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The Inyo eruptive episode 1350 C.E., Long Valley caldera, California

Explosive eruption occurs as the result of magma rising to the surf...
Explosive eruption occurs as the result of magma rising to the surface and encountering groundwater.(Credit: Stovall, Wendy K. Public domain.)

Sequence of explosive activitiy: steam-driven explosions precede and follow explosive eruption of magma

When rising magma encountered groundwater beneath the Inyo chain, a series of steam-driven explosions blasted rock debris into the air. The eruptions occurred from a vent located beneath the South Deadman flow. This early activity was followed by much stronger explosive eruptions at the South Deadman vent, which ejected pumice and ash fragments during at least 2 separate episodes. This early activity also generated a pyroclastic flow that spread at least 6 km (3.7 mi) from the vent. Soon after this activity at South Deadman vent, explosive eruptions began at Obsidian vent and then at the Glass Creek vent, located 5 km (3.1 mi) and 3 km (1.8 mi) to the north, respectively. Finally, the explosive phase ended with a series of steam-driven explosive eruptions.

Geologists have carefully studied the layers of ash, pumice, and rock debris formed by these steam-driven explosions and magmatic explosive eruptions in order to determine the sequence of activity and relative size of the Inyo eruptions. Maps of the distribution of rock debris ejected by the eruptions provide a reference for the area that could be effected by similar activity in the future.

Distribution of tephra and a pyroclastic flow from the South Deadman vent. (Miller, C.D. Public domain.)


Magma first reaches the surface at South Deadman vent

During the first explosive episode of the Inyo eruptions, the prevailing wind was blowing toward the northeast. Pumice and ash rising in the eruption column were blown northeast, and then fell to the ground to form a layer that is more than 200 cm (78 in) thick near the vent and about 10 cm (4 in) thick at a distance of about 12 km (purple isopach) lines in upper right). In the South Deadman map illustration, the colored isopach lines show areas where the thickness of a tephra deposit are the same.

A second explosive episode erupted a series of pyroclastic flows that extend at least 6 km (3.7 mi) to the northeast and a few kilometers to the west (light-blue area on map). The pyroclastic-flow deposits are more than 10 m (32 feet) thick near the vent along Deadman creek.

During a third explosive episode, the prevailing wind carried tephra toward the south-southwest. This episode ejected about 4 times more tephra into the air than the first one. Near the vent, the resulting deposit is more than 4 m (13 ft) thick. At a distance of 12 km (7.5 mi) from the vent, the deposit is 20 cm (7.9 in) thick (blue isopach lines in lower left for South Deadman map illustration).


Magma continues to rise, causing more explosive eruptions

Magma next reached the surface at Obsidian vent, where a stong explosive eruption also ejected pumice and ash high into the air. The prevailing wind carried the tephra toward the northeast (dark green isopach lines in upper right of Obsidian Flow Vent illustration). Geologists have found thin layers of pyroclastic flow deposits atop the tephra layer near the vent.

The largest and final magmatic explosive activity of the Inyo eruptions occurred at the Glass Creek vent, located between Obsidian and South Deadmanvents. Wind carried tephra from the eruption column toward the south-southwest. Near the vent, the resulting deposit is more than 8 m thick. At a distance of 12 km (7.5 mi) from the vent, the deposit is more than 50 cm (20 in) thick (light green isopach lines in lower left in Obsidian Flow Vent illustration).


Distribution of tephra and a pyroclastic flow from the Obsidian and Glass Creek vents. (Miller, C.D. Public domain.)

Rising magma beneath Deer Mountain stalls, causing steam-driven explosions

Sometime after the explosive eruptions at the South Deadman, Obsidian, and Glass Creek vents, magmamoved upward toward Deer Mountain. It never reached the surface. Instead, a series of steam-driven explosions blasted rock debris through layers of older volcanic rock to form craters at the surface--the Inyo Craters and two craters atop Deer Mountain. The explosions were triggered when superhot groundwater, heated by the rising magma, suddenly flashed to steam, like a geyser. The resulting explosions fractured rocks above the magma and hurled large blocks and pulverized rock fragments onto the surface. The shattered rock debris fell back to the ground and formed layers loose rocks around the vents.

As part of an effort to understand the magma conduit system beneath the Inyo vents, scientists drilled several holes beneath the chain in the mid-1980's. One hole was drilled beneath the South Inyo Crater to intersect the "frozen" magma at the top of the dike. The drill passed through a narrow, highly fractured zone of rocks at a depth between 600-650 m (approximately 2000 to 2140 ft) below the crater, but above the actual feeder dike. Some of the broken rocks were of the same composition as the debris that erupted onto the surface high above. Why didn't the magma continue to rise to the surface? Scientists have suggested that the influx of groundwater into the conduit system may have "quenched" or cooled the magma at an early stage, helping to prevent the progression of strong explosive activity that occurred at the South Deadman, Obsidian, and Glass Creek vents.

Aerial view of Inyo Craters and Deer Mountain.
Aerial view of Inyo Craters and Deer Mountain, California. (Credit: Miller, C. D.. Public domain.)