming, it’s the Ordovician Big Horn Dolomite that we talked about March 19. And even further east, deep in the subsurface of the Williston Basin of eastern Montana and western North Dakota, there’s the Silurian, in its proper, expectable position beneath the Devonian.

What it amounts to is this unconformity, which seems like it’s between perfectly parallel strata in the Cambrian and the Devonian out here where I know it well, is really an angular unconformity after all – a very very low angle angular unconformity. The angle is so tiny that you don’t see it until you look at a really large swath of territory.

What this all seems to indicate is that sometime after Ordovician and Silurian rocks were deposited in what was generally a wide, shallow, warm sea- after that, there was a very gentle titling of the continent, a slight uplift – nothing to make mountain ranges, but enough to raise what is now western Montana above sea level, so it could be eroded. Think of the Cincinnati Arch, that we discussed March 10, but on a broader, gentler scale. And that low uplifted area was eroded, slowly, gently, over 75 or more million years. And then slowly, gently, it all subsided below sea level so the Devonian could be deposited.

So is that the answer? I think it’s probably the best overall explanation, but don’t take it as gospel. There’s almost too many slowlys and gentlys in there for me. And we can only examine the details in the rocks where they are still present – the edges, the erosional zero edge is what we call it – of the Ordovician and Silurian rocks. Where they are gone, they are gone – and we can’t look at the details to figure out what exactly was going on. So I’m not about to say that it was absolutely positively a case of sediments deposited and lithified and uplifted and eroded. There might have been some places out there where the Silurian was never deposited. We’d need a time machine to tell with absolute certainty – but that doesn’t mean we’re guessing. The total body of information we do have points to the idea of deposition and later erosion as really pretty likely, at least for the best overview of what happened.

At least that’s what I think, until someone tells me they’ve found some rocks that have information to the contrary. That’s how science works. 

—Richard I. Gibson

April 15. Clinton Iron Ore

You remember the Taconic Mountains?  They were uplifted toward the end of the Ordovician by the collision between the North American continent and a probable volcanic island arc terrane. As with all mountains, they began to be eroded as soon as they were uplifted above sea level. We talked about some of the erosion products – the thick pile of sediments called the Queenston Delta.  

Some of the rocks in the uplifted Taconic Mountains, which originally extended from New England south into Georgia, must have contained a lot of iron. Dissolved in water, iron came into Silurian sediments in a long belt from Alabama to New York to eventually produce the only significant iron ore in the 48 United States apart from those in the Precambrian around Lake Superior. The rocks go by various names, but generally the iron-rich package is called the Clinton Iron Ore. Iron was necessary, of course, to early settlers, and even moreso as the nation entered the Civil War, so this resource was aggressively exploited in the 1700s and 1800s.

Oolitic hematite

Some of the richest deposits are near Birmingham, Alabama, where the rock is called the Red Mountain Formation because of its rusty color due to the iron content. The iron occurs right along with fossil crinoids and brachiopods, which together with other aspects of the rock indicates that the iron came in as the rock was solidifying. Don’t visualize an iron-rich sea—crinoids and such probably couldn’t survive in such water. The iron became part of the rock in a process called diagenesis, which is all of the action related to a rock solidifying from loose sediment to solid rock. That includes things like compaction, dewatering, and dissolution of some material and redepositing it as the cement that holds the rock grains together.

A good bit of the iron in the Clinton rocks is oolitic. Oolitic comes from a Greek word for eggs, and they are typically little round or oval grains, a millimeter or two across, that grow by precipitation of mineral matter in concentric layers. They usually start on some nucleus, a grain of sand or a fossil fragment, and oolites are often just calcium carbonate, calcite, the stuff many fossils are made of. But in the Clinton, the material is iron oxide, the mineral hematite, pretty much the same as rust on the underbelly of your car. This oolitic hematite is actually the state mineral of Alabama.

Industrialization after the Civil War really drove the iron and steel industry, and while most of it was centered in the Lake Superior area and the steel mills of Pittsburgh and elsewhere, Birmingham, Alabama, also became an important iron and steel center. In 1940, Birmingham produced 40% of America’s pig iron, high-carbon iron produced in smelters as an intermediate product that would ultimately become wrought iron or steel.

