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Ground cracks and faults associated with the Inyo eruptions.

Scientists designed a simple experiment to demonstrate how the cracks and faults along the Inyo chain might have formed above a shallow intrusion of magma. This physical model helped scientists understand the relationship between characteristics of ground cracks and the depth and size of the dike intrusion. The results of the model and descriptions of labels F, D, and E are described below.


Summary

Rising magma stalled beneath Deer Mountain more than 600 m below the ground. Scientists infer that molten rock rose to near the surface beneath Deer Mountain and the Inyo Craters based on the location of (1) explosion craters; and (2) associated fissures and faults. In the mid-1980's, a hole was drilled so that it passed directly beneath the South Inyo Crater between 600 and 650 m below the ground. The hole did not intersect the magma, but it did pass through highly fragmented rock above the dike. Scientists interpret the broken rock as evidence of subsurface explosions caused by the heating of groundwater above the hot magma and the subsequent "flashing" of hot, pressurized water to steam.

Several fissures and faults continue south from the Inyo Craters along the so- called "Earthquake Fault." These features suggest that subsurface dike(s) also extend south of the Inyo Craters to an area just west of the town of Mammoth Lakes. These may have formed by multiple dike intrusions over the past few thousand years. If a dike were to intrude into the Inyo chain in the future, we can expect to see similar ground cracking and to record an intense swarm of earthquakes as the molten rock makes its way toward the surface.


Map of fissures and faults near Inyo Craters


Layered model demonstrates fissures and faults above a dike

To help study the way in which these fissures and faults may have formed above an expanding dike, scientists devised a model using a small box filled with a mixture of all-purpose flour, white granulated sugar, and corn meal. A mixture of flour and sugar (3:1 ratio by volume) was packed and, in some experiments, layered with corn meal in a box with plexiglass panels on the front and back. The corn meal served as marker beds to make fissures and faults more visiblethrough the plexiglass panels. A "dike" made of two linoleum sheets taped together at the top overlay a slit in the bottom of the box and extended about halfway into the flour mixture. Expansion of the "dike" was modeled by inserting sheets of cardboard between the linoleum sheets.

About 60 experiments were run in which patterns of fissures and faults were photographed and mapped at successive stages of growth. Scientists also made detailed measurements of the amount of extension across the surface of the model and the distance between fissures and faults and the top of the "dike." For these results, see the reference below.

Features on the surface: photographic summary


As the "dike" expands, fissures develop in two areas that run parallel to the trend of the dike (locations A and B in left photo). Branched fissures may form when a new fissure joins an older fissure (location C in middle photo). Growth of fissures on opposite sides of the "dike" is accompanied by the development of a broad, shallow trough just above the "dike" (note shallow crease between the fissure zones in middle photo). With additional expansion of the "dike," the trough subsides even further into a depression called a graben (location D, right photo). The graben is bounded on both sides by fault scarps along which it has subsided (locations E and F).

Features beneath the surface: photographic summary


As the "dike" expands and fissures form at the surface (location a in left photo), indistinct vertical fractures and zones of disaggregation develop around the top of the "dike" (location b in left photo). As the shallow trough develops at the surface (black line marks original surface), the subsurface also subsides and new fractures appear above the outer margins of the "dike" (locations c and d). With additional expansion of the "dike," movement along these fractures form faults as indicated by the arrows. Continued slip along the faults connect fractures in adjacent layers (location e in right photo) and with fissures that have formed at the surface (location f in right photo).

Experiment Reference

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.