The Yellowstone region is a wonderful place to study geology first-hand, because the rise of the Rocky Mountains shows us so much material that might be buried elsewhere in the world. Glaciers and rivers have also carved swaths through billions of years of prehistory recorded in the rocks.
During the Cambrian period, 570 to 75 million years ago, the Cambrian and Cretaceous periods saw the flooding and recession of shallow inland seas at least twelve times. Sandstone, shale, limestone and dolomite, sedimentary rocks, were formed as sand and remains of ancient living things were crushed under the weight of new layers of rocks.The oldest rocks in Yellowstone were formed about 2.7 billion years ago, during Precambrian times. They include gneisses, schists and granite. Gneisses are metamorphic rock, formed by heat and pressure, and characterized by stripes of various kinds of minerals. Schist is a kind of metamorphic rock that contains a great deal of mica and which flakes easily. Granite is a form of igneous rock, the kind of rock formed from the cooling of magma. It was grains of various kinds of minerals, giving it a gray, black, pink or white color with grains of different colors include throughout.
The earliest fossil forms found in the area are Precambrian, dating from the proterozoic era 2.5 billion to 543 million years ago. The fossils are microbial, and are believed to have been similar to some of the microbes living in Yellowstone’s hot springs today. Most rock from the period is metasedimentary, meaning that it was formed from sediments that gradually changed from pressure or heat.
During the cretaceous period much of North America was covered by a shallow inland sea. From about 100 million years ago to about 65 million years ago, Yellowstone was under a few hundred feet of water. The branch of the sea that covered Yellowstone is called the Western Interior Seaway. Sand settled to the bottom, eventually becoming cemented together to form sandstone. Siltstone was formed from rivers flowing into the seaway. Ammonites, small shelled marine animals that somewhat resembled today’s snails, inhabited the inland sea and left fossils in this sedimentary rock. Other fossils from the inland sea include the remains of a pleisiosaurus, a marine dinosaur of the time.
Other fossils, from various prehistoric time periods, include invertebrates such corals, bryozoans, brachiopods, trilobites, gastropods and crinoids. Vertebrate fossils include a dinosaur shell fragment, some dinosaur and fish bones, fish scales and teeth, an ancient horse, and an ancient bison. The list of fossilized plants is made up of over 150 species, including ferns, horsetail rush, conifers and deciduous trees.
Beginning about 75 million years ago, orogeny, or the building up of mountains, began and lasted for about 20 million years. Streams changed their courses during that time, causing erosion to shift directions. The upward movements of the earth’s crust probably set the scene for the Yellowstone volcanoes.
From 55 million to 40 million years ago several large volcanoes erupted and had lava flows. The magma cooled to form the Yellowstone plateau that covers the area today. Another period of uplift 10 million years ago formed mountains, canyons and mesas. Three million to about 70,000 years ago huge eruptions formed the calderas that are seen today. From 300,000 to 8,000 years ago three glaciers crawled over Yellowstone, creating giant moraines and boulder areas, gouging and smoothing the land, creating ridges and lakes. Some of the glaciers were present at the time of the lava flows, which must have made a spectacular sight.
Today a variety of rocks color Yellowstone. Geyserite, a glass-like rock chemically similar to opal, is formed where hot water percolates through volcanic rocks, dissolving the silica. When it cools silicon dioxide is formed. The formation is going on and can be seen at Crater Hills Geyser, Old Faithful, Castle Geyser, the Fountain Paint Pot area and Shell Spring, which is actually a small geyser.
Mammoth Hot Spring Terraces are made up of white travertine, a form of calcium carbonate, which is carried up with rising hot water that goes through limestone. When the calcium carbonate cools it forms the travertine that can be seen near the north entrance to the park. The travertine formations are still growing today.
Rhyolite, formed from lava flows, is a kind of granite with pieces of glittery black obsidian. It can be seen along the roads in Dunraven Pass and Canyon Village, on Mount Washburn, on Elephant Back Mountain, and around Lewis and Shoshone Lakes and the geyser basins. Yellow rhyolite is found along the Yellowstone River, which gave the area its name. Zircon and quartz crystals can also be found embedded in lava rocks.