Craters of the Moon

"The 'Valley of the Moon' lies in a region literally combed with underground caves and passages, bewildering in their immensity, mystifying in their variety of strange formations, where there are natural bridges as yet unknown to geographers, where bear tracks hundreds of years old may be traced for miles across cinder flats. Here are craters of dormant volcanoes half a mile wide and seemingly bottomless, huge cups in which the five-story Owyhee hotel might be placed to resemble a lone sugar loaf in a huge bowl. Here are strange ice caves with stalactites and ice-encrusted walls, caves that contain as much ice in the middle of August as they do in the winter."

In the heart of the Snake River Basin lies the Craters of the Moon National Monument and Preserve. Consisting of three major lava fields spanning almost half a million acres, the land primarily is covered by sagebrush steppe and grasslands. Today, the Bureau of Land Management oversees livestock grazing on some of the park's land.

Of the three lava fields, the Craters of the Moon field is the largest and the oldest. In fact, at 618 square miles, it is the "largest young basaltic lava field in the lower 48 states." There are six flows that comprise the field, ranging in age from 15,000 to 2,000 years old. More than 25 volcanic cones are scattered throughout the field, including some flawless specimens of spatter cones. Two younger lava fields, the Kings Bowl and the Wapi, lie on the "southern edge of the Snake River Plain." Both of these fields are about 2,200 years old. The volcanic eruptions that formed these two fields were most likey witnessed by the Shoshone people who lived in the area at that time. According to Shoshone legend, "a serpent on a mountain who, angered by lightening, coiled around and squeezed the mountain until liquid rock flowed, fire shot from cracks, and the mountain exploded. (http://vulcan.wr.usgs.gov/Volcanoes/Idaho/CratersMoon/description_craters_moon.html)"

All three of the lava fields within the preserve are located along the Great Rift (62 miles long), providing "some of the best examples of rift cracks in the world." As a whole, the preserve contains and displays stunning amounts of pahoehoe, slabby pahoehoe, shelly pahoehoe, spiny pahoehoe, aa, block lava, rafted blocks, tree molds, lava tubes, and other volcanic features.

The Craters of the Moon National Monument was established by President Calvin Coolidge on May 2, 1924. Although the preserve has been expanded on five separate occasions owing to the Antiquities Act, its largest expansion came on November 9, 2000 when President Bill Clinton "signed a proclamation enlarging it 13-fold." The preserve currently is comprised of 715,000 acres of federally owned land.

So how did a lava field just randomly sprout up in the middle of Idaho? Geologists wondered the same thing, and currently use the "mantle plume theory" as an explanation. According to the theory, a "hot spot" (see definition below) is located beneath the crust of the Snake River Plain. "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. (http://vulcan.wr.usgs.gov/Volcanoes/Idaho/CratersMoon/description_craters_moon.html)" When these volcanoes first erupt, they are quite violent. with rhyolitic flow. Later, they produce a basaltic, runny flow which covers the rhyolite.

Craters of the Moon essentially straddles the Great Rift, which is a 62-mile long crack in the Earth's crust. As a result of this instability, volcanic action has been extremely prevalent in the area. Over the past 15,000 years, volcanic explosions have occured eight times along the rift. When the eruptions occured, cinder and lava shot from the rift. When they finally cooled, lava fields studded with cinder and spatter cones remained. Unable to be seen at first glance, underground ice caves and lava tubes also formed as a result of the eruptions. The following sections highlight in much greater detail the specific types of lava and volcanic formations.

Before We Begin...Some Important Vocabulary!
Cinder Cone	A volcanic cone built entirely of loose fragmented material (pyroclastics.)
Spatter Cone	A low, steep-sided cone of spatter built up on a fissure or vent. It is usually of basaltic material.
Pahoehoe	A Hawaiian term for lava with a smooth, billowy, or ropy surface.
Aa	Hawaiian word used to describe a lava flow whose surface is broken into rough angular fragments.
Block (lava)	Angular chunk of solid rock ejected during an eruption.
Lava Tube	A tunnel formed when the surface of a lava flow cools and solidifies while the still-molten interior flows through and drains away.
Kipuka	An area surrounded by a lava flow.
Bomb	Fragment of molten or semi-molten rock, 2 1/2 inches to many feet in diameter, which is blown out during an eruption. Because of their plastic condition, bombs are often modified in shape during their flight or upon impact.
Fissures	Elongated fractures or cracks on the slopes of a volcano. Fissure eruptions typically produce liquid flows, but pyroclastics may also be ejected.
Hot Spot	A volcanic center, 60 to 120 miles (100 to 200 km) across and persistent for at least a few tens of million of years, that is thought to be the surface expression of a persistent rising plume of hot mantle material. Hot spots are not linked to arcs and may not be associated with ocean ridges.
Tephra	Materials of all types and sizes that are erupted from a crater or volcanic vent and deposited from the air.

