Source: NOVA: "Deadliest Volcanoes"
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Geologists are scientists who study the history of Earth and the ongoing processes that have shaped the planet. One way they do this is to analyze rocks that have been deposited in a specific region. Rocks provide a detailed record of events throughout Earth’s history, including volcanic eruptions that have shaped the landscapes seen today. By understanding eruptions from the past, geologists can better predict how these events could affect humans in the future.
One of the most well-known volcanic complexes is the Yellowstone Plateau, located in Wyoming, Montana, and Idaho. Through studying the rocks at the Yellowstone volcanic field, geologists discovered that over the past 2.1 million years, there have been three major eruptions. Each eruption produced between 280 and 2,450 cubic kilometers of magma—molten rock that is ejected from Earth’s interior during a volcanic eruption. These eruptions have classified Yellowstone as a supervolcano. The term refers to a volcano that has produced over 1,000 cubic kilometers of material during an eruption at one point in time (the material would fill at least 400,000,000 Olympic-size swimming pools). For comparison, the catastrophic 1980 eruption of Mount St. Helens in Washington State produced about 0.25 cubic kilometers of material. For Yellowstone, the loss of that much material from beneath the surface years ago resulted in the overlying rock collapsing to fill the empty void, forming a volcanic depression called a caldera. But don’t be fooled by the flattened landscape seen at Yellowstone today—the volcano is still very active.
Researchers use various methods to study the Yellowstone volcanic complex. In this video, scientists use seismometers to record earthquake signals from around the world and then use these seismic waves to scan the subsurface through a technique called tomography. Scientists can calculate the average speed of seismic waves traveling from an earthquake to a seismometer by knowing the distance the waves travel and the time it takes them to reach a seismic station. Different materials affect the speed of seismic waves—they travel faster through colder rock and more slowly through hot materials like magma. Therefore, it is inferred that seismic waves arriving late to a seismic station have traveled through warmer Earth material that slowed them down. By looking carefully at arrival times and other signatures of seismic waves, scientists can map three-dimensional features beneath the surface. As networks of seismometers become denser, mapped images become clearer and reveal more information about the structure of the planet and the types of materials beneath the surface involved in Earth processes. At Yellowstone, seismic tomography reveals a long column of partially molten rock that is pushing its way through the surface, feeding a volcano in the middle of the North American Plate. This is the same process responsible for creating the Hawaiian Island volcanic chain.
Here are some of the main ideas students should take away from this video:
The Earth Science Literacy Principles highlight the big ideas and supporting concepts in Earth Science. Use this video to highlight Big Idea 1: Earth scientists use repeatable observations and testable ideas to understand and explain our planet.
NARRATOR: One of the greatest mysteries surrounding the Yellowstone supervolcano is its location. The world is made up of giant plates of rock, called tectonic plates. Most of the world's volcanoes lie on the edges of or between these plates, where it's easier for magma to form and reach the surface.
The most active edges are the ones surrounding the Pacific Ocean. This is the "Ring of Fire." It includes some of the most populated countries on Earth, including Indonesia, Japan and the U.S.A. Three-quarters of the world's active volcanoes are on the Ring of Fire.
But Yellowstone is different. It isn't on the edge of a plate. It's right in the middle of one, the North American Plate. How can there be a volcano here, so far from a plate boundary?
We know, from looking at the rocks of Yellowstone, that the volcano has been active for a long time. Two million years ago, Yellowstone produces one of the largest eruptions ever seen on the face of the earth. It blasts away the peaks we normally associate with volcanoes and leaves this strange flattened landscape. Yellowstone hasn't had a major eruption on this cataclysmic scale for over 640,000 years, but the hot springs and geysers are signs that there's a lot of heat under the surface.
This volcano is very much alive, and it is intensely monitored. Here, seismometers are vital.
Bob Smith is the scientist in charge.
BOB SMITH (University of Utah): I'm opening the lid, this is going to fall down, of a seismic vault. And inside is a seismometer, which sends a signal on to a telemetry network. And that antenna is pointed to a receiver about 20 miles down, on the lake. So anything that we're doing right now, it's recorded right now, on my computer at Salt Lake City, as are the entire seismic and GPS networks.
NARRATOR: The easiest way underground magma reaches the surface is through a channel, created by fracturing the rock above it. As the rock cracks, it creates vibrations, small earthquakes that occur days or weeks before an eruption. It's these vibrations that seismometers detect.
Smith is confident that the seismometers will give some kind of warning of the next eruption, but when that might happen, no one knows. At the moment, there's no major increase in activity. Yellowstone is sleeping.
But scientists also use these earthquakes and GPS to reveal what's inside the volcano and solve the mystery of why Yellowstone is here, in the middle of North America.
BOB SMITH: The seismographs do two things: they record the earthquakes that are active, and then they're used to scan the subsurface that we can't otherwise see.
NARRATOR: Seismic waves travel faster through cold, solid rock and slower through hot, molten rock. As seismic waves from distant earthquakes travel through underground magma reservoirs, they slow down. By measuring the waves at multiple points, it's possible to build up a three-dimensional picture of where the magma underneath Yellowstone is located.
BOB SMITH: We can construct a three-dimensional picture, like an M.R.I. scan or CAT scan, by recording thousands of earthquakes into our array of seismographs.
NARRATOR: The seismic network at Yellowstone is extensive, with over 150 seismometers, spread out over 100,000 square miles.
Bob Smith is able to look deeper than ever before, hundreds of miles down. And what he's discovered is striking: a huge structure of partly molten rock beneath Yellowstone.
This is a mantle plume, a rising column of magma, stretching at least 400 miles into the earth.
HENRY HEASLER (Geologist, Yellowstone National Park): This is the same concept as a lava lamp. You heat up the wax in the base, over a bulb, and then it slowly rises up because it's less dense. That's the basic concept for a mantle plume.
NARRATOR: For scientists, seeing this for the first time is a breakthrough. It solves the mystery of why the Yellowstone volcano is here. This plume is powerful enough to force its way through the middle of the North American Plate and create a volcano where there wouldn't normally be one.
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