Episode 393 The Mountains of the Moon

Today we’re going to the Mountains of the Moon – but not those on the moon itself. We’re going to central Africa.

There isn’t really a mountain range specifically named the Mountains of the Moon. The ancients, from Egyptians to Greeks, imagined or heard rumor of a mountain range in east-central Africa that was the source of the river Nile. In the 18^th and 19^th centuries, explorations of the upper Nile found the sources of the Blue Nile, White Nile, and Victoria Nile and identified the Mountains of the Moon with peaks in Ethiopia as well as 1500 kilometers away in what is now Uganda. Today, the range most closely identified with the Mountains of the Moon is the Rwenzori Mountains at the common corner of Uganda, the Democratic Republic of Congo, and Rwanda.

This location is within the western branch of the East African Rift system, an 8,000-kilometer-long break in the earth’s crust that’s in the slow process of tearing a long strip of eastern Africa away from the main continent. We talked about it in the episode for December 16, 2014.

The long linear rifts in east Africa are grabens, narrow down-faulted troughs that result from the pulling apart and breaking of the continental crust. The rifts are famously filled in places by long, linear rift lakes including Tanganyika, Malawi, Turkana, and many smaller lakes.

Virunga Mountains (2007 false-color Landsat image, annotated by Per Andersson : Source)

When rifting breaks the continental crust, pressure can be released at depth so that the hot material there can melt and rise to the surface as volcanoes. In the Rwenzori, that’s exactly what has happened. The Virunga volcanoes, a bit redundant since the name Virunga comes from a word meaning volcanoes, dominate the Rwenzori, with at least eight peaks over 10,000 feet high, and two that approach or exceed 4,500 meters, 15,000 feet above sea level. They rise dramatically above the floors of the adjacent valleys and lakes which lie about 1400 meters above sea level.

These are active volcanoes, although several would be classified as dormant, since their last dated eruptions were on the order of 100,000 to a half-million years ago. But two, Nyiragongo and Nyamuragira, have erupted as recently as 2002, when lava from Nyiragongo covered part of the airport runway at the town of Goma, and in 2011 with continuing lava lake activity. Nyiragongo has erupted at least 34 times since 1882. The volcanic rocks of these and the older volcanoes fill the rift enough that the flow of rivers and positions of lakes have changed over geologic time.

Lake Kivu, the rift lake just south of the volcanoes, once drained north to Lake Edward and ultimately to the Nile River, but the volcanism blocked the outlet and now Lake Kivu drains southward into Lake Tanganyika. Local legends, recounted by Dorothy Vitaliano in her book on Geomythology, Legends of the Earth (Indiana University Press, 1973), tell the story of demigods who lived in the various Virunga volcanoes. As demigods do, these guys had frequent arguments and battles, which are probably the folklore equivalent of actual volcanic eruptions. The stories accurately reflect – whether through observation or happenstance – the east to west migration of volcanic activity in the range.

The gases associated with the volcanic activity seep into the waters of Lake Kivu, which has high concentrations of dissolved carbon dioxide and methane. Generally the gases are contained in the deeper water under pressure – Lake Kivu is the world’s 18^th deepest lake, at 475 meters, more than 1,500 feet. But sometimes lakes experience overturns, with the deeper waters flipping to the surface. When gases are dissolved in the water and the pressure reduces, they can abruptly come out of solution like opening a carbonated beverage bottle. This happened catastrophically at Lake Nyos in Cameroon in 1986, asphyxiating 1700 people and thousands of cattle and other livestock. The possibility that Lake Kivu could do the same thing is a real threat to about two million people.

The critically endangered mountain gorilla lives in the Virunga Mountains, which also holds the research institute founded by Dian Fossey.

—Richard I. Gibson

Episode 392 Ophiolites

Today’s episode focuses on one of those wonderful jargon words geologists love to use: Ophiolites.

It’s not a contrived term like cactolith nor some really obscure mineral like pararammelsbergite. Ophiolites are actually really important to our understanding of the concept of plate tectonics and how the earth works dynamically.

The word goes back to 1813 in the Alps, where Alexandre Brongniart coined the word for some scaly, greenish rocks. Ophiolite is a combination of the Greek words for snake and stone, and Brongniart was also a specialist in reptiles. So he named these rocks for their resemblance to snake skins.

