The Hot Spot
The Grey "Cap" that you see on the hill before you was deposited after a volcano erupted violently 3 million years ago. Volcanic ash was still so hot when it landed, it welded together as layers of stone above you.
The volcanic eruption that left behind this grey cap was one of over 100 volcanic eruptions that occurred across Southern Idaho in the past 16 million years. That series of eruptions left a footprint across southern Idaho - the Snake River Plain. The same source of volcanic activity (the "hot spot") now lies beneath Yellowstone national Park in western Wyoming. This volcanic activity creates the geysers and hot springs that make Yellowstone famous.
In many places throughout the United States, geothermal waters were used by Native Americans for bathing, cooking, warmth, and medicinal purposes. Hot springs served as a neutral ground for warring tribes. Native Americans have a history at every major hot springs in the United States (U.S. Department of Energy, 2002). Some of the hot springs of Idaho were popular places for Native Americans to congregate as indicated by artifacts and petroglyphs on nearby rocks. Some of the hot springs have recorded histories of over 100 years. Dansart et al. (1994) reported that Heise Hot Springs has been using geothermal water for recreation purposes since the late 1800’s.
Eleven miles west and slightly north of Cress Creek are the Menan Buttes. The buttes rising 800 feet above the surrounding plain, are extinct volcano cones built through violent eruptions 10,000 years ago. The eruptions that formed the Menan Buttes were unusually violent. Most of the recent volcanic eruptions in this region were like those that we see in Hawaii today, where red-hot lava emerges from a crack in the earth and then flows slowly along the surface. Many such lava flows have covered eastern Idaho in black “lava rock” over a mile deep.
The eruptions that formed the Menan Buttes were unusual and rare because they occurred through cold water of the Henry’s Fork of the Snake River. An abandoned river channel west of North Menan Butte indicates that the Snake River actually flowed where the North Butte now stands and at this time the river probably contained much more water than it does today. The violent eruptions of the Buttes blocked the flow of the river and forced it to the south. The Menan Buttes are the only eruptions in the United States that occurred through water. Because of the sudden chilling that occurred when the molten magma came in contact with the cold water in the river, no molten lava flowed out. Instead, the sudden chilling changed the molten magma into small fragments of glass called tachylite, which were cemented together to form the volcanic rock known as tuff. The Menan Buttes are truly glass mountains, being made of small fragments of solid glass formed by the sudden chilling of the magma. The glass is of olivine basalt composition, which lends to the dark color of the tuff.
Most of the area’s rocks came from lava and ash flows. However, these rocks are light-colored, not like darker volcanic rocks, such as those found at Craters of the Moon National Monument, are called basalt. The rocks here, called rhyolite (RYE-o-light), are usually light grey.