We had a request to take a look at one of the largest recent basalt fields in the US, the Craters of the Moon National Monument, Idaho. As timing would have it, I was doing a little head scratching on the subject, so why not?
Before I begin, there is another Craters of the Moon in Taupo, NZ. We will leave that for another day. http://www.cratersofthemoon.co.nz/
The Craters of the Moon occupy the Snake River Plain, the eastern half of which looks like a swath blasted through the mountains of Idaho by a series of chained calderas due to the activity of the Yellowstone hot spot over the last 15 Ma.
This one includes some 1,600 km2 of basalt lava flows, scoria cones, shield volcanoes, spatter cones and rift eruptions. There are over 60 separate lava flows erupted in eight periods between 15 Ka and 2 Ka. The most recent eruption was about 2,000 years ago. Each eruptive episode emits between 4 – 5 km3 of material for an estimated total of 30 km3 over the last 15 Ka. There are 25 cinder cones, mostly along a 45 km long rift system called the Great Rift volcanic zone.
Eruptive periods are typically separated by 2,000 years. Que the breathless reporting that Craters of the Moon are now overdue for an eruption
Based on previous episodes, the system is predicted to erupt another 4 – 5 km3 of material during the next episode. Finally, erupted magmas have become more silicic over time, meaning eruptions are shifting away from primarily effusive to more explosive over the years.
Craters of the Moon are some 300 km SW of Yellowstone. It is located on the flats of the Snake River Plain in Idaho. There are some 93,000 living within 100 km of the Monument.
It is also very close to the Idaho National Laboratory, one of the US nuclear labs. This one specialized in reactors for nuclear powered ships. It now works on small, modular reactor designs for civilian power. It was also the home of the nuclear rocket program cancelled in 1972.
Most of the activity is clustered around the Great Rift, an 85 km long, 2 – 8 km wide belt of shield volcanoes, cinder cones, fissures, associated lava flows and non-eruptive fissures. Typical fissure zones on the Snake River Plain measure 120 km long by 10 km wide. The Great Rift is perpendicular to the long axis of the Snake River Plain, generally NNW to SSE.
Eruptions have not yet been observed, but are thought to begin with a vigorous fissure eruption with lava fountains that peter out and become localized. During the early stages, there are volcanic gasses that lead to formation of spatter ramparts and tephra. As the pressure decreases, spatter and cinder cones build up around the lava fountains. Later stages are marked by quiet but voluminous lava emissions. Extended eruptions build shield volcanoes. Little explosive activity is involved in the shield building stage. The majority of lava is pahoehoe though aa lavas are also present.
It is important to note the volume of each of the eight eruptive episodes, as the 4 – 5 km3 lavas erupted on average close to a cubic mile of stuff each eruptive sequence. As a reference, the Holuhraun eruption in Iceland in 2014 produced 1.4 km3 in a six-month long eruption.
The field contains a variety of cinder cones, spatter cones, tephra blankets, small shield volcanoes, fissure vents along the Great Rift. There is at least one crater along the Great Rift, the Kings Bowl thought to be result of a phreatic explosion as magma and ground water interacted.
There is no shortage of lava tubes among the fields due to the volume of pahoehoe lavas erupted. In turn, due to the altitude of the National Monument there is no shortage of ice in the caves at unexpected times of the year. Due to the fractured nature of the lava flows surrounding the Great Rift, little water remains on the surface. It all sinks quickly into the lava.
There are open crack rifts between the main lava fields and the Kings Bowl Lava Field that did not produce any basalt.
Lessons learned in the study of basalt eruptive activity on the Big Island of Hawaii and Iceland have been used to analyze what took place at Craters of the Moon.
The eruptive periods for Craters of the Moon are numbered with letters, the oldest being farthest from the letter “A.”
Oldest eruptive period “H” was around 15 Ka. Mostly pahoehoe lavas were produced. They were covered by successive activity. Source vents have not been identified, though flow directions indicate a source on the Great Rift, possibly Echo Crater of Crescent Butte.
Eruptive period “G” took place some 12.5 Ka. Its location was close to the mountains bounding the northern part of the Plain. Flows traveled as much as 19 km east on the Plain. Lavas are some of the least evolved of any erupted along the Great Rift. Some of these flows traveled as far as 53 km SW. Sunset cone is thought to be at or near the source for these lavas.
Eruptive period “F” erupted lavas along the SW margin of the lava field starting some 10.1 Ka. Radiocarbon dating suggests a source vent on the Great Rift south of Sheep Trail Butte near Crescent Butte and Echo Crater.
Eruptive period “E” dates to 7.8 Ka with the eruption of Lava Point aa flows near the southern bouday of the National Monument. Source points to the Great Rift NW of Echo Crater. There was cinder cone complex at Grassy Cone that formed after the initial aa flows.
