If a period of unrest does lead to an eruption in the Long Valley area, its potential effects will depend critically on several factors. These include the location and number of vents, the size and style of the eruption, and the direction of the prevailing winds during the eruption. Most likely, the next eruption will be of the same style and small to moderate size of previous eruptions along the Mono-Inyo Craters chain over the past 5,000 years (note on eruption size). Based on these recent eruptions, the likely sequence of a future eruption would proceed in the following manner.
Initial Eruptions: Often Steam Explosions
Volcanic eruptions often begin with a series of steam
explosions (phreatic explosions) caused by the
interaction of magma and groundwater. As magma rises toward
the surface, it heats groundwater to its boiling temperature and
triggers the "flashing" of water to steam. The resulting expansion
and explosion of the steam can shatter rocks, hurl large blocks
and volcanic ash hundreds of meters in the air, and excavate a
crater in the ground. The Inyo Craters are examples of the
craters excavated by phreatic explosions about 600 years
ago. | |
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Aerial view of the Inyo Craters, which were formed by a series of steam-driven eruptions at the end of the the Inyo eruption sequence about 600 years ago. The South Deadman flow is in the background. |
Next: Explosive Eruption of Magma
When gas-rich rhyolitic magma (the type that has produced
most of the Mono-Inyo eruptions) reaches the surface, strong
violent explosive eruptions typically occur. The explosive
activity blasts apart the magma into small fragments called
tephra (pumice and volcanic ash), which may rise to
more than 10 km (30,000 feet) above the vent. The tephra may
drift tens to hundreds of kilometers downwind. With increasing
distance from the vent, tephra that falls to the ground will
generally become smaller in size and form a thinner layer.
Modest accumulations of tephra pose no immediate threat to life or
property (especially in areas where most structures are
built to withstand substantial snow loads), but even a thin
dusting of fine ash can seriously disrupt social and
economic activities for weeks or months after an eruption. | |
Mount Pinatubo, Philippines |
Huge cloud of volcanic ash and gas rising from Mount Pinatubo, Philippines, on June 12, 1991, three days before this volcano's cataclysmic eruption. This eruption cloud is comparable in size to the cloud that was generated by the eruption that formed the Inyo Craters and nearby lava domes in the Long Valley Caldera some 550 to 600 years ago. |
An explosive magmatic episode may also produce occasional pyroclastic flows. These are hot, gas-rich clouds of pumice and ash that move laterally from vents at speeds of 100 to 200 km/hour (over 100 miles/hour). Such flows pose a serious threat to life and property. Pyroclastic flows have extended up to 10 km (6 miles) in narrow, tongue-like patterns from several of the Mono-Inyo vents over the past 5,000 years. Given the likely distribution of vents, however, the chances are small that future pyroclastic flows would directly impact a local population center. | |
Mount St. Helens, Washington |
A small pyroclastic flow rushes down the north flank of Mount St. Helens during an eruption on August 7, 1980. The view is from Johnston Ridge, located 8 km north of Mount St. Helens. Photograph by P. Lipman. |
Final Eruptions: Often Lava Flows
The final stage of the recent Mono-Inyo volcanic activity
typically involved the non-explosive eruption of viscous
magma to form one or more mound-shaped lava domes. Evidence
collected by scientists suggests that the magma erupted quietly because
water dissolved in the magma was released during its upward
journey instead of being trapped. For example, such lave
flows include the Obsidian flow, Glass Creek flow, Wilson Butte,
and the Panum Dome). | |
Mount St. Helens, Washington |
Lava emerged from the vent in the crater of Mount St. Helens (top image) hours after a series of explosive eruptions generated eruption columns and pyroclastic flows October 16-18, 1980. During the next few days, the slow-moving lava spread outward to form a mound-shaped lava dome about 300 m in diameter (lower image). Photographs by T. Leighley. |
Mount St. Helens, Washington |
Lava Fountains Also Possible in the Long Valley Area
Effusive, Hawaiian type eruptions of basaltic magma have
also occurred in the area and could occur again. Such
eruptions typically begin with lava fountains that form
small cinder cones a few hundred meters or more in diameter.
These eruptions also produce hot, fluid lava flows that may
extend several kilometers down-slope from the vents. The Red
Cones, located 3 km south-southwest of Mammoth
Mountain, were produced by eruptions of this type some 5,000
years ago. Similar flows were erupted from vents around the
base of Mammoth Mountain and in the west moat of the caldera
between 60,000 and 400,000 years ago. While such flows can
destroy most structures in their paths, they seldom advance
faster than a brisk walk and usually don't endanger people.
As in Hawaii, this kind of eruption is likely to become a
tourist attraction. | |
Kilauea Volcano, Hawaii |
An effusive eruption of basaltic lava in Makaopuhi crater on Kilauea volcano, Hawaii, on March 5, 1965. The eruption broke out from a fissure several hundred yards long that cut across the west wall of the crater. View from the south rim of the crater looking down to the northwest. The main fountain is about 100 feet high, and the lava is flowing in to the bottom of the crater forming a lava lake. Photograph by D.P. Hill.
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