• About 2 million years ago,
• then 1.2 million years ago,
• and then again 600,000 years ago.

Thermal activity here is extensive both over time and distance. Terrace Mountain, northwest of Golden Gate, has a thick cap of travertine. The Mammoth Terraces extend all the way from the hillside where we see them today, across the Parade Ground, and down to Boiling River. The Mammoth Hotel, as well as all of Fort Yellowstone, is built upon an old terrace formation known as Hotel Terrace. There was some concern when construction began in 1891 on the Fort site that the hollow ground would not support the weight of the buildings. Several large sink holes (fenced off) can be seen out on the Parade Ground. This area has been thermally active for several thousand years.

Norris sits on the intersection of three major faults. The Norris-Mammoth Corridor is a fault that runs from Norris north through Mammoth to the Gardiner, Montana, area. The Hebgen Lake fault runs from northwest of West Yellowstone, Montana, to Norris. This fault experienced an earthquake in 1959 that measured 7.4 on the Richter scale (sources vary on exact magnitude between 7.1 and 7.8). These two faults intersect with a ring fracture that resulted from the Yellowstone Caldera of 600,000 years ago. These faults are the primary reason that Norris Geyser Basin is so hot and dynamic. The Ragged Hills that lie between Back Basin and One Hundred Springs Plain are thermally altered glacial moraines. As glaciers receded, the underlying thermal features began to express themselves once again, melting remnants of the ice and causing masses of debris to be dumped. These debris piles were then altered by steam and hot water flowing through them.

The Grand Canyon of the Yellowstone

The specifics of the geology of the canyon are not well understood, except that it is an erosional feature rather than the result of glaciation. After the caldera eruption of about 600,000 years ago, the area was covered by a series of lava flows. The area was also faulted by the doming action of the caldera before the eruption. The site of the present canyon, as well as any previous canyons, was probably the result of this faulting, which allowed erosion to proceed at an accelerated rate. The area was also covered by the glaciers that followed the volcanic activity. Glacial deposits probably filled the canyon at one time, but have since been eroded away, leaving little or no evidence of their presence.

The canyon below the Lower Falls was at one time the site of a geyser basin that was the result of rhyolite lava flows, extensive faulting, and heat beneath the surface (related to the hot spot). No one is sure exactly when the geyser basin was formed in the area, although it was probably present at the time of the last glaciation. The chemical and heat action of the geyser basin caused the rhyolite rock to become hydrothermally altered, making it very soft and brittle and more easily erodible (sometimes likened to baking a potato). Evidence of this thermal activity still exists in the canyon in the form of geysers and hot springs that are still active and visible. The Clear Lake area (Clear Lake is fed by hot springs) south of the canyon is probably also a remnant of this activity.

According to Ken Pierce, U.S. Geological Survey geologist, at the end of the last glacial period, about 14,000 to 18,000 years ago, ice dams formed at the mouth of Yellowstone Lake. When the ice dams melted, a great volume of water was released downstream causing massive flash floods and immediate and catastrophic erosion of the present-day canyon. These flash floods probably happened more than once. The canyon is a classic V-shaped valley, indicative of river-type erosion rather than glaciation. The canyon is still being eroded by the Yellowstone River.

Evidence of the geological forces that have shaped Yellowstone are found in abundance in this district. The hills surrounding Old Faithful and the Upper Geyser Basin are reminders of Quaternary rhyolitic lava flows. These flows, occurring long after the catastrophic eruption of 600,000 years ago, flowed across the landscape like stiff mounds of bread dough due to their high silica content.

Evidence of glacial activity is common, and it is one of the keys that allows geysers to exist. Glacier till deposits underlie the geyser basins providing storage areas for the water used in eruptions. Many landforms, such as Porcupine Hills north of Fountain Flats, are comprised of glacial gravel and are reminders that as recently as 13,000 years ago, this area was buried under ice. Signs of the forces of erosion can be seen everywhere, from runoff channels carved across the sinter in the geyser basins to the drainage created by the Firehole River.

The West Thumb Geyser Basin, including Potts Basin to the north, is unique in that it is the largest geyser basin on the shores of Yellowstone Lake. The heat source of the thermal features in this location is thought to be relatively close to the surface, only 10,000 feet down! The West Thumb of Yellowstone Lake was formed by a large volcanic explosion that occurred approximately 150,000 years ago (125,000-200,000). The resulting collapsed volcano, called a caldera ("boiling pot" or caldron), later filled with water forming an extension of Yellowstone Lake. The West Thumb is about the same size as another famous volcanic caldera, Crater Lake in Oregon, but much smaller than the great Yellowstone caldera which formed 600,000 years ago. It is interesting to note that West Thumb is a caldera within a caldera.

