CRATERS OF THE MOON
ARCO - IDAHO, USA
MAY 6TH 2020
Craters of the Moon is located within the Snake River plain and consists of 60 mapped lava flows within the volcanic field. Craters Of The Moon is a polygenetic lava field, so it erupts more than once in its lifetime. Unlike Hell's Half Acre which is a nearby Shield Volcano that is classed as monogenetic since it won't erupt again. The Crates Of The Moon lava field is shadowed by a looming future eruption, though that would be over1,000 - 3,000 years away which is rather short in geologic times.
The legacy the Yellowstone Hotspot left behind is visible throughout the region, as 9 ancestral calderas are located in Idaho alone 7 of which are located in the Snake River Plain. 10 million years ago was the eruption of the Picabo Caldera which was a past eruption of the Yellowstone Hotspot. The Yellowstone Caldera system is an extensive time-progressive hotspot track, which moves with the North American Plate. Craters Of The Moon itself doesn't sit within in the caldera but the smaller Wapi Lava Field and Kings Bowl Rift Set do sit within the Picabo Caldera. The Caldera is almost invisible because of its large size, weathering and the extension of the Basin and Range.
Spatter Cones Trail
Since the end of the last Ice Age, the area around Craters Of The Moon has been home to paleo-Indians dating back 12,000 to 14,000 years ago but artifacts are scarce. The Shoshone tribe inhabited the upper Columbia River Basin and are a separate branch to the Northern Shoshone, the Bannock tribe is related to the Northern Paiute Tribe.
Both tribes coexisted together in the harsh environment of the Snake River Plain despite the slight language differences. Although neither exhibited a highly structured tribe, instead several families searched for food and other resources throughout the summer months. Craters Of The Moon itself was never a permanent settlement as it lacked the resources for sustainable living conditions.
Artifacts have been found within Lava Tubes, Sagebrush and Limber Pines and date to around 2,000 years in age, many older sites are possibly covered by newer lava flows.
A local Shoshone legend suggests the tribe witnessed one of the many eruptions from Craters Of The Moon. The story speaks of a serpent sunning itself in today's location of the lava field it was angered by lightning striking nearby, in turn, it coiled around the mountain until lava flowed and fire was shot into the air as the mountain exploded.
The mining of Tachylyte occurred close to the current location of the Visitors centre while obsidian was collected from around Southern Idaho, both were used to make projectile points. Tachylyte was used when obsidian was unavailable as it's not as glassy in appearance, has a larger grain size along with a higher iron content as its made from basalt lava flows.
Hunting blinds were constructed to catch and kill game throughout the summer usually in sagebrush. The Rock Rings below could represent a past camp settlement, the rocks were possibly used to construct a temporary windbreaker and tie it down so it wasn't picked up by the wind. It is also possible that the structures were once hunting blinds but its truly unknown.
The Lava Flows From Crates Of The Moon are basaltic and are formed from ejected lava from a single vent. The ejected cinders form a cone around the erupting vent that rarely exceeds a 1,000 feet in height (304 meters). The eruptions are usually gas-rich and contain an above-average amount of Sodium/potassium.
Scoria (Cinders) are a vesicular, low-density basalt. These type of eruptions are driven by Lava Fountains (Curtin Of Fire) formed from gas bubbles expanding in all directions, the only way to release the build-up of pressure is up the vent that is why lava Fountains erupt vertically. Later in the eruption cinders start to pile up around the vent and eventually form cinder cones, The Big Craters are classed as a cinder cone complex as it consists of 9 separate vents with smaller eruptive vents located within larger ones.
If the location changes slightly during an eruption the cone loses its simplicity and its circular shape, that later complexity is visible in crosscutting within Cinder Cones. Even if the wind was blowing during an eruption the shape can change to an asymmetric formation. This is visible when looking at Inferno Cone and Paisley Cone, the wind was moving in from the west and southwest this pushed tephra and thin lava flows downslope to the east and northeast.
Big Cinder Butte
Big Craters on the North Crater Hiking Trail
The cinders that form cinder cones are comprised of many different colours, from brown, red, black and even iridescent purple and gold. The red colouring visible on some of the cinder cones is the result of iron oxidation, this happens when cooling cinders come in contact with steam. Iridescent purple and gold colours come from a very thin layer of glass that forms when hot cinders come in contact with cold air.
Rafted Blocks are the broken remnants of crater walls that have been carried away in A'a lava flows, they are the only flow type that can move the large blocks away from their original location. There is speculation about the destruction of several cinder cones near the North Crater, there is evidence of 4 flows that came from a vent at or near the North Crater. The thick A'a Lava flowed in-between Sunset Cone and Grassy Cone and was violent enough to destroy the north flank of North Crater and some other cinder cones in its path.
Spatter Cones in Craters Of The Moon are formed from the same process that produces Cinder Cones, except that the lava ejected has less gas to force it out the erupting vent. This happens when erupting spatter collects around the vent and builds up over time. Spatter Cones are usually aligned with regional rift zones, in Craters of the moon that would be the Great Rift Zone with the spatter cones trail located within the Crystal Fissure system.