Fast forward about 150 years, to the 1960s. Geophysical data, deep-sea sampling, and other work was leading to the understanding that the earth’s crust is fundamentally different beneath the continents and beneath the oceans—and we found that the rocks in the oceanic crust are remarkably similar to the greenish, iron- and magnesium-rich rocks that had been labeled ophiolites long ago and largely ignored except by specialists ever since.

Those rocks that form the oceanic crust include serpentine minerals, which are soft, often fibrous iron-magnesium silicates whose name is yet another reference to their snake-like appearance.  Pillow basalts, iron-rich lava flows that solidify under water with bulbous, pillow-like shapes, are also typical of oceanic crust. The term ophiolite was rejuvenated to apply to a specific sequence of rocks that forms at mid-ocean ridges, resulting in sea-floor spreading and the movement of plates around the earth.

The sequence usually but not always includes some of the most mantle-like minerals, such as olivine, another iron-magnesium silicate, that may settle out in a magma chamber beneath a mid-ocean ridge. Shallower, relatively narrow feeders called dikes toward the top of the magma chamber fed lava flows on the surface – but still underwater, usually – that’s where those pillow lavas solidified.

There are certainly variations, and interactions with water as well as sediment on top of the oceanic crust can complicate things, but on the whole that’s the package. So why not just call it oceanic crust and forget the jargon word ophiolite? Well, we’ve kind of done that, or at least restricted the word to a special case.

Pillow Lava off Hawaii. Source: NOAA. 

The word ophiolite today is usually used to refer to slices or layers of oceanic crust that are on land, on top of continental crust. But wait, you say, you keep saying subduction is driven by oceanic crust, which is denser, diving down beneath continental crust, which is less dense. Well, yes – but I hope I didn’t say always.

Sometimes the circumstances allow for some of the oceanic crust to be pushed up over bits of continental crust, despite their greater density. One area where this seems to happen with some regularity is a setting called back-arc basins, which are areas of extension, pulling-apart, behind the collision zone where oceanic crust and continental crust come together with the oceanic plate mostly subducting, going down under the continental plate. It took some time in the evolution of our understanding of plate tectonics for the idea to come out that you can have significant pulling apart in zones that are fundamentally compression, collision, but they’re recognized in many places today, as well as in the geologic past.

It seems to me that back-arc basins are more likely to develop where the interaction is between plates or sub-plates that are relatively weak, or small, and more susceptible to breaking. An example is where two oceanic plates are interacting, with perhaps only an island arc between them. The “battle” is a closer contest than between a big, strong continent and weaker, warmer, softer, oceanic crust, so slices of one plate of oceanic crust may be squeezed up and onto the rocks making up the island arc. This happens in the southwest Pacific, where the oceanic Pacific Plate and the oceanic part of the Australian Plate are interacting, creating back-arc basins around Tonga and Fiji and elsewhere.

It also happens where continental material is narrower, or thinner, or where the interaction is oblique or complex. One example of this today is the back-arc basin in the Andaman Sea south of Burma, Myanmar, where the Indian Ocean plate is in contact with a narrow prong of continent, Indochina and Malaya.

We’ve now recognized quite a few ophiolites on land, emplaced there long ago geologically. At Gros Morne National Park in Newfoundland, the Bay of Islands ophiolite is of Cambrian to Ordovician age. The area is a UNESCO World Heritage Site for the excellent exposures of oceanic crust there, not to mention fine scenery.

On Cyprus, the Troodos Ophiolite represents breaking within the Tethys Oceanic plate as it was squeezed between Gondwana, or Africa, and the Anatolian block of Eurasia, which is today’s Turkey. The Troodos Ophiolite is rich in copper sulfides that were probably deposited from vents on a mid-ocean ridge. In fact, the name Cyprus is the origin of our word copper, by way of Latin cuprum and earlier cyprium.  

On the island of New Caledonia, east of Australia and in the midst of the messy interactions among tectonic plates large and small, the ophiolite is rich in another metal typical of deep-crust or mantle sources: nickel. There’s enough to make tiny New Caledonia tied with Canada for third place as the world’s largest producer of nickel, after Indonesia and the Philippines.

There’s a huge ophiolite in Oman, the Semail Ophiolite, covering about a hundred thousand square kilometers. It’s one of the most compete examples anywhere, and it was pushed up on to the corner of the Arabian continental block during Cretaceous time, around 80 million years ago. Like the one in Cyprus, this one is also rich in copper as well as chromite, another deep-crustal or mantle-derived mineral.