Ring fractures formed as the magma chamber bulged up under the surface of the earth and subsequently cracked, releasing the enclosed magma. This created the source of heat for the West Thumb Geyser Basin today.

Yellowstone Lake

Geologists indicate that large volcanic eruptions have occurred in Yellowstone on an approximate interval of 600,000 years. The most recent of these (600,000 years ago) erupted from two large vents, one near Old Faithful, the Mallard Lake Dome, and one just north of Fishing Bridge, the Sour Creek Dome. Ash from this huge explosion, 1,000 times the size of Mt. St. Helens, has been found all across the continent. The magma chamber then collapsed, forming a large caldera filled partially by subsequent lava flows. Part of this caldera is the 136-square mile basin of Yellowstone Lake. The original lake was 200 ft. higher than the present-day lake, extending northward across Hayden Valley to the base of Mt. Washburn.

It is thought that Yellowstone Lake originally drained south into the Pacific Ocean via the Snake River. The lake currently drains north from its only outlet, the Yellowstone River, at Fishing Bridge. The elevation of the lake's north end does not drop substantially until LeHardy Rapids. Therefore, this spot is considered the actual northern boundary of Yellowstone Lake.

In the last decade, geological research has determined that the two volcanic vents, now known as "resurgent domes", are rising again. From year to year, they either rise or fall, with an average net uplift of about one inch per year. During the period between 1923 and 1985, the Sour Creek Dome was rising. In the years since 1986, it has either declined or remained the same. The resurgence of the Sour Creek dome, just north of Fishing Bridge is causing Yellowstone Lake to "tilt" southward. Larger sandy beaches can now be found on the north shore of the lake, and flooded areas can be found in the southern arms.

The Hayden Valley was once filled by an arm of Yellowstone Lake. Therefore, it contains fine-grained lake sediments that are now covered with glacial till left from the most recent glacial retreat 13,000 years ago. Because the glacial till contains many different grain sizes, including clay and a thin layer of lake sediments, water cannot percolate readily into the ground. This is why the Hayden Valley is marshy and has little encroachment of trees.

The thermal features at Mud Volcano and Sulphur Caldron are primarily mud pots and fumaroles because the area is situated on a perched water system with little water available. Fumaroles or "steam vents" occur when the ground water boils away faster than it can be recharged. Also, the vapors are rich in sulfuric acid that leaches the rock, breaking it down into clay. Because no water washes away the acid or leached rock, it remains as sticky clay to form a mud pot. Hydrogen sulfide gas is present deep in the earth at Mud Volcano. As this gas combines with water and the sulfur is metabolized by cyanobacteria, a solution of sulfuric acid is formed that dissolves the surface soils to create pools and cones of clay and mud. Along with hydrogen sulfide, steam, carbon dioxide, and other gases explode through the layers of mud.

The geology of the Tower district is incredibly varied. Major landforms are expressions of geologic events that helped shape much of the Yellowstone area. Absaroka volcanics, glaciation, and erosion have left features as varied as Specimen Ridge's petrified trees to the gorges along the Yellowstone River's Black Canyon and the Grand Canyon of the Yellowstone.

Mt. Washburn and the Absaroka Range are both remnants of ancient volcanic events that formed the highest peaks in the Tower District. Ancient eruptions, perhaps 45 to 50 million years ago, buried the forests of Specimen Ridge in ash and debris flows. The columnar basalt formations near Tower Fall, the volcanic breccias of the "towers" themselves, and numerous igneous outcrops all reflect the district's volcanic history.

Later, glacial events scoured the landscape, exposing the stone forests and leaving evidence of their passage throughout the district. The glacial ponds and huge boulders (erratics) between the Lamar and Yellowstone rivers are remnants left by the retreating glaciers. Lateral and terminal moraines are common in these areas. Such evidence can also be found in the Hellroaring and Slough creek drainages, on Blacktail Plateau, and in the Lamar Valley.

The eroding power of running water has been at work in the district for many millions of years. The pinnacles of Tower Fall, the exposed rainbow colors of the Grand Canyon of the Yellowstone at Calcite Springs, and the fearsome gorge of the Black Canyon all are due, at least in part, to the forces of running water and gravity.