If the spatter cone is not welded the formation can become delicate and is prone to erosion, unlike welded spatter cones which will keep the formation together and preserve the shape of the cone for a longer period of time. Welded cones are also steeper and can be almost vertical as the particles stick together, the cones on the Spatter Cones trail are not welded and have lost 2 feet of elevation over 50 years.
Snow Cone is known to hold snow all summer as the sides of the spatter cone shadow the vent and keep the snow from melting when it was deposited during winter. Ice is found in other locations in the monument, like caves and depressions in very rough and broken lava flows.
During thawing periods in Autumn and Spring, water seeps into crevasses and turns to ice as cold air circulates in caves which keep the ice from melting, or a draft entering the cave freezes any water inside. Some locations slowly thaw out throughout the summer months, but others stay frozen year-round.
Separating all 60 different lava flows is only possible by looking at the subtle differences of the lava surfaces along with weathering and differing vegetation. The only issue with this is that individual flows can have uneven vegetation and are prone to differential erosion. The only way to properly determine what mapped flow is visible is by geochemistry.
The Craters Of The Moon lava field is the most studied lava field in the area, it is comprised of basalts and has been enriched by iron, phosphorus, titanium and alkali elements. Lava flows in Craters Of The Moon range from 15,000 to 2,000 years old and were deposited during 8 separate eruptive episodes, which formed 25 separate cinder cones in a 28-kilometre long zone.
Pahoehoe Lava Flows
Majority of the lava flows within Craters Of The Moon are pahoehoe with some other a'a and blocky lava flows, the flows have a ropy texture that represents the fluid nature of the lava flow.
pahoehoe flows advance in the formation of toes, the lava continues to move out of cracks and build forward in the same pattern. The flows generally move at a speed of 2-5 cubic meters per second or 1-10 meters per hour on average as the lava is formed from a low-effusion type of eruption emplaced at a low volumetric flow rate.
The top centimetre is comprised of dense to vesicular volcanic glass that forms in blue/green iridescent colours the most notable being the Blue Dragon Flow. Later pahoehoe lava flows were deposited through lava tubes, once the flow field was more organized and well developed, these lava flows are prone to collapse and create skylights in lava tubes. This type of flow also creates pressure ridges and pressure plateaus which are large scale lava flow formations.
The Blue Dragon Flow
The Blue Dragon Flow is one of the youngest in the National Monument and is around 2,030±80 years old. The Flow consists of an outer layer that contains titanium and magnetite crystals, these crystals reflect blue light and give the flow a blue tint. Where the flow has been broken the underlying red layers are visible, the red colouring is from the high iron content within the basaltic lava flows. Iron oxidizes when it's in contact with oxygen and water, the reaction makes the iron rust. When the lava flow cools it traps gases and water vapour beneath the glassy surface of the flow, this forms an ideal environment for rust.
Most of the Lava Flows in the Monument were fed through Lava Tubes which is how the flow field extends so far out from the vents.
Lava Tubes are formed by molten lava after the crust has hardened above the flow protecting it from cooling, towards the end of the eruption lava exits the cave and leaves a hollow opening. Sometimes lava is diverted elsewhere and drains downslope, this leaves behind partially empty conduits and high-lava marks on the walls, along with forming mostly flat floors.
Gas escapes from cracks within the lava tube, along with through skylights where the roof of the lava tube has collapsed. Strong hot escaping fumes concentrate at skylights and make the area very dangerous to approach.
If the flow is still active skylights can be a way to view the flowing lava beneath the surface, when the flow hardens and comes to an end those skylights start to collapse to the floor of the lava tube. This is visible in Craters Of The Moon with the best example being Indian Cave.
Lava Stalactites & Re-melt Structures
Although these are not the traditional Stalactite formations, they are formed in a similar way. When the lava was draining from the ceiling of the tube stalactites started to form as liquid lava dripped from the roof of the lava tube to the floor of the cave.
At the time of formation lava may have still been active within the tube system but not flowing at full speed or hight, this is evident with the patterns on the floor. The patterns look as if the lava was close to solidifying but not quite, it resembles slow-moving lava that didn't have a high velocity. The cave would have been extremely hot but starting to cool down at the time of formation.
Lava close to the temperature of solidification takes little time to turn to rock and is a very poor conductor of heat, several minutes after the flow surrounds a tree it will harden. Hot lava creates a chamber of air between the tree and the mould which accelerates the cooling process, the addition of tree sap forms steam and cools the lava down which is why dead trees rarely form moulds.
Lava must flow slowly and at the right temperature to preserve tree moulds, once the burned tree rotted away the hole where its trunk once stood is all that's left. Many of the holes left behind can be 10 to 20 ft deep, the depth is dependent on how deep the lava flow was during the eruption and the number of cinders and other rocks that have fallen into the mould. Sometimes a tree will collapse after a lava flow has moved through the area, this can preserve patterns on the outside of the tree as the branches fall into the cooling lava.
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Oregon State University. (2019, June 13). Lava Tubes. Oregon State University | Volcano World.