Episode 394 The Mangrullo Formation of Uruguay

Today we’re going back about 280 million years, to what is now Uruguay in South America.

280 million years ago puts us in the early part of the Permian Period. Gondwana, the huge southern continent, was in the process of colliding with North America and Eurasia to form the supercontinent of Pangaea. South America, Africa, Antarctica, India, and Australia had all been attached to each other in Gondwana for several hundred million years, and the extensive glaciers that occupied parts of all those continents were probably still present in at least in highlands in southern South America and South Africa, as well as Antarctica.

But the area that is now in Uruguay was probably in cool, temperate latitudes, something like New Zealand or Seattle today. The connection between southern South America and South Africa was a lowland, partially covered by a shallow arm of the sea or perhaps a broad, brackish lagoon at the estuary of a major river system that was likely fed in part by glacial meltwater from adjacent mountains. We know the water was shallow because the rocks preserve ripple marks produced by wave action or currents.

The basin must have been near the shore because delicate fossils such as insect wings and plants are among the remnants. It looks like this shallow sea or lagoon became cut off from the ocean, allowing the waters to become both more salty, even hypersaline, and anoxic, as the separation restricted inflows of water, either fresh or marine, that could have continued to oxygenate the basin. In the absence of oxygen, excellent preservation of materials that fell to the basin floor began, and there were few or no scavenging animals to disrupt the bodies.

The rocks of the Mangrullo Formation, as it’s called today, include limestones and siltstones, but the most important for fossil preservation are probably the extremely fine-grained claystones and oil shales. These rocks contain some of the best preserved fossil mesosaurs known anywhere. That’s mesosaurs, not the perhaps more well-known mosasaurs, which are large whale-like marine reptiles that lived during Cretaceous time. Here, we’re in the Permian, well before the first dinosaurs.

Mesosaur by Nobu Tamura (Creative Commons license & source) 

Mesosaurs were aquatic reptiles, and they are the earliest known. They evolved from land reptiles and were among the first to return to the water to adopt an aquatic or amphibious lifestyle. They were once thought to be part of a sister group to reptiles, a separate branch of amniotes, which are animals that lay their eggs on land or bear them inside the mother, like most mammals do. In that scheme, mesosaurs and reptiles would have diverged from a common, earlier ancestor. But more recent studies categorize them as reptiles that split off from the main genetic stem early in the history of the class, so they’re pretty distant cousins to dinosaurs and all modern reptiles, but they’re still reptiles. There is ongoing debate among evolutionary paleontologists as to exactly where mesosaurs fit.

The fossils in Uruguay are so well preserved that we can identify the gut materials of mesosaurs, and we know they mostly ate crustaceans, aquatic invertebrates related to crabs, shrimp, and lobsters. The preservation is so exceptional that in some cases, soft body parts are preserved including major nerves and blood vessels in mesosaurs and stomachs and external appendages in the crustaceans. The earliest known amniote embryos also come from these fossil beds.

Mesosaurs had a short run in terms of their geologic history, only about 30 million years. They were extinct about 270 million years ago, well before the great extinction event at the end of the Permian, 250 million years ago. But the presence of coastal-dwelling mesosaurs in both South America and Africa was a contributing idea in the early development of the theory of continental drift, since it was presumed that they could not have crossed the Atlantic Ocean as it is today.

—Richard I. Gibson

Links:

Piñeiro et al. 2012 Environmental conditions 

Paleogeography from Ron Blakey 

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