Today, 99% of U.S. iron ore production comes from Michigan and Minnesota, but there’s still a steel industry in Alabama. By some estimates, as much as 10% of U.S. iron ore came from the Silurian Clinton ores, with most of that from Alabama and most of it before 1950.

—Richard I. Gibson

Reference: http://geology.teacherfriendlyguide.org/index.php?option=com_content&view=article&id=196&Itemid=207

Photo by Dave Dyet via Wikipedia (public domain) 

April 14. The Caledonian Orogeny

Today I’m going to tackle the Caledonian Orogeny. It’s gonna be a long one – it’ll make up for the short episodes I’ve had recently. You recall, I hope, that orogeny means “mountain building,” and mountains often result from collisions. The Caledonian Orogeny represents some really complex collisions during the Silurian, especially in Europe. The Caledonian gets its name from the Latin name for Scotland, given to it by the Romans. 

Before I get into it I want to define three terms. First, a terrane. That’s a relatively coherent block of geological material. It could be a continental fragment or an island arc or a slice of oceanic crust, or some combination of them all, but generally, it remains one structural piece over some period of geologic time, and has more or less a fairly uniform history over that time.

Next, a suture. That’s the place where two terranes come together, just as a surgical suture is where two pieces of the body are joined. It might be a fault line, or it might be a narrow belt of rocks, or it might be more complicated. Sometimes when two strong blocks like continents come together, the suture between them is a narrow belt of oceanic crust trapped between them as they came together.

And last, accretion. That’s just the process of things coming together, amalgamating. It’s a continent growing by accreting little terranes around its margins.

Simplified map of the Caledonian Orogeny

So – The Caledonian. Back in the Ordovician, on March 16, we talked about the Avalonian Terrane, a long narrow strip of continental material that lay between Laurentia – that’s the core of North America – and Baltica, the heart of Europe. Avalonia included most of what is now England, as well as southern Ireland, together with southeastern Newfoundland where the Avalon Peninsula lies and gives its name to this terrane, plus northern Nova Scotia and some bits of northern coastal New England.

Avalonia probably began as a long narrow microcontinent that rifted away from the supercontinent of Gondwana, probably from what is now more or less northeastern Africa. A modern analogy might be East Africa today, where the East African Rift System is tearing eastern Africa away from the main continental mass of Africa. Eventually, it may become a long, narrow microcontinent chugging across the Indian Ocean.

Most of what is now northern Scotland and northwestern Ireland were parts of North America, probably pretty close to what is now southeastern Greenland. North America, or Laurentia, was tilted from its present orientation and much further south, so that the Silurian equator ran pretty much along what is now the eastern United States. Baltica and Avalonia were to the south, and the sea between North America and the other two is called the Iapetus Ocean. Some call it the proto-Atlantic Ocean, but it really has nothing directly to do with the modern Atlantic.

As we discussed in the Late Ordovician on March 24 there was probably a volcanic island arc, something like modern Indonesia, off the coast of North America. Its collision with the continent near the end of the Ordovician created the Taconic Orogeny.

Avalonia was working its way across the Iapetus Ocean in complex ways. Britain, pretty much at one end of Avalonia, began to collide with the main Baltica continent probably in the Early to Middle Silurian, about 430 million years ago. Once that end of Avalonia was attached to Baltica, Europe had a long trailing peninsula, perhaps something like the modern Malay Peninsula, but bigger and longer. The Iapetus Ocean was getting narrower and Baltica, with Avalonia attached, continued to move toward North America.

Toward the end of the Silurian, the true continent-continent collision that we call the Caledonian Orogeny was in progress. The mountains that formed were prominent especially in what is now northeastern Greenland and in Norway – and the Iapetus Ocean that was once between Greenland and Norway was gone. Two continents had collided, squeezing a mountain range up between them much like the Himalayas scrunched between colliding India and Asia. There is some evidence in the rocks to indicate that the Greenland-Norway phase of the Caledonian Orogeny actually got started in the Ordovician or even earlier, but I think it’s fair to say that the culmination of the mountain building was during the Silurian. These things take many millions of years.

Further along the colliding belt, the far north of Scotland and northwestern Ireland, which were sitting there on North America, were sutured to Avalonia and touched the rest of Britain for the first time.

