What last happened here about 2,000 years ago looked much like this photograph of a volcanic eruption in Hawaii. Bubbling, pooling, and flowing lava blanketed the landscape as molten materials poured or gushed out of the Earth. Most volcanic phenomena preserved at Craters of the Moon have been seen in action in Hawaii.
Geology of the Craters of the Moon
A 400-mile-long arc known as the Snake River Plain cuts a swath from 30 to 125 miles wide across southern Idaho. Idaho’s official state highway map, which depicts mountains with shades of green, shows this arc as white because there is comparatively little variation here compared to most of the state. Upon this plain, immense amounts of lava from within the Earth have been deposited by volcanic activity dating back more than 14 million years. However, some of these lavas, notably those at Craters of the Moon National Monument, emerged from the Earth as recently as 2,000 years ago. Craters of the Moon contains some of the best examples of basaltic volcanism in the world. To understand what happened here, you must understand the Snake River Plain.
Basaltic and Rhyolitic Lavas. The lavas deposited on the Snake River Plain were mainly of two types classified as basaltic and rhyolitic. Magma, the molten rock material beneath the surface of the Earth, issues from a volcano as lava. The composition of this fluid rock material varies. Basaltic lavas are composed of magma originating at the boundary of the Earth’s mantle and its crustal layer. Rhyolitic lavas originate from crustal material. To explain its past, geologists now divide the Snake River Plain into eastern and western units. The following geologic story relates to the eastern Snake River Plain, on which Craters of the Moon lies.
On the eastern Snake River Plain, basaltic and rhyolitic lavas formed in two different stages of volcanic activity. Younger basaltic lavas mostly lie atop older rhyolitic lavas. This portion of the plain runs from north of Twin Falls eastward to the Yellowstone area on the Wyoming-Montana border. Drilling to depths of almost 2 miles near the plain’s midline, geologists found ½ mile of basaltic lava flows lying atop more than 1½ miles of rhyolitic lava flows. How much deeper the rhyolitic lavas may extend is not known. No one has drilled deeper here.
Crossing Idaho in an arc, the Snake River Plain marks the path of the Earth’s crustal plate as it migrates over a heat source unusually close to the surface. It is believed that the heat source fueling Yellowstone’s thermal features today is essentially the same one that produced volcanic episodes at Craters of the Moon ending about 2,000 years ago.
This combination—a thinner layer of younger basaltic lavas lying atop an older and thicker layer of rhyolitic lavas—is typical of volcanic activity associated with an unusual heat phenomenon inside the Earth that some geologists have described as a mantle plume. The mantle plume theory was developed in the early 1970s as an explanation for the creation of the Hawaiian Islands. According to the theory, uneven heating within the Earth’s core allows some material in the overlying mantle to become slightly hotter than surrounding material. As its temperature increases, its density decreases. Thus it becomes relatively buoyant and rises through the cooler materials—like a tennis ball released underwater—toward the Earth’s crust. When this molten material reaches the crust it eventually melts and pushes itself through the crust and it erupts onto the Earth’s surface as molten lava.
The Earth’s crust is made up of numerous plates that float upon an underlying mantle layer. Therefore, over time, the presence of an unusual heat source created by a mantle plume will be expressed at the Earth’s surface—floating in a constant direction above it—as a line of volcanic eruptions. The Snake River Plain records the progress of the North American crustal plate—350 miles in 15 million years—over a heat source now located below Yellowstone. The Hawaiian chain of islands marks a similar line. Because the mechanisms that cause this geologic action are not well understood, many geologists refer to this simply as a heat source rather than a mantle plume.
Two Stages of Volcanism. As described above, volcanic eruptions associated with this heat source occur in two stages, rhyolitic and basaltic. As the upwelling magma from the mantle collects in a chamber as it enters the Earth’s lower crust, its heat begins to melt the surrounding crustal rock. Since this rock contains a large amount of silica, it forms a thick and pasty rhyolitic magma. Rhyolitic magma is lighter than the overlying crustal rocks, therefore, it begins to rise and form a second magma chamber very close to the Earth’s surface. As more and more of this gas-charged rhyolitic magma collects in this upper crustal chamber, the gas pressure builds to a point at which the magma explodes through the Earth’s crust.