The Coast Range Ophiolite in California is Jurassic, about 170 million years old, and formed at roughly the same time as the Sierra Nevada Batholith developed as a more standard response to subduction. It’s likely that western North America at that time was somewhat like the southwestern Pacific today, with strings of island arcs, small irregular continental blocks, and diverse styles of interaction – the perfect setting for a long band of oceanic crust to be pushed up and over other material. The whole thing ultimately got amalgamated with the main North American continent. I talked a bit more about these events in the episode on the Franciscan, November 7, 2014.

—Richard I. Gibson

LINKS: 

Nice images from Oregon State 

Oman Virtual Field Trip 

Episode 391 Valles Marineris

In today's episode we’re going to space. Specifically, Mars. You didn’t really think that earth science is really limited to the earth, did you? Our topic today will be the Valles Marineris.

The Valles Marineris is a long series of canyons east of Olympus Mons, the largest mountain in the solar system. These canyons are about 4,000 km long, 200 km wide and up to 7 km (23,000 ft) deep. On terrestrial scales, the Valles Marineris is as long as the distance from New York to Los Angeles. That’s about the same as Beijing to Hong Kong or Madrid to Copenhagen for our international listeners. They are as wide as central Florida, central Italy, or the middle of the Korean peninsula. Two and a half times deeper than Death Valley, though only about 60 percent of the depth of the Marianas Trench, the lowest point on earth.

Valles Marineris Image Courtesy NASA/JPL-Caltech

Not to be outdone, our planet, Earth, has even bigger valleys. These occur at the oceanic ridges, where plate spreading takes place. The longest rift valley on earth lies in the middle of the Mid-Atlantic Ridge, and it is more than double the length of the Valles Marineris. But let’s not belittle Mars. After all, while we have a pretty good idea for how oceanic rifts form on earth, there is quite a bit of debate about how Mars’ great valley formed.

The most popular theory suggests that the Valles Marineris are an analog to our oceanic rifts, and formed by the same process. As the volcanoes of the nearby Tharsis region developed, the Martian crust bowed down toward the center of the planet due to the weight of the new volcanic rocks. In time, the crust began to crack. This crack is what we see in the Valles Marineris. Unlike on Earth, this rift valley did not continue expanding, but shut down as the Tharsis Region, and Mars as a whole, cooled. Remember that unlike Earth, Mars does not have plate tectonics. It doesn’t have a continual process of hot material (like lava) rising to the surface, while relatively cold material (like the oceanic crust) is brought down towards the planet’s center.

More recent work has used satellite images, and high resolution elevation data to develop new insight into how the Valles Marineris formed. While images from the 1970’s Mariner 9 orbiter were quite blurry by today’s standards, new missions in the late 90’s to early 2000’s have given us a better view of the Martian surface than we have available for the earth. The Mars Reconnaissance Orbiter can take images where each pixel is about 0.5 m or 20 inches. That is, the color on each image is an average of an area of 0.25 square meters, or 2.5 square feet. It can then use image pairs to estimate the elevation of any point on the Martian surface with a pixel size of 0.25 m, or about 10 inches.

These new satellite images include multispectral data, or images that look at different wavelengths of light. The camera on your phone works in the same way: There are sensors that pick up, red light, green light, and blue light. Your phone records the intensity of each color in each part of the image, and then plays it back on your phone’s screen to create a picture.

Some of the satellites orbiting Mars take this to the next level. They don’t just take different slices of colored light, but also longer wavelength, infrared light. If you’ve ever seen an image from a thermal imaging camera, you know what this is. Parts of you show up as hotter or colder on the screen. It’s the same with the surface of the earth, or Mars. Scientists can compare the intensity of different wavelengths of light from each point on the surface. They can then compare these values, with what would be expected for different rock types. In other words, we’re able to roughly determine the types of rocks on the Martian surface without ever setting a boot, or rover tread, on the red planet.

Data from these images has shown that the Valles Marineris have layered rock formations both on the sides of the canyons, and within them. The great valley has seen many landslides over the last 3.5 Billion years of its existence, as well as new and smaller canyons carved into it. Scientists now speculate that rather than just forming as a big crack in the Martian surface, the Valles Marineris have been sculpted by flowing water, either in its liquid form as rivers, or in its solid form as glaciers.

An alternative hypothesis proposes that the Valles Marineris formed as a crack during a massive, planetary scale landslide. This landslide was about half the size of the US or China. How do you form a landslide that big? Well, you need a large pile of relatively weak rock, and high elevations for the landslide to flow from.

