AnnFoszz: West Thumb Geyser Basin

Yellowstone National Park sits on top of a giant volcano. The last time it blew its top was 600,000 years ago. The eruption left behind a 40-mile wide crater called a caldera. Over time, the lowest part of the caldera filled with water, forming Yellowstone Lake.

Hundreds of thousands of years later, pressure in the underground magma chamber found a weak spot in the earth's crust just to the west of the lake. The magma chamber "burped" and left behind a small caldera (a caldera within a caldera). The hole filled with water, uniting with Yellowstone Lake to form its "West Thumb".

The earth's crust is still thin here. Thermal activity abounds. Visitors are warned to stay on the boardwalks lest they scorch their feet on the hot ground.

Yellowstone is home to many different kinds of geothermal features. "Geothermal" means earth + heat. When you add water into the equation, you get a "hydrothermal" feature (water + heat). The classification of the feature depends on the amount of water, the chemicals in the water, and the feature's "plumbing".

Hot Spring: This is the most common hydrothermal feature in the park. Water gets heated underground and travels to the surface. It typically forms a pool at the surface.

Geyser: This feature is cyclical. The process starts with the accumulation of water in a chamber underground. The water gets heated. But unlike hot springs, it can't freely travel to the surface because the feature's plumbing has a constriction (like a nozzle on a garden hose). Pressure builds until the water flashes to steam and erupts out of the opening. Eventually, the pressure is relieved and the eruption stops. Groundwater re-accumulates and the cycle starts over.

Fumarole: If you remove the large supply of water from a hot spring, you're left with a steam vent called a fumarole.

Travertine Terraces: Sometimes the water in a hot spring contains high amounts of calcium carbonate (dissolved limestone). When the water hits the air, it loses carbon dioxide and can no longer hold onto its calcium carbonate (just like we learned in the caves a few days ago). The mineral gets deposited in a series of constantly shifting terraces.

Mudpots: Sometimes the water in a hot spring contains acid (often due to hydrogen sulfide being produced underground and percolating up through the water table). The acid breaks down the surrounding rock into clay. If there's enough water available, the clay becomes a mudpot. The surface of mudpots is frequently disturbed by bubbles of escaping gas.

Bacteria mat: When an area is constantly covered with warm water, colonies of microorganisms begin to form. These microorganisms are known collectively as "thermophiles". Examples include algae, bacteria, viruses, and fungi. The term "bacteria mat" refers to any thermophile, bacteria or not. Each species is adapted to live in a narrow temperature and pH spectrum. And each species exhibits a distinct color. So, we can use color to approximate the temperature of a hot spring and its runoff.

Different areas of the park are known for different features. West Thumb Geyser Basin has a few geysers and lots of hot springs.

This is the Abyss Pool. It's 53 feet deep.
The color tells us that archaea lives here. That means the pool's temperature is incredibly hot, about 199 degrees.

Note: at this elevation, water boils at approximately 200 degrees. So, that's why many pale blue hot springs are covered in steam.

This bacteria mat is gold and orange. So, assuming the pH is alkaline, we can approximate the temperature to be between 113-167 degrees.

(That's Yellowstone Lake in the background.)

Surging Spring

This pool is so hot that it actually boils in the middle. See the ripples?

Some of the thermal features are located in the lake itself.

This is Fishing Cone. Long ago, fishermen would reel in their catch, drop it into the boiling water of Fishing Cone, and pull out a fully cooked dinner.

Fishing Cone was also once a geyser. It quit erupting several decades ago and is now classified as a hot spring.

Remember those elk we almost hit earlier? Well, evidently they're part of a herd that frequents West Thumb Geyser Basin. The warm water attracts the mothers and their offspring.

The bad news for the elk is that the silica in the hot spring water wears away their teeth. But that's not a huge problem in Yellowstone. These elk are destined to be some wolf's or bear's dinner long before they die of old age.