By the late 18th century, scientists knew quite a lot about the planet we live on. They knew the dimensions of the Earth and even its distance from the sun and other planets. One would think that determining the age of the Earth would be relatively straightforward. However, there would be many discoveries (i.e. splitting of the atom, invention of television, nylon, and instant coffee) before scientists could figure out the age of our home planet. How could this be?
First of all, not a lot of people were studying geology. It simply was not thought of as an exciting subject. James Hutton, who happened to be in the minority, is given credit for creating the science of geology. In 1795, he wrote about the slow processes that shaped the planet. However, this landmark writing did little good in advancing the field because the writing was so boring no one could understand it. Hutton did, in fact, ask one important question:
Why are ancient clamshells and other marine fossils so often found on mountaintops?
Hutton came to the conclusion that the marine fossils had risen along with the mountains themselves. He also deduced that it was the heat within the Earth that created new rocks and continents. According to Hutton, it was also this heat that caused land to erupt from the surface and form mountain chains. The amazing thing is all of this conjecture was 200 years before plate tectonics would be adopted as a scientific theory.
Geology, as a field of science, did not spark much interests in the U.S. until Charles Lyell, the greatest geologist of the 19th century, came to give a series of lectures in Boston in 1841. Lyell had published The Principles of Geology in 3 volumes between 1830-33. This book was so influential that even Darwin took a copy of it with him on the Beagle voyage.
The Greater Yellowstone Ecosystem offers the rare chance for people (not just professional scientists) to immerse themselves in geology. Yellowstone’s geology is unique because you can actually see geological processes at work in real time. To get the most out of this geological journey it is important to know some basic points.
Geological time is divided into four big chunks known as eras: 1) Precambrian; 2) Paleozoic (541 mya); 3) Mesozoic (252 mya); and 4) Cenozoic (66 mya). There have been Precambrian fossils found in some national parks (see here for a complete list). However, the oldest layers deposited in Yellowstone are from the Paleozoic. Dinosaurs roamed much of the Yellowstone area during the Mesozoic. During the Cenozoic, most of the current geological features formed in Yellowstone, including eruptions that covered live trees in ash, mud, and debris around 50 mya. This caused trees to petrify.
Yellowstone has had 3 caldera-forming eruptions (2.1mya, 1.3mya, and 640,000 years ago). The eruption that occurred 2.1 mya spilled more than 600 cubic miles of ash. This ash was 2400X the volume of ash produced during the Mount St. Helens eruption in 1980.
The most recent eruption 640,000 years ago formed the current Yellowstone caldera. Typically, most of the ejected material from an eruption falls back and spreads laterally as turbulent flows of ash, gas, and rock. This is called pyroclastic flow. The ash/debris eventually forms a rock type called tuff. Ash-fall tuff is the deposited material from the initial blast while ash-flow tuff is deposited material from the pyroclastic flows.
So, what does an eruptive cycle look like (see Figure 1)? First of all, basaltic magma begins to form 125 miles below the Earth’s surface. This happens as the upper mantle is heated by the thermal plume. Next, this basaltic magma slowly flows up and accumulates in a magma chamber at the base of the crust. The continental crust (see Figure 2), made of granite, melts and forms another chamber of magma called rhyolite. Finally, the climax of an eruption is a great outpouring of hot gas, ash, magma, and chunks of rock that are ripped off the walls of the vents.
Figure 1-By Kbh3rd [Public domain], via Wikimedia Commons
Thermal features in Yellowstone are just holes in the Earth’s crust from which hot fluids (water, but also steam and gases) escape. Most of the water in Yellowstone’s thermal features comes from rain and snow melt. Hot bedrock in the subsurface heats the water, lowering it’s density and causing it to circulate back upward to the thermal features at the surface. The bedrock is heated both by the thermal plume beneath Yellowstone and by the rhyolite magma that is present below the surface. In some places this magma is located as close as 2 miles below the surface.
Enjoy this place not only for its magnificent wildlife, but also for its geology.
The video below gives a 1.5 minute overview of the geologic processes that have shaped and continue to shape Yellowstone and the Grand Tetons.