At Craters of the Moon the black rocks are lava flows. The surface lava rocks, basaltic in composition, formed from magma originating deep in the Earth. They are named for their appearances: Pahoehoe (pronounced “pah-hoy-hoy” and meaning “ropey”), Aa (pronounced “ah-ah” and meaning “rough”), or Blocky. Geologists have seen how these flows behave in modern volcanic episodes in Hawaii and elsewhere.
Pahoehoe lava
Pahoehoe More than half the park is covered by pahoehoe lava flows. Rivers of molten rock, they harden quickly to a relatively smooth surface, billowly, hummocky, or flat. Other pahoehoe formations resemble coiled, heavy rope or ice jams.
Aa lava
Aa Aa flows are far more rugged than pahoehoe flows. Most occur when a pahoehoe flow cools, thickens, and then turns into aa. Often impassable to those traveling afoot, aa flows quickly chew up hiking boots. Blocky lava is a variety of aa lava whose relatively large silica content makes it thick and often dense, glassy, and smooth.
Blocky lava
Bombs Lava pieces blown out of craters may solidify in flight. They are classed by shape: spindle, ribbon, and breadcrust. Bombs range from ½ inch to more than 3 feet long.
Tree Molds When molten lava advances on a living forest, resulting tree molds may record impressions of charred surfaces of trees in the lava.
Blue Dragon Flows lava
Breadcrust bomb
Spindle bomb
Wood-like lava
Tree mold
Lava river
Mt. St. Helens erupts in 1980. Because the lava contained a large amount of silica, its explosive eruption contrasts sharply with recent basaltic flows in volcanic activity in Hawaii.
Basaltic flows in Hawaii.
Upon reaching the surface, the gases contained within the lava easily escaped and produced rather mild eruptions. Instead of exploding into the air like earlier rhyolitic activity, the more fluid basaltic lava flooded out onto the surrounding landscape. These flows were fairly extensive and often covered many square miles. After millions of years, most of the older rhyolitic deposits have been covered by these basaltic lava flows.
The Great Rift and Craters of the Moon. Craters of the Moon National Monument lies along a volcanic rift zone. Rift zones occur where the Earth’s crust is being pulled in opposite directions. Geologists believe that the interactions of the Earth’s crustal plates in the vicinity of the Snake River Plain have stretched, thinned, and weakened the Earth’s crust so that cracks have formed both on and below the surface here. Magma under pressure can follow these cracks and fissures to the surface. While there are many volcanic rift zones throughout the Snake River Plain, the most extensive is the Great Rift that runs through Craters of the Moon. The Great Rift is approximately 60 miles long and it ranges in width from 1½ to 5 miles. It is marked by short cracks—less than 1 mile in length—and the alignment of more than 25 volcanic cinder cones. It is the site of origin for more than 60 different lava flows that make up the Craters of the Moon Lava Field.
Eight Major Eruptive Periods. Most of the lavas exposed at Craters of the Moon formed between 2,000 and 15,000 years ago in basaltic eruptions that comprise the second stage of volcanism associated with the mantle plume theory. These eight eruptive periods each lasted about 1,000 years or less and were separated by periods of relative calm that lasted for a few hundred to more than 2,000 years. These sequences of eruptions and calm periods are caused by the alternating build up and release of magmatic pressure inside the Earth. Once an eruption releases this pressure, time is required for it to build up again.
Eruptions have been dated by two methods: paleomagnetic and radiocarbon dating. Paleomagnetic dating compares the alignment of magnetic minerals within the rock of flows with past orientations of the Earth’s magnetic fields. Radiocarbon dating makes use of radioactive carbon-14 in charcoal created from vegetation that is overrun by lava flows. Dates obtained by both methods are considered to be accurate to within about 100 years.
A Typical Eruption at Craters of the Moon. Research at the monument and observations of similar eruptions in Hawaii and Iceland suggest the following scenario for a typical eruption at Craters of the Moon. Various forces combine to cause a section of the Great Rift to pull apart. When the forces that tend to pull the Earth’s crust apart are combined with the forces created as magma accumulates, the crust becomes weakened and cracks form. As the magma rises buoyantly within these cracks, the pressure exerted on it is reduced and the gases within the magma begin to expand. As gas continues to expand, the magma becomes frothy.