Practicing Geology Exercise

Are the Siberian Traps a Smoking Gun of Mass Extinction?

The mass extinction at the end of the Permian period, dated at 251 million years ago, marks the transition from the Paleozoic era to the Mesozoic era, as described in Chapter 8. The flood basalts of Siberia—the product of the largest continental volcanic eruption in the Phanerozoic eon—have also been dated at 251 million years ago. Is this just a coincidence, or was the eruption of the flood basalts responsible for the end-Permian mass extinction?

Let’s first consider the size and rate of the Siberian eruption. Geologic mapping of these flood basalts, called the Siberian Traps, shows that they once extended across much of the Siberian platform and craton, covering an area exceeding 4 million square kilometers. Although much has been eroded away or buried beneath younger sediments, the total volume of the basalts must have originally exceeded 2 million cubic kilometers and may have been as much as 4 million cubic kilometers. Isotopic dating indicates that the basalts were extruded over a period of about 1 million years, implying an average eruption rate of 2 to 4 km³/year.

To appreciate how large this rate really is, we can compare it with the volcanism at rapidly diverging plate boundaries. Enough basalt is extruded along mid-ocean ridges to form the entire oceanic crust, so the production rate of seafloor spreading is given by the formula

The fastest spreading we see today is along the East Pacific Rise near the equator, where the Pacific Plate is separating from the Nazca Plate at an average rate of about 140 mm/year, or 1.4 × 10⁻⁴ km/year (see Figure 2.7), creating a basaltic crust with an average thickness of 7 km. The length of the Pacific-Nazca plate boundary is about 3600 km, so the production rate along this spreading center is

From this calculation, we see that the Siberian eruption produced basalt at a rate comparable to that of the entire Pacific-Nazca plate boundary, the largest magma factory on Earth today!

You can sail on the tropical sea surface over the Pacific-Nazca plate boundary and be completely unaware of the magmatic activity deep beneath you. Most of the magma generated by seafloor spreading solidifies as igneous intrusions to form the basaltic dikes and massive gabbros of the oceanic crust (see Figure 4.15). The basalts that are extruded onto the seafloor are quickly quenched by sea-water to produce pillow lavas, and the gases that are emitted dissolve into the ocean.

But if you were visiting Siberia about 251 million years ago, you would probably not be so comfortable. The Siberian basalts were erupted directly onto the land surface through fissures in the continental crust, flooding millions of square kilometers. This exceptionally rapid extrusion of lavas would have generated huge pyroclastic deposits—much more than typical flood basalt eruptions, such as those of the Columbia Plateau—and it would also have discharged massive amounts of ash and gases, including carbon dioxide and methane, into the atmosphere. Such an eruption could have triggered changes in Earth’s climate of a magnitude that might have led to the end-Permian mass extinction, in which 95 percent of the species living at the time were completely wiped out (see Chapter 11).

Some geologists have argued for years that the end-Permian mass extinction was the result of this intense Siberian volcanism, possibly caused by the sudden arrival of a “plume head” at Earth’s surface (see Figure 12.28). Others have preferred alternative hypotheses, such as a meteorite impact or a sudden release of gases from the ocean. However, recent isotopic dating with improved techniques has shown that the Siberian volcanism occurred immediately before or during the end-Permian mass extinction. The finding that these extreme events so precisely coincide has convinced many more geologists that the Siberian Traps are the “smoking gun” behind the largest killing of species in Earth history.

PROBLEM: The Big Island of Hawaii, which has a total rock volume of about 100,000 km³, has been formed by a series of basaltic eruptions over the last 1 million years. Calculate the production rate of the Hawaiian basalts and compare it with that of the Siberian Traps. What length of the Nazca-Pacific Plate boundary produces basalt at a rate equivalent to the Hawaiian hot spot?