The largest lava field within the preserve, Craters of the Moon is a monogenetic (built by a single eruption) volcanic field, comprised mainly of olivine basalt. The field is composed of about 60 lava flows and 25 tephra cones (see definition in above table). Despite the high olivine content, the basaltic material found within the Craters of the Moon field are surprisingly diverse in chemical composition. The magma from which the flow formed is believed to be the same as the other fields within the preserve, however crustal contamination from older rocks and crystal fractioning has lead to different composition. The Craters of the Moon field differs from the other fields in another major way also. It was formed as the result of eight separate eruptions over a period of almost 13,000 years, while the smaller fields represent outcomes from single volcanic events.

The Kings Bowl and Wapi Lava Fields make up the remainder of the preserve. They are identical in age, and date back about 2,200 years. Despite being formed at the same time, the fields experienced very different means of creation.

The Kings Bowl was formed from a single explosion of volcanic activity, possibly within the span of only about six hours. (http://www.nps.gov/crmo/geology/geology-1.htm) The bowl is marked with a single, eruptive fissure approximately four miles long. That fissure is bordered by two subparallel but non-eruptive fissures. The actual Kings Bowl is 280 ft. x 100 ft. x 100 ft. and was formed when lava came in contact with groundwater and created a massive steam (phreatic) explosion. Directly next to the bowl is a lava lake whose crust was broken by blocks following the aforementioned phreatic explosion. The interior of the lake was still molten, so when it was broken many gas-enriched lava squeeze-ups came to the surface. Today they are dramatic candy-kiss shaped lava formations. Lying along the Great Rift in the Kings Bowl field are beautiful fissure caves. The Crystal Ice Cave and Creons Cave are the largest and most well-known.

Unlike the Kings Bowl field, the Wapi Lava Field was formed slowly, as the result of a fissure eruption with prolonged activity. The Wapi field is a shield volcano with a flattened dome shape. The slow eruption developed a central vent complex, which is what created the shield volcano shape. The complex is made up of six large and five small vents. The largest vent is comprised of many layered flows and is known as the Pillar Butte.

Rising 250 meters above the Snake River Plain, the Big Cinder Butte is the largest cone in the preserve. Interestingly, there is no summit crater at the top of the butte. Around the base the remains of five or six older volcanoes are visible. The Big Cinder Butte produced several flows in its lifetime, the evidence of which can be seen all around it. One of the youngest flows originated from a fissure on the northern side of the butte. At this fissure Blue Dragan pahoehoe lava is visible. Blue Dragon lava is characterized by its shiny blue luster, which is formed as a result of the outer surface of pahoehoe lava fusing into glass.

Native Americans were the first to experience what is now the Craters of the Moons preserve, and they most likey witnessed the creation of both the Kings Bowl and Wapi Lava Fields. It was not until 1901 that scientific research was first conducted in the region. Israel Russel of the United States Geological Survey (USGS) set out to investigate south-central Idaho. After his research, Russel wrote his account of the region entitled "Cinder Buttes." Russel brought attention to the area, and in 1910 Era Martin, a resdident of Idaho, explored and marked many of the caves now in the preserve. The largest exploration of the area came in 1921 when Robert Limbert, a taxidermist from Boise, ID, visited the area and subsequently published an article in National Geographic describing his discoveries. In 1923, the National Park Service asked USGS employee Harold Stearns to delineate boundaries for the area and write a petition justifying its status as a National Monument. It was declared federal land in 1924.

Despite the seemingly barren appearance of the landscape at Craters of the Moon, there are many types of wildlife and plantlife that exist there. A kipuka is an area of growth around which lava flowed. The background for this entire webpage is a kipuka in the large, Craters of the Moon Lava Field. Here are some pictures of things you might find in a kipuka: