 Illustration from J.H. Fink and modified by J. Johnson photograph by S.R. Brantley on August 2, 1998 |
| 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. This view is from atop Mammoth Mountain toward the north. The lower part of the diagram shows how the "dike" might appear if we could remove a square portion of the Earth's crust along the axis of the Inyo vents. |
| When magma began rising beneath the southern end of the Mono-Inyo Craters volcanic chain about 600 years ago, a remarkable series of eruptions and ground cracking formed what are now familiar features -- lava flows, craters, layers of pumice and ash, and ground cracks between Mammoth Mountain and Obsidian Flow (map). 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. |
 Experiment demonstrates pattern of gound cracking caused by shallow intrusion of molten rock. |
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 long and 10 m wide in places. The ground above the dike was significantly cracked and faulted. |
 The final series of explosive eruptions blasted the Inyo Craters in the ground. |
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 from the South Deadman vent. This explosive activity was followed by smaller steam-driven explosions that formed the Inyo Craters. |
 Large circular-shaped lava flows oozed from three vents after the explosive activity stopped, including Obsidian Flow. |
After the explosive eruptions, molten rock continued to rise toward the surface. But instead of erupting explosively into the atmosphere, the magma oozed onto the ground to form thick rounded lava flows (also referred to as domes because of their mound-like shape). The molten rock forming these lava flows contains less dissolved water and other volcanic gasses (volatiles) than the earlier, explosively erupted magmas. |
 Map of the explosion craters, lava flows, and faults that formed about 600 years ago. Map modified from C.D. Miller by J. Johnson. |
 Shaded-relief map of the area shown at left. |
Fink, J.H., 1985, Geometry of silicic dikes beneath the Inyo domes, California: Journal of Geophysical Research, v. 90, n. B13, pp. 11,127-11,133.
Mastin, L.G., and Pollard, D.D., 1988, Surface deformation and shallow dike intrusion processes at Inyo Craters, Long Valley, California: Journal of Geophysical Research, v. 93, no. B11, pp. 13,221-13,235.
Miller, C.D., 1985, Holocene eruptions at the Inyo volcanic chain, California -- implications for possible eruptions in the Long Valley caldera: Geology, v. 13, pp. 14-17.