Eruptive period “D” has aa flows that appear to come from Silent Cone. The eruptions destroyed the north side of Silent Cone. It dates to 6.6 Ka.
The next eruptive period “C” appears to have come from a 9 km long segment of the Great Rift between Sheep Trail Butte and Echo Crater. These eruptions were spread out by as much as a century starting some 6.1 Ka.
Eruptive period “B” began some 4.5 Ka and lasted for 1,000 years. The vent complex was inundated by voluminous lava flows so its size and shape are unknown. The source appears to be some 10 km SE of Black Top Butte cinder cone. There is a broad lava cone that was topped with a pair of lava lakes that appears to be source for spatter vents and thin blankets of tephra as far as 9 km NW and SE of the cinder cone.
Final eruptive period “A” is in some ways the most interesting. It began 2.3 Ka. Eruptions in this sequence were more violent as some of the magmas erupted was more evolved. One eruption was so violent that it destroyed much of North Crater. A second sequence moved activity south along the rift and formed the Kings Bowl and Wapi lava fields to the south of the older activity.
The Kings Bowl and Wapi lava fields are more typical of Snake River Plain eruptions than the rest of the sequence described above.
The eastern Snake River Plain is characterized by 1 – 2 km basalt overlying rhyolite and welded tuff from the passage of the North American Plate over the Yellowstone hot spot. Basalt thickness ranges from less than 100 m to over 1,500 m. The rhyolite underlying extends in depths exceeding 3,000 m.
The rhyolite eruptive centers are younger from SW to NE, with Yellowstone being the most recent. Basalt eruptions seem to follow the rhyolite activity by a million or two years.
The Twin Falls eruptive center (caldera) dates 10.0 – 8.6 Ma and the Picabo eruptive center (10.2 Ma) lie to the SW of Craters of the Moon. Closest is the Heise eruptive center (6.7 – 4.3 Ma). To the NE is the Island Park – Yellowstone eruptive center (1.8 – 0.6 Ma).
Movement of the hot spot under the North American Plate is some 3 – 5 mm/yr.
There is a sill in the underlying crust of what appears to be partially molten material measuring some 10 km thick by 90 km wide underlying the eastern Snake River Plain. It thickens as it approaches the active rhyolite centers and the hot spot. It is thought to basaltic melts intruded into the crust or remaining high temperature rhyolites caused by partial melting of the crust. The rifting of the plain due to the dual actions of deflation and the continental Basin and Ridge rifting open cracks in the eastern Snake River Plain that allow some of this material to reach the surface. The plain is littered with the remnants of these sorts of basalt eruptions.
The depression that is the eastern Snake River Plain measures some 100 km across and lies generally on a NE – SW axis. The existence of the depression is explained by the relaxation of the crust previously warmed and expanded by the passage over the hot spot cooling, contracting and subsiding afterwards. During and after the passage, a few kilometers of new material in the form of rhyolite, basalt and intra-crustal gabbros added to the surface and under the surface also serve to depress the plain.
Rifting is explained by continental rifting in this part of North America, the Basin and Ridge Province continental rifting. It is the rifting that drives the formation of volcanic rift zones throughout the length of the Snake River Plain. The rifts are generally parallel to the line of mountain ridges on either side of the plain. They are also generally perpendicular to the long axis of the plain.
The Axial Volcanic Zone lies NE of the Craters of the Moon. It includes at least five rhyolite domes ranging from 1.4 Ma to 300 Ka. It is also the source of at least four smaller lava fields. The domes were formed by rhyolite intrusions through the layer of basalt lava flows overlying the rhyolite produced by the passage of the hot spot NE along the plain as the North American Plate moved SW.
The Basin and Range Province of western North America is an area of active continental expansion centered between eastern California and Utah. The Province has mountain ranges extending generally N-S separated by valleys. An accelerated period of continental expansion started some 16 Ma and spread to eastern Idaho by 2 Ma. The Province is characterized by higher than normal heat flow indicating hotter, less dense material closer to the surface, perhaps mantle upwelling.
The Basin and Range Province is also an area of thinning continental crust. It is mostly centered in Nevada, though extends well into Mexico to the south and west Texas to the east. One explanation for it was the shift from compressional forces on western North America due to subduction of small ocean plates off the west coast of California to a more extensional regime.
When the Pacific Plate came in contact with North America, its motion was strike slip, generally NNW along the line of the coast. The small extensional force rifted the entire region, which broke into hundreds of blocks and was uplifted by the hotter mantle under the thinning crust. Action started in the southern part of the province and worked its way north over time. This thinning and fractured crust is one of the reasons geothermal energy potential is high in that region of North America.
Craters of the Moon are the legacy of the passage of the Yellowstone hot spot under southern Idaho. It has been quite active in the last 15,000 years and given the presence of a magma body under it there is no reason to believe it is extinct. And the eruptions tend to be large in terms of total volumes of lava produced.