Geology for Teachers
Many Questions Arise
The Snake River Plain
Western Snake River Plain
This rift valley originated about 17 million years ago and the last eruptions along the rift occurred between 1 million and 300,000 years ago. Because this landform continues to shrink as it cools, and because lava and sediments are periodically deposited on it, the area is in a state of continual subsidence. Although the Western Snake River Plain is still linked geographically to the Eastern Snake River Plain, geologists now find their origins to be distinctly different.
Eastern Snake River Plain
Calderas are massive craters up to several hundred square miles in area formed when explosive eruptions emptied magma chambers beneath the earth's crust, causing the surface to collapse. Calderas formed by rhyolitic eruptions are present on the Eastern Snake River Plain. Vestiges of numerous rhyolitic lava flows composed of small pieces of ash (ash flow tuffs) are also apparent along the margins of the Eastern Snake River Plain. They are all that is visible of a thick rhyolitic lava flow overlain by more recent basaltic lavas. These basaltic and rhyolitic lava flows have been measured by drilling to a depth of 2 1/2 miles. One half mile of basalt rests on more than two miles of rhyolite.
The rhyolitic volcanic deposits range in age from 16 million years on the western edge of the Plain, to 600,000 years at Yellowstone. Any theory about the origin of this area must explain the progressively younger ages of the rhyolitic deposits as you travel west to east, as well as the fact that later basaltic eruptions buried the rhyolite. Most geologists consider the Mantle Plume Theory the best explanation for the formation of the Eastern Snake River Plain.
Mantle Plume Theory
The plumes remain stationary while the plates that make up the earth's crust move over them. Thus volcanic activity above a plume is expressed as a line of eruptions creating volcanic features which grow older the further they are from the hot spot. This relationship was first identified in the Hawaiian Island chain, and many geologists see the same pattern expressed on the Snake River Plain. Eruptions occurred 10 million years ago at Twin Falls, 5 million years ago at Arco, and finally, 600,000 years ago in Yellowstone.
The events that occur during an eruption associated with a mantle plume can be broken into two distinct stages.
The Great Rift
The most extensive system of fissures on the Snake River Plain is called the Great Rift, which passes through Craters of the Moon. This volcanic rift zone is 60 miles long and from 1.5 to five miles wide. At Craters of the Moon it is characterized by short surface cracks, more than 25 cinder cones, and is the point of origin of over 60 lava flows. Geologists believe that the formation of the Great Rift is related to Basin and Range type faulting.
Basin and Range
The release of this tension has resulted in about 150 mountain ranges and valleys in the Basin and Range province, all aligned approximately in a north/south direction. They are spaced approximately at 16 mile intervals.
There are Basin and Range mountain ranges to the north and south of the Snake River Plain. The faults that occur at the edge of each the mountain ranges are known as "border" or "range-front" faults, and extend beyond the base of the mountains and out beneath the lavas. The extension of the range-front faults onto the Eastern Snake River Plain is marked by zones of parallel cracks that may run for tens of miles, and are known collectively as volcanic rift zones.
The Great Rift is not a typical Snake River Plain volcanic rift zone, because it cannot be readily identified as a continuation of a Basin and Range border fault. However, the distance from the Great Rift to the Lost River Mountains is slightly more than the 16 miles normally found between Basin and Range structures. Furthermore, gravity and seismic information indicates a fracture extending from the Great Rift into the Pioneer Mountains to the north. This leads some geologists to conclude that the Great Rift is an extension of a typical Basin and Range fault system.
The Great Rift is the conduit through which a tremendous amount of lava reached the surface to form the Craters of the Moon lava field.
Eruptions at Craters of the Moon
During a typical eruption at Craters of the Moon, the force of rising magma causes a section of the Great Rift to pull apart. As magma rises through the crack, gases contained within the magma expand. The frothy magma is very fluid and charged with gas. Eruptions begin as a long line of tall fountains along a crack that may extend more than a mile. These are called "curtain of fire" eruptions and produce downwind blankets of frothy cinders.
After hours or days, the initial expansion of gases decreases and the eruption becomes less violent. Some sections of the fissure seal off and the eruption becomes more localized, at this point the cinders are often thrown even higher into the air and as they rain back down around the vent(s) build up in piles forming cinder cones.
As the amount of gas contained in the magma continues to drop, the volcanic activity again changes. Huge outpourings of lava flow from various fissures and vents. These lava flows typically continue for days to a few months, but may continue for years. They are the source of most of the rock produced during an eruption. The flows gradually subside and all activity stops.
What Does the Future Hold?
Did You Know?
Watch out for bombs! Before they cooled, volcanic bombs were hot globs of lava that were hurled from volcanoes along the Great Rift. They form a variety of interesting shapes described as "breadcrust", "spindle" and "ribbons" by geologists. More...