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Geology and History

Geologic history of the Inyo Craters volcanic chain


Inyo Craters viewed from atop Mammoth Mountain toward the north with cartoon of magma supply below ground.
Inyo Craters viewed from atop Mammoth Mountain toward the north with cartoon of magma supply below ground. The lower part of the diagram shows how the "dike" might appear if a square portion of the Earth's crust were removed along the axis of the Inyo vents. (Credit: Fink, J. H.. Public domain.)

A long, narrow tabular body of molten rock beneath the ground called a "dike" fed eruptions that formed the Inyo lava flows and several explosion craters around Deer Mountain. The magma began rising beneath the southern end of the Mono-Inyo Craters volcanic chain n about 1350 C.E. and formed a remarkable series of lava flows, craters, layers of pumice and ash, and ground cracks between Mammoth Mountain and Obsidian Flow (see the map at the bottom of the page).

This volcanic activity probably occurred during a short period of time, perhaps lasting as long as a few weeks to a few months. The sequence of these eruptions can help us to anticipate the type of activity that is likely to occur again from along the chain, and also to recognize the events that will almost certainly precede future eruptions.


Sequence of events & effects of the eruptions

Aerial view of Inyo Craters, CA.
Inyo Craters, part of the Mono-Inyo Chain, Long Valley region, California. (Credit: C.D. Miller. Public domain.)

1. Moving magma causes cracks in the ground

Magma moved upward toward the surface both before and during the sequence of eruptions that occurred at the southern end of the Mono-Inyo Craters volcanic chain. The molten rock first broke through existing rock and then traveled as an elongate tabular body known as a dike. By spreading both horizontally and upward, the dike eventually reached about 11 km (6.8 mi) long and 10 m (32.8 ft) wide in places. The ground above the dike was significantly cracked and faulted.


2. Explosive eruptions create craters, ash fall, and pyroclastic flow

The first significant volcanic activity was a series of explosive eruptions at three separate vents. The eruptions ejected pieces of molten and solid rock, created small craters, and generated tall eruption columns above the vents. Pumice and ash fell to the ground near the vents and covered extensive areas with a layer of rock debris downwind. The explosive activity also produced a pyroclastic flow that traveled about 6 km (3.7 mi) from the South Deadman vent. This explosive activity was followed by smaller steam-driven explosions that formed the Inyo Craters.


Obsidian Flow, a large circular-shaped lava flow covered in snow.
Obsidian Flow, a large circular-shaped lava flow, is part of the Mono-Inyo Chain, California. (Credit: Bailey, R. A.. Public domain.)

3. Lava flows erupt quietly

After the explosive eruptions, molten rock continued to rise toward the surface. But instead of erupting explosively into the atmosphere, the magmaoozed onto the ground to form thick rounded lavaflows (domes). The molten rock forming these lava flows contains less dissolved water and other volcanic gasses (volatiles) than the earlier, explosively erupted magmas.


Characteristics of deposits from Inyo eruptions about 600 years ago

The eruption of magma in the form of tephra (chiefly pumice and volcanic ash), pyroclastic flows, and lavaflows occurred about 600 years ago from the Inyo chain at the Glass Creek, Obsidian, and South Deadman vents.

Vent Tephra     Pyroclastic flows     Lava Flows  
  length, km area, km2 volume, km3 length, km area, km2 volume, km3 area, km2 volume km3
Glass Creek >190 >9000 0.10 -- -- -- 1.0 0.10
Obsidian >25 >140 0.02 <2 -- -- 1.8 0.17
South Deadman1 >20 >220 0.05 6 >15 0.05 1.1 0.13

Two tephra lobes were produced during the explosive activity of South Deadman vent; the values reported represent combined area covered and volume erupted.

Map of the Inyo explosion craters, lava flows.
Map of the Inyo explosion craters, lava flows, and faults that formed about 600 years ago. (Credit: Miller, C. D. Public domain.)