Yellowstone Journal

In a little over a month from now my family and I will be on vacation. It is the first vacation in a very long time that is intended solely for enjoyment. We aren't going someplace to visit family or friends, we are going to sight-see and have fun. We are going to Yellowstone National Park. Each of us came up with two places we would be happy going to see. That totalled out to 6 places. Each of us then ranked the places from 1 to 6 as to the order of how much we wanted to see it. The numbers for each place was totalled and the one with the lowest total won ... Yellowstone.

Since Yellowstone was from my list I feel like it is my responsibility to make sure my wife and son have a good time. In order to do that I am trying to become as familar with the region before we go as I can. I thought it might be a good thing to write up some blogs about Yellowstone before I go. This first one will deal with the formation of the area.

When I think of Yellowstone, the first thing that comes to mind is geysers. Yellowstone contains 300 to 500 geysers of which 200 to 250 erupt every year. That is about half of all the geysers in the world. The story of how this area formed is an interesting one.

As most people know, deep below the surface of the earth is molten rock ... the mantle. The crust of the earth is actually a thin layer of relatively light material that floats on this molten sea. The deeper part of the mantle is hotter, and hot things expand. As they expand they become lighter and in a fluid medium light things will rise. This means that the mantle has convection currents in it. These currents move the crust.

Over deep time the movement of the earth has caused the crust to be broken up into a series of plates. The currents in the mantle cause these plates to move. Some plates are forced together while others are forced apart. Usually it is at the edge of these plates that earthquakes and volcanos occur.

Yellowstone, however, is an exception. It is not located particularly close to a plate boundary. It's geothermal activity is associated with a "hotspot". A hotspot is thought to be a place where a superheated plume of magma rises from the lower parts of the mantle toward the surface. These plumes seem to remain at the same site for millions of years. The Hawaiian Islands are part of sea chain archipelago of mountains that are thought to have been formed over such a hotspot over many millions of years:

The base rock of Yellowstone is 2.7 billion year granite and gneiss. These are metamorphic rock that form under high pressures (at least 10 miles below the surface). They are thought to be the root rock of a 3 billion year old mountain range that has long since eroded away. This erosion has made it impossible to say much about the natural history of the Yellowstone region for the next 2.9 billion years.

There are some rocks that seem to be the roots of another long since eroded mountain range (1.6 billion years old) to the west of the park. There are a few sedimentary rock formations date to 350 million years ago. But it was not until 100 million years ago was there a period of geologic activity that is important in the formation of Yellowstone as we now know it.

Some 600+ miles west of Yellowstone the Juan de Fuca tectonic plate is being pushed underneath the North American plate. This process is called SUBDUCTION. As the crust from the Juan de Fuca place is forced into the hot magma of the upper mantle it heats up and becomes molten itself. It rises up and forms volcanoes. Present-day volcanos involved in this process include such famous ones as Mt Ranier and Mt St. Helens. In earlier times this process helped form the Sierra Nevada mountains to the west of Yellowstone and the two major mountain chains that lie within Yellowstone ... the Gallatin range on the west and the Absaroka range on the east. This period of volcanic activity ended around in the Yellowstone area about 40 million years ago.

Then about 2 million years ago plate tectonics moved the Yellowstone area over the geologic hotspot. It is then that things got really interesting. As molten rock from the plume invades the native rock it caused the area to bulge upward. In Yellowstone a huge magma chamber was created. Water in the area becomes steam and enough steam can build up in the magma chamber to cause high pressures. This pressure can be released in cataclysmic explosion in which huge amounts of material can be blown out into the atmosphere leaving a deep hole in the ground. This hole is called a CALDERA.. In the Yellowstone region there have been 3 such explosions.

The first one is called the Huckleberry Ridge Caldera event. It happened 2 million years ago. This was an incredibly huge explosion. It is estimated that it ejected 600 cubic miles of material into the atmosphere. Within Yellowstone there are areas in which ash from this explosion is 550 feet thick. It is still 3 inches thick in parts of Iowa.

The caldera does not just stay there. It fills up with magma. It so happens that the magma in the Yellowstone region is RHYOLITE. Rhyolite has a large content of silica which makes it very viscous and tacky. As the plume from the hotspot sends up more magma the process was repeated.

About 1.3 million years ago the second cataclysmic eruption took place. This was the Mesa Falls Caldera event (also called the Island Park Caldera event) the smallest of the three eruptions but still put 60+ cubic miles worth of material into the atmosphere.

The third eruption formed the Yellowstone Caldera. It happened 640,000 years ago. It was not as strong as the Huckleberry Ridge Caldera event, but it was still impressive ejecting 240 cubic miles worth of material into the atmosphere. To put it into perspective, it blew out 1000 times more material than did the 1980 Mt St Helens eruption as shown in the following video:

If you were paying close attention you may have noticed that these three eruptions occurred about 700,000 years apart. The last one being 640,000 years ago. Given the error inherent in measuring the exact time of these events, is 640,000 years ago uncomfortably close to 700,000 suggesting that another such "supervolcano" event can happen soon? It is true that the hotspot plume is still there ... that is the ultimate cause of Yellowstone's geothermal features. Geysers are steam that is being vented from localized magma pits. However, while such an event could happen theoretically, it is VERY unlikely to happen within our lifetimes or the lifetimes of anybody we will ever know. Furthermore, for all we know there may never be another such event.

The formation of the Yellowstone Caldera sparked the formation of one of Yellowstone's most scenic features ... the Grand Canyon of the Yellowstone.

Prior to the caldera being formed the Yellowstone river ran through the Absaroka range north of the park. Volcanic ash deposited by the eruption channeled the river to its present course.

The final geologic mechanism that formed Yellowstone into what it is today is glaciation. Glaciers during the last two ice ages (at least) covered nearly the entire park. Some of these glaciers were a half mile thick. Their movement carved out the valleys of the park. The glaciers last retreated (for the most part) about 10,000 years ago ... about the same time that human activity began in the park region. But that will be left for another blog.

Cheers,

Darwin's Beagle