Southeastern Newfoundland and northern Nova Scotia, which were part of Avalonia, joined with New Brunswick and the Northern Peninsula of Newfoundland, which were already parts of North America. In North America, this is sometimes called the Acadian Orogeny, for the French name of Nova Scotia. And to be accurate, it was really only getting started during the Silurian. We’ll talk a bit more about it next month, during the Devonian, but it was part of this great complex collision often lumped together as the Caledonian Orogeny.

Don’t visualize all this as a nice straightforward head-on impact. It looks like the assemblage was at least in part a result of complex glancing blows and sliding, oblique collisions. At times, it might have been something like western North America today, where the Queen Charlotte Islands off British Columbia are sliding along a major strike-slip fault, a break where two distinct terranes slide past each other. But don’t even visualize that as a nice smooth slide, either. Think of fits and starts, with collision here, pulling apart there, and generally a lot of diverse action making a geological mess of things.

That’s what we have now in Newfoundland and Scotland – Newfoundland is partly the edge of the core of North America and partly Avalonia, and Scotland is part of North America attached to Avalonia – which had already been attached to the Precambrian core of Europe, which we’ve been calling Baltica.

All these pieces that fused together during the Caledonian Orogeny are far apart today, because of the opening of the Atlantic Ocean. That’s a rifting apart that was near, but not exactly along the old suture zone, the join line, between the continents. That’s why bits of North America got left in northern Scotland and bits of the Avalonian part of Europe got left in Newfoundland. But that rifting, that leaving behind of bits of continents, didn’t happen until the Jurassic. We won’t be talking about that until October. In fact, there was still ocean outboard of most of North America south of New England. It won’t be closed until the fat lady sings, and in this case the fat lady is Africa – or more properly, Gondwana. Gondwana is coming, but it won’t be here for almost 200 million years. We’ll get to that in June.

While the main mountain-building action was between Greenland and Norway, and down through what is now Scotland and Maritime Canada, there was another branch of the Caledonian Orogeny in northern Germany and Poland, along the linear margin of Baltica. This one might have been another that was mostly sliding rather than head-on collision. As for the rest of Europe, most of France, Spain, Italy, the Balkan region – it wasn’t there yet.

One last thing – don’t visualize all this mess as coming to a sharp end at some precise time. Additional small continental fragments seem to have been rifting off Gondwana, starting a trek across the remaining ocean basin. One such block is called the Meguma Terrane, which is most of southern and central Nova Scotia and the adjacent offshore. It collided, well after the end of the Silurian Period, with the southern edge of the Avalonian Terrane, which itself had already been accreted to North America in places. So the process was ongoing, over many millions of years.

I won’t be surprised if I got something in all that wrong – so Caledonian experts, please speak up!

—Richard I. Gibson

Primary references for this compilation:
1. Petroleum Geology of the North Sea, K.W. Glennie, ed., Blackwell Science, 1998
2. Evolution of the Arctic-North Atlantic and Western Tethys, Peter A. Siegler, AAPG Memoir 43, 1988.
3. Silurian Paleogeographic map by Ron Blakey 
You can find plenty more about the Caledonian on the web.

Map above by Woudloper via Wikipedia under Creative Commons license 

April 13. Waldron Shale

 

Eucalyptocrinus calyx.
The lower portion is
commonly preserved.

One of the most famous assemblages of Silurian fossils in the United States is the Waldron Shale, which crops out in east-central Indiana and western Ohio. Shale is a very fine grained rock that solidified from mud, often including clay-sized particles that are just a fraction of a millimeter across. Some parts of the Waldron include limestones and dolomites, and that’s where most of the richest fossils are found. In some places the animal fossils amount to small reefs. 

The life included the usual suspects for a warm, shallow Silurian sea – trilobites, crinoids, brachiopods, corals, and gastropods – those are snails. My collection includes a specimen of Eucalyptocrinus – a type of crinoid that had a relatively rigid calyx, or cup, where the organs were centered. It was given to me by an 8th grader who found it while on a field trip led by the Indiana University geology club. On the top of the cup-like calyx there’s a snail, a gastropod, common in the Waldron. The gastropod’s name is Platystoma, and it’s found commonly in close association with crinoid calyxes. Such close association, in fact, that it’s thought that they lived in a symbiotic relationship with the crinoids. Crinoids’ mouths and their anuses were both in the calyx, next to each other, and the interpretation is that the snail Platystoma lived on top of the crinoid, eating the crinoid’s waste products. That gives it the technical name coprophagus – which means poop eating. An alternative explanation is that there was no symbiosis at all – the snail was there because it was eating, and killing, the crinoid. 

