This sign is located at the Lost Lake Viewpoint on the Big Obsidian Flow Trail.
Marker Name: The Eruption
Marker Text: First came a violent eruption of pumice and ash. Then glassy lava oozed from the ground.
Magma Chamber
From deep hot regions, liquid rock called magma accumulated in a chamber 2 to 4 miles (3 to 6 km) below the surface. Temperature of the chamber was a roasting 1600 degrees F (900 degrees C). The overlying rock trapped water and gases in the magma.
Conduit
Fingers of magma searched for hidden weak places in their underground prison. When one of these found an unguarded path to the surface, the magma began its escape. In its journey out of confinement, the magma frothed as the trapped water and gases made billions of bubbles, like taking the cap off a shaken bottle of soda pop.
Violent Eruption
The highly gas-charged magma erupted explosively; pumice and ash blasted out of the vent at tremendous speed. Areas of forest in Newberry Crater were swept away, leaving gray plains of ash and pumice.
Passive Eruption
Halfway through the eruption, magma reaching the surface held much less gas. The violent eruption gave way to a passive one. A mixture of stiff, sticky liquid and froth oozed out of the ground, and a great pile (a dome) grew over the vent.
Flowing Over The Land
Slowly and sluggishly, the lava moved inches (or cm) per hour. A 100-foot (30 m) wall of lava moved like a glacier over the land, burying shallow valleys and low ridges. The low fluidity of the flow was due to its high silica content.
Middle Of Flow
Near the top of the flow, the lava developed at least three layers: white pumice, obsidian, and gray pumice. The pumice layers formed where the flow frothed and foamed. In the gray pumice layer under the obsidian layer, gases formed giant bubbles up to 50 feet (15 m) in diameter, which floated up and exploded.
Lower End Of Flow
The layers of pumice and obsidian became wildly contorted as the flow thickened. Layers folded and refolded, shattered and sheared, flowed up and over themselves. Pumice attempted to float, obsidian tried to sink, and great pressure ridges rose up like wrinkles on a rumpled rug.
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