Topography

Craters of the Moon lies at the north edge of the eastern Snake River Plain, a broad flat arc, concave to the north, which covers nearly 10,000 square miles of southern Idaho. It extends from the Yellowstone Plateau and the Teton Mountains on the east to the Oregon state line on the west. The Snake River borders the southern edge of the plain, which occupies almost a quarter of the surface of Idaho and contrasts markedly with the mountainous terrain that dominates the northern, central, and far southern parts of the state. The eastern Snake River Plain is essentially flat in this vicinity: vertical relief is a few hundred feet at Craters of the Moon and less than that elsewhere.

There are two distinct landforms in the monument: the foothills of the Pioneer Mountains in the north give way to the low relief of the lava flows in the rest of the monument. The monument's highest elevation, 7,729 feet above sea level, is in the Pioneer Mountains. Elevations gradually decrease from north to south; the lowest elevation is about 5,330 feet in the southeast corner of the monument. Within the lava flows, cinder cones provide the greatest vertical relief. The highest cinder cone is Big Cinder Butte which stands more than 700 feet above the surrounding plain. Nineteen other cinder cones are at least 100 feet high. The Great Rift is apparent from the linear alignment of the cinder cones.

Volcanic Features

The primary resource value of Craters of the Moon is the great diversity of basaltic features in a small area. Almost all the features of basaltic volcanism are visible at the monument.

Much of the volcanism of the Snake River Plain was confined to volcanic rift zones. A volcanic rift zone is a concentration of volcanic landforms and structures along a linear zone of cracks in the earth's crust. The Great Rift volcanic rift zone is a zone of cracks running approximately northwest to southeast across almost the entire eastern part of the Snake River Plain. The entire Great Rift is 62 miles long.

The Great Rift is an example of basaltic fissure eruption. This type of volcanic activity is characterized by extrusion of lavas from fissures or vents that is relatively quiet in comparison with highly explosive eruptions such as the 1980 Mount Saint Helens eruption.

Where the Great Rift intersects the earth's surface, there is an array of cinder cones, lava cones, eruptive fissures, fresh-appearing lava flows, noneruptive fissures, and shield volcanoes.

Of the more than 60 lava flows of the Craters of the Moon lava field, 20 have been dated: their ages were found to range from about 15,000 years before present to about 2,100 years before present. The flows were laid down in eight distinct eruptive periods that recurred on an average of every 2,000 years. On the basis of recent eruptive history, the Craters rift set is due for another eruption within the next thousand years, perhaps as soon as within 200 years.

Crates of the Moon lava flows are classified as aa, pahoehoe, or blocky pahoehoe.

Aa lava has rough, jagged surfaces with sharp points.

Pahoehoe, which is more fluid before hardening, spreads into sheets with smooth, glistening surfaces that are often twisted into ropelike wrinkles, pleats, and folds.

A continued flow of pahoehoe lava may break the twisted surface into jagged blocks that resemble aa but do not have the sharp surface projections and spines characteristic of aa.

New basaltic lava generally has a dark brown surface. As lavas age and weather, the surface color may change. Disturbance to lava alters the normal dark surface is evident because it exposes the underlying oxidized area. Pahoehoe lava may take on a glossy, iridescent veneer due to chemical composition. The Blue Dragon and Green Dragon pahoehoe flows and the Vermilion Chasm are named for the striking lava colors in those areas.

The Hot Spot

One explanation for the existence of the Snake River Plain and the Craters of the Moon lava field is called the mantle plume theory. This theory states that beneath the crust of the Snake River Plain lies a "hot spot" or localized heat source. Periodically, this hot spot consists of a "plume" of molten rock (magma) which rises buoyantly to the surface of the earth. The hot spot does not move but rather remains in a fixed position. What does move is the crust of the earth; as the North American plate slides southwestward over the hot spot. As the plate moves over the hot spot volcanic eruptions occur on the surface.

Initially these eruptions are very violent and produce a lava known as rhyolite. Huge calderas of up to 30 miles in diameter are formed when these devastating eruptions take place. Later a more fluid lava known as basalt flows onto the surface and covers the rhyolitic flows. Yellowstone National Park, the area where the hot spot is believed to be located at this time, is the place where catastrophic rhyolitic eruptions last occurred 600,000 years ago. Craters of the Moon represents the second stage of the eruptions where fluid basaltic lava covered the landscape as recently as 2,000 years ago.