A key player here is salt. Salt is relatively weak as compared to rock, and can deform easier when squeezed. It can also hold water, which can be driven off by heating. On Earth, weak salt layers are partly responsible for undersea landslides in the Gulf of Mexico. The Opportunity rover had found some salt layers during its mission on Mars, so we know salt is present on the red planet.

Some scientists interpret the layers on the sides of the Valles Merinaris to be made of salt, and possibly include pockets of ice. This would imply that those layers are weak, and could potentially move downhill under the right circumstances.

Heating in the Tharsis region helped de-water salts under the future landslide, melted ice pockets, and created high elevations on one side of it. Think of it like putting a can on a wet metal sheet. If you raise one side of the sheet, the can will slide to the lower side. Just like that, the salty Martian crust broke, and slid downhill.

A crack in the side of this landslide allowed massive amounts of underground water to escape. As the water flowed downhill, it eroded the crack to form a massive canyon. This canyon is the Valles Marineris. The flood that helped form the Valles Marineris was probably bigger than any seen on earth. Bigger than the massive glacial outburst floods that formed the channeled scablands of the northwestern United States. Dick Gibson discussed outburst flooding in the December 27, 2014 episode. Unlike the Earth, the Martian surface has been relatively quiet since the Valles Marineris formed 3.5 billion years ago.

—Petr Yakovlev

This episode was recorded at the studios of KBMF-LP 102.5 in beautiful and historic Butte, Montana. KBMF is a local low-power radio station with twin missions of social justice and education. Listen live at butteamericaradio.org.

Episode 390 Mud Volcanoes

As the name implies, mud volcanoes are eruptions of mud – not molten rock as in igneous volcanoes.  They’re found all around the world, amounting to about a thousand in total number known. The one thing they have in common is hot or at least warm water, so they occur in geothermal areas especially, but they also are found in the Arctic.

They range in size from tiny, just a few meters across and high, to big things that can cover several square miles. In Azerbaijan some mud volcanoes reach 200 meters, 650 feet, in height, and around the world many of them do have conical, volcano-like shapes. But there are others that are just low mounds, more like a shield volcano.

A little (15-cm) mud volcano in New Zealand.
Photo by Richard Gibson.

The mud is often enough just a slurry of suspended fine-grained sediment that mixes with the hot water. And by hot water, we don’t necessarily mean incredibly hot – mud volcano temperatures as cold as a couple degrees Centigrade are known, but most are associated with temperatures approaching the boiling point of water.  In some places, like Yellowstone, the water is acidic which helps it dissolve rocks down to the tiny fragments in mud, and in other places it may just be the weathered soil and debris picked up by the water that makes the mud.

Mud volcanoes can erupt violently, or seep slowly, and emissions can last from minutes to years. I think it’s fair to think of some of them as geysers in which the water contains a lot of sediment, while others are more like thick, viscous muddy warm springs.

Besides water and fine sediment, mud volcanoes often contain natural gas – most commonly methane, but sometimes carbon dioxide, nitrogen, or other gases. The pressure of these gases is often the driving force behind eruptions, and with a hydrocarbon gas like methane present you might think mud volcanoes would be associated with oil and gas fields, and you’d be right. The hundreds of mud volcanoes in Azerbaijan and in the adjacent Caspian Sea are in the midst of the first great oil province to be exploited, and some of the petroleum deposits there are related to structures in the rocks and sediments caused by the upward force of the mud, which can bend its confining rocks as it rises, just as a salt dome can do. And since methane is flammable, often enough there are flames associated with mud volcanoes. In 2001, near Baku, Azerbaijan, flames shot 15 meters, near 50 feet, into the air. Gobustan in Azerbaijan is a World Heritage Site for its abundant rock carvings dating to 5000 to 20,000 years ago or more. The flaming methane eruptions of mud volcanoes in Azerbaijan have been linked to the development of the Zoroastrian religion, and in fact the name Azerbaijan derives from words meaning Land of the Eternal or Sacred Fire.

The most destructive mud volcano eruption in recent years was on the island of Java, in Indonesia, in May 2006. It erupted in the middle of a rice paddy, and ultimately killed 20 people, caused nearly 3 billion dollars in damage, and displaced 60,000 people. The mud it erupted covers about seven square kilometers, nearly three square miles, and in 2018 it continues to erupt something like 80,000 cubic meters of mud every day – that’s almost 3 million cubic feet, 32 Olympic swimming pools each day.