—Richard I. Gibson 

Eucalyptocrinus drawn by James Hall (1881)

Links:
Waldron shale 
Probable coprophagus snail
Coprophagus snail 
Waldron fossils 

April 12. Silurian scorpion

Scorpions are among the oldest arachnids known. They date to the Silurian, but most of them were probably aquatic at that time. They had gills rather than lungs, and Silurian scorpions have been found in Great Britain and New York – which, you must remember, were not really very far apart during the Silurian. 

This 2008 paper has some great photos of fossil scorpions from the Silurian – showing pretty clearly that they haven’t changed much in more than 400 million years.

—Richard I. Gibson

Illustration from Extinct Animals by Sir Edwin Ray Lankester (1905, public domain)

April 11. Land life

Pneumodesmus

On March 9, back in the Ordovician, we talked about life invading the land. But I think you need to not think of it as an invasion, but more of a gentle encroachment. There wasn’t much beyond moss-like plants and algae, maybe some fungus, and we really only know about them from spores.

It wasn’t until the Silurian that we find “real” plant and animal fossils that are definitely from land-dwellers, including air-breathing animals. Plants had to evolve mechanisms to maintain water once they were out of the sea. That included a skin of sorts, to keep the water in, and pores to manage the water content. By the Silurian, plants were developing vascular systems to send water (and the nutrients it carried) here and there throughout their bodies. Plants called Cooksonia and related varieties are the oldest such plants we know.  They’ve been found in Silurian rocks of Wales and England.

The first true air-breathing animals were things like millipedes and centipedes, and possibly spiders and scorpions – all arthropods. The oldest of all is called Pneumodesmus, and it’s a probable millipede from Scotland that lived about 428 million years ago, about the middle of the Silurian Period. It was discovered in 2004 and there’s only one specimen – but it has structures that indicate it was an air-breather. It would take 50 million years before the first vertebrate came ashore to live – during the Devonian, which we’ll get to next month.

—Richard I. Gibson

Link:
Silurian life

Photo by Xenarachne via Wikipedia under Creative Commons license

April 10. Crinoids

First, an update. This week which is early April 2014, scientists from China, England, and the United States announced a new discovery from the Chengjiang faunal of China. Those are the remarkably well preserved Cambrian fossils that we talked about on February 8. 

The report details an arthropod that’s so well preserved, they have been able to describe the animal’s cardiovascular system. I’ve put a link to this paper on the February 8 episode.

_________________

Crinoids are echinoderms whose plant-like appearance gives then the name “sea lilies,” but they are animals related to starfish. And the name itself comes from the Greek word for a lily. Crinoids originated during the Ordovician biodiversification, but they expanded dramatically during the Silurian. As fossils, their disk-like segments, from their stems, are really common. Their more fragile, cup-shaped calyxes, where the animal’s feeding and digestive organs were located, are more rarely preserved. But when they are, they are some of the coolest fossils around.

Most crinoids were attached to the sea floor, though a few were free-floating, and most of the modern species are free-floating as well. The sea-floor attachments are called "holdfasts," but they really do look like the root system of a plant.

—Richard I. Gibson

Crinoid drawn by James Hall (1881). Natural height about 5 inches.

Link:

Earliest crinoids 

Blog extra: Today, April 10, in 1815, Tambora erupted in Indonesia. The eruption was violent, and put so much dust and ash in the atmosphere that it caused the "year without a summer" when it snowed in Washington D.C. in July 1816.

April 9. Almadén’s Mercury Ore

This is the 100th daily episode of the History of the Earth. I appreciate all my listeners and readers! Thanks! If you are listening on iTunes and have a suggestion for ways I can improve the program, please leave a review on iTunes. You can also post comments and questions on the blog.

I’m going to try hard to keep the daily episodes going, but once it becomes summer here in Butte, Montana, my life gets a lot fuller. I do historic walking tours and drive the Chamber of Commerce tourist trolley, and they take up a lot of my time. I’ll try, though, to keep the podcasts coming on a daily basis. Thanks again for your interest.

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Today, we focus on a mineral deposit that’s hosted in Silurian rocks. As with many mineral deposits, it’s often not entirely clear exactly when mineralization happened, and often enough it can take place over many millions of years. But we do know the age of the rocks that the mineral resources are in.

