When does Yellowstone start?
When looking at the geologic history of Yellowstone, we must ask ourselves when we would like to start. At what point did this region become the Yellowstone we know? I like to ask folks what comes to mind when they think of Yellowstone. (This is actually a fun exercise to do with any area you feel connected to, so long as you come ready for any answer with as little judgment as you can muster.) If we define Yellowstone by the existence of the park, its we can squarely point to 1872. If we want to say it’s the geysers, we have to push it back to after the ice receded some 12,000 years ago. If we say Yellowstone is everything resulting from the volcanic hotspot, we must go back nearly 30 million years.
The myopic bipedal primate in me says that the only things that matter are things that can be tied to what is in the park currently. But maybe that is everything?
What is Geologic Time?
Eras and periods are often distinguished by the unique fossilized flora and fauna characteristic of each timeframe. Throughout Earth's history, there have been six major events marked by rapid and significant declines in biodiversity, the variety of species present from one geological layer to another. These profound reductions are termed mass extinctions.
Among these, the most famous is the K-T extinction event, which abruptly ended the Cretaceous period and brought the age of the dinosaurs to a close. This extinction is widely attributed to a massive asteroid impact. However, the most devastating mass extinction was the Permian Mass Extinction, during which approximately 96% of all species on Earth vanished, profoundly reshaping the planet's biological landscape.
Geologic time is indeed challenging to grasp because it spans billions of years, far beyond a human lifespan. To make sense of this vast timeline, scientists divide it into hierarchical time units. The largest commonly used units are eras, which are further divided into periods.
Eras provide a broad framework for Earth's history and are often named using Latin roots combined with "-zoic," which means "life." This suffix reflects major changes in life forms during each era:
Paleozoic Era (“old life”): This era, ranging from about 541 to 252 million years ago, saw the emergence of most early life forms, including the first fish, insects, and amphibians.
Mesozoic Era (“middle life”): Lasting from 252 to 66 million years ago, this era is famous for dinosaurs and the rise of the first birds and mammals.
Cenozoic Era (“new life”): Starting 66 million years ago and continuing to today, this era is characterized by the dominance of mammals and the development of humans.
Within these eras, periods offer finer divisions, often named after geographic regions where rocks from that time were first studied or notable features of the time (e.g., Jurassic, named after the Jura Mountains). By breaking down geologic time this way, we create a manageable framework to better understand Earth's deep history and the evolution of life.
Volcanism in Yellowstone
Yellowstone is one of the most well-known volcanic regions in the world, primarily known for its supervolcano—a massive volcanic system with the potential to produce extremely large and powerful eruptions.
Types of Lava in Yellowstone
One can find both mafic and felsic rock types in Yellowstone, depending on where one looks. Mafic rocks tend to be darker and color and cool into rocks like basalt. Felsic rocks tend to be lighter in color and cool into granites or rhyolites. Felsic magma is nearly entirely molten silica, and because of this is exceptionally viscous.
Eruption Types in Yellowstone
Yellowstone’s eruptions align with the Plinian eruption style. These explosive eruptions produce large ash clouds that can rise into the stratosphere and cause widespread ashfall. Historically, Yellowstone's supereruptions have ejected thousands of cubic kilometers of volcanic material, drastically reshaping the landscape and impacting climate on a global scale.
Yellowstone had other effusive eruptions between these massive explosive events. Some of these eruptions were akin to the eruptions we see in Hawaii that resemble rivers rushing downhill 10-20 feet deep; others would have been more than 500-1,000 feet in depth and moving just a few inches per day. They may have resembled massive lines of Play-Doh slowly being pushed from the extruder or honey spreading from a jar dropped on the floor.