What caused the violent and extensive eruption of the Lusi Mud Volcano, also called the Sidoarjo mud flow, on Java is not clear. It may be simply part of the ongoing natural tectonic and magmatic processes in the region, which is dotted with many real volcanoes, the kind that carry molten rock to the surface as lava, and there’s a fault system that may provide a conduit for hot water from a volcano about 50 kilometers away. Lusi may be an entirely natural phenomenon. But there are also interesting possible trigger mechanisms. One suggests that a large earthquake two days before the mud volcano erupted changed the plumbing system enough to spur the eruption. That’s reasonable, since we know that earthquakes can have significant effects on geyser systems. Old Faithful in Yellowstone changed its eruption period following the strong Hebgen Lake earthquake in 1959. The other possible trigger is nearby drilling by a gas exploration company, which may have encountered an open pocket of gas or some other feature that ultimately may have allowed enough pressure to build up to make the mud volcano erupt. Good science on all sides of this issue have not resolved its origin with certainty, but on the whole I think the consensus is that the mud eruption was indeed triggered by the drilling. Studies continue, and there are legal cases in progress too, of course.

Sidoarjo Mud Flow, Indonesia, 2008

NASA image created by Jesse Allen, using data from NASA/GSFC/METI/ERSDAC/JAROS, and the U.S./Japan ASTER Science Team. Caption by Michon Scott, based on interpretation by Geoffrey S. Plumlee, U.S. Geological Survey Crustal Imaging and Characterization Team. Source 

Another mud volcano that was recently in the news is in Taiwan. Taiwan has at least 17 mud volcanoes which have been known for centuries, and the flammable natural gas associated with them was used in brick-making in southern Taiwan. The gas is probably methane, and it sometimes ignites naturally. The Wandan mud volcano in this area has a sporadic history, dormant for 9 years in the 1980s but erupting with damage in 2011 and 2016. Taiwan is on the subduction zone between the Philippine plate and Eurasia, complicated by a change in orientation of the subduction zone where Taiwan sits. This complex tectonic setting, together with the heat liberated by subduction, is probably the ultimate cause of the earthquakes, geologically recent volcanism, and the mud volcanoes on Taiwan.

Mud volcano eruptions are probably no more predictable than real volcanoes or earthquakes, but their similarity to geysers might give at least an element of predictability to them. A mud volcano that erupted in Trinidad in February 2018 seems to have a period of about 25 to 30 years, but that’s obviously a pretty wide range. The most recent event at Trinidad’s Devils Woodyard mud volcano covered an area about 100 meters across and tossed mud six meters into the air. Like the features in Azerbaijan, the mud volcanoes in Trinidad are closely associated with hydrocarbon deposits, including Trinidad’s famous pitch lake – thick tarry oil at the surface of the land.

Most of the hot mud activity in Yellowstone isn’t really what you’d call mud volcanoes. It’s more boiling mud-rich hot springs like the Fountain Paint Pots, but every now and then they can make small cones, less than a meter high, and in the past there have been mud-rich geyser eruptions at Yellowstone.

By some estimates there are many more mud volcanoes on the sea floor than there are on land. The known offshore mud volcanoes are often associated with methane hydrates – methane gas frozen into ice in the sediment beneath the sea floor. So it would be no surprise that as those ice-methane complexes melt they might drive the development of mud volcanoes underwater.

—Richard I. Gibson

Links:

Trinidad 

Taiwan 

Indonesia 

Cretaceous and Cenozoic Vertebrates compilation

Smilodon and dire wolves (drawing by Robert Horsfall, 1913)

Running time, 1 hour. File size, 69 megabytes.

This is an assembly of the episodes in the original series from 2014 that are about Cretaceous and Cenozoic vertebrates.

I’ve left the references to specific dates in the podcast so that you can, if you want, go to the specific blog post that has links and illustrations for that episode. They are all indexed on the right-hand side of the blog.

Thanks for your interest and support!

Triassic and Jurassic Vertebrates compilation

Morganucodon, a possible early mammal from the Late Triassic. Length about four inches.Drawing by FunkMonk (Michael B. H.) used under Creative Commons license. 

Running time, 1 hour. File size, 68 megabytes.

This is an assembly of the episodes in the original series from 2014 that are about Triassic and Jurassic vertebrates.

As usual, I’ve left the references to specific dates in the podcast so that you can, if you want, go to the specific blog post that has links and illustrations for that episode. They are all indexed on the right-hand side of the blog.

Thanks for your interest and support!