Lecture 13 - Yellowstone National Park

Outline

I. Tectonic Setting of Yellowstone National Park

II. Hydrothermal Features- Geysers and Hot Springs

III. A Field Trip Through Yellowstone

 

I. Tectonic Setting of Yellowstone National Park

About one half of Yellowstone is occupied by a series of three large calderas that formed in gigantic, catastrophic eruptions 2, 1.2,and .6 million years ago. The hydrothermal activity (geysers and hotsprings) that Yellowstone is famous for result from the elevated temperatures that still exist within the most recent caldera. We knowof these 3 huge eruptions from the large volume of pyroclastic material they deposited with ages of 2, 1.2 and .6 million years. The first eruption (2 million years ago) produced about 2,500 km3 of ash called the Huckleberry Ridge Tuff (rock made of ash is called tuff),the second eruption (1.2 million years ago) was much smaller, about280 km3 of ash, and produced the Mesa Falls Tuff. Much of the calderas and the deposits from these earlier eruptions are covered by deposits and the caldera built by the third eruption .6 million yearsago. This eruption deposited about 1,000 km3 of Lava Creek tuff and produced the present Yellowstone Caldera which is 45 kmx 75 km. Let'scompare the size of these eruptions to others that we have studied or might know about:

Name

Caldera Size

Volume of erupted material

Yellowstone (.6 my ago)

75km x 45km

~ 1,000 km3

Mt. Mazama (Crater Lake)

7km x 8km

~75 km3

Mt. Pinatubo, Philippines

None

~10 km3

Mt. St. Helens

None

~1-2 km3

Remember that caldera eruptions are always very large because theloss of a lot of magma from a reservior below the surface causes thesurface to collapse leaving the caldera structure

What kind of magma was erupted at Yellowstone and how was it produced?

The magma at Yellowstone is rhyolite, the most silica rich and therefore explosive of all magmas. Like Hawaii, Yellowstone volcanism occurs within the middle of the North American Plate and is believed to also be due to a hot spot in the mantle below Yellowstone. Thishot spot is believed to melt large quantities of mantle material but unlike Hawaii, the basaltic magma beneath Yellowstone must travel through 30-40 km of continental crust. Contamination of the basaltic melt through the crust produces the rhyolite that is explosively erupted. Do we have the same kind of evidence for a hot spot origin as we see in Hawaii? Yes, we know that the North American Plate is moving southwesterly and we do see an age progression of volcanic centers, with the age of the volcanism getting younger in the direction opposite to the plate motion. The trace of the hot spot has produced a region known as the Snake River Plain that extends through Idaho to terminate at Yellowstone. Craters of the Moon National Monument is located along the Snake River Plain. However the composition of the volcanic rocks there are basaltic. Since the basalts are younger than the rhyolites, this suggests that there has been an evolution of the magma along the hot spot chain from rhyolitic to basaltic.

There is certainly plenty of evidence that magma and its heat still exist beneath Yellowstone and that future eruptive activity is very possible. This evidence is:

  1. High heat flow- We have many measurements worldwide of heat flow coming out of the Earth. This value is high at volcanically active regions and even higher at Yellowstone.
  2. Seismic wave velocities- Earthquakes generate seismic waves that travel through the Earth at different speeds depending on the properties of the rock. These waves travel much slower through melted or partially melted rock. Seismic wave velocities of rocks beneath Yellowstone Caldera suggest that magma (melted rock) exists.
  3. Gravity measurements- Gravity measurements are sensitive to the density of rock. Molten or melted rock is much less dense than solid rock and gravity measurements near Yellowstone Caldera confirm the seismic measurements that motlen rock or magma is present beneath the surface there.
  4. Earthquake activity- Earthquake swarms occur in Yellowstone, suggesting magma movement at depth
  5. Ground deformation- two small domes in the caldera are expanding, suggesting that pressure is building beneath the caldera.

II. Hydrothermal Features- Geysers and Hot Springs

Yellowstone is one of the best regions to observe abundant hydrothermal activity which is the interaction of groundwater with hot rock. Yellowstone has well over half the world's know geysers, thousands of hot springs, mud pots and fumaroles. What are these features and how do they work?

  1. Geyser- intermittent hot spring that erupt jets of hot water and steam
  2. Hot Springs- Springs with temperature above body temperature (98.6 š F).
  3. Fumaroles- vent emitting gas
  4. Mud pot- hot springs with relatively little water and a lot of altered rock

Water sinks into the ground getting heated as it contacts the hot rock at depth. As it heats up, the great pressure above the rock keeps the water from boiling (turning to steam), however it does become less dense and begins to rise by convection. As pressure decreases as the water travels toward the surface, the hot water begins to boil. If pressure release is gradual, boiling is mild and ahot spring develops at the surface. However, if pressure is contained for a while and then released suddenly, boiling occurs suddenly and steam and water erupt in a geyser. The nature of the conduits thatthe water travels through determine if the boiling of the water will be fast or slow. If the conduits are very narrow, gas bubbles may block them and pressure builds up until it is violently released. After eruption of a geyser, the system has cleared itself and theprocess may repeat itself in a semi-regular fashion once a new influx of water has been supplied to the system. Old Faithful is an example eof a geyser that erupts fairly regularly. (Once ever 55-80 minutes depending on how much water and steam was erupted in the previous eruption).

Geysers tend to be associated with rhyolitic terranes because the hot water in contact with the rhyolite tends to dissolve some of the silica into solution. As the water cools, it reprecipitates the silica and this lines the conduits and makes them very narrow, preventing hot water from escaping. The silica deposits are called sinter and are also responsible for the crusts visible near geyser vents. Hydrothermal deposits outside of the caldera are travertine(chemically precipitated calcite -CaCO3) because the water here circulates through limestones and dissolves this to reprecipitate it.

III. A Field Trip Through Yellowstone (Slide Show)

[return icon]Return to Earth Sciences 3 Lectures Home Page


This page was last reviewed on 2/03/04.