In this case, we are talking about the largest single deposit of mercury ore anywhere on earth. It’s at Almadén, Spain, where more than 250,000 tons of mercury have been produced over the past 2,000 years or so. It’s one of the longest operating mine complexes anywhere in the world. The name of the location, Almadén, comes from Arabic, meaning, “The Mine.”

Cinnabar

The ore is mostly the mineral cinnabar, mercury sulfide. Cinnabar is a bright red mineral, and if you know where mercury is on the periodic table – atomic number 80, with a high molecular weight – you might not be surprised to learn that cinnabar is a very heavy mineral. Its specific gravity – that’s a measure of mineral density which is the ratio between the density of the mineral compared to that of water – is 8.1, three times the specific gravity of quartz. If you have a specimen that’s red and feels anomalously heavy as you hold it in your hand, there’s a good chance you have some cinnabar. Don’t eat it. Mercury is bad for you, although there are some medical compounds using mercury that are highly beneficial.

The Romans ground the red mineral to use as pigment. Alchemists in the Middle Ages were fascinated by mercury – it is a liquid metal, after all – and by the 16th century, the discovery was made that mercury could combine with gold and silver, which were otherwise relatively inert, in a process called amalgamation. That made mercury important in extraction of gold from rock – and the Spanish were discovering tremendous deposits of gold and silver in the Americas in the 1500s. The mines at Almaden were owned by German bankers in the 1500s and 1600s, in return for loans they gave to the Spanish government. Beginning about 1566 convict labor was used to mine the ores. They were fed at least moderately well, but about a quarter of the convicts died, usually from mercury poisoning. 

The cinnabar ore at Almadén is in cracks and pores in Silurian quartzites. Quartzite can be a metamorphic rock – basically, pure quartz – or it can be a sedimentary rock that’s really densely compacted. At Almadén, the rock was most likely originally a pretty clean quartz sand laid down in a delta, near a large river mouth. The rock is pretty brittle, easy to fracture, and the mercury ores at Almadén are in fractures within the quartzite.

There’s speculation that the mercury ultimately came from older black shales of Ordovician age. Something heated the water in these rocks, which percolated through the shale, picking up mercury, which was deposited in the fractures in the overlying quartzite. Alternatively, the mercury came from the earth’s mantle, much deeper than any of the near-surface sedimentary rocks. 

Wherever the mercury ultimately came from, it seems that magmatic activity – moving molten rock – probably was the driving force that mobilized the mercury, or the hot waters that moved it around. The richest deposits are quite close to an explosive volcanic vent whose rocks are more or less basaltic in composition. The volcanic activity is almost certainly the most important factor in concentrating the mercury ore.

But why is there so much, right here in this one small district? By some estimates, a third of all the mercury on earth is at Almadén, Spain. You can talk about modes of concentration, volcanoes and fractures, but to my mind the question of why so much is here is unanswered. What I tell tourists about the incredibly rich deposit of copper, silver, and many more minerals here at my home in Butte, Montana, is that I think it’s the luck of the draw – that there were inhomogeneities in the early earth, essentially blobs of the good stuff, that have been altered, changed, and moved around by geologic processes, but that must have been persistent concentrations from way back in geologic time. That’s my guess for the mercury at Almadén. But trust me, I definitely don’t know for sure.

Because mercury is so toxic, its use has declined precipitously in the past couple decades. The last time primary mercury was produced in the United States was in 1992, from a mine in Nevada. Mercury does have a lot of uses, however, and the biggest consumer in the U.S. is the production of caustic soda. Electronics and fluorescent light bulbs also use mercury, but all uses are decreasing because of mercury’s toxicity and persistence in the environment.

The European Union ended mercury production in 2003, and the mines at Almadén were closed. Almost 2,000 tons per year are still mined around the world – mostly in China, which produces almost three-quarters of the world’s mercury, with Kyrgyzstan in second with about 14% of the total.

—Richard I. Gibson

Cinnabar photo by Rob Lavinsky via Wikipedia, under Creative Commons license.


More on mercury uses can be found in my other book, What Things Are Made Of (2011)

Further reading about the Almaden mine's geology:
http://www.geocomunicaciones.ing.uchile.cl/2013/junio/publicaciones3a.html

http://econgeol.geoscienceworld.org/content/85/3/482.abstract