|Lassen National Forest Hot Spot Map 08-26-12|
|Prospect Peak Shield volcano MODIS Satellite Imagery|
|Hot Spot Topo Map Of Prospect Peak: |
Inside Northern boundary of Lassen Volcanic National Park.
|Lassen National Park Hot Spot Location Map|
Shield volcanoes are formed slowly from the build-up of many basalt lava flows that spread thinly over a great distance. Flow after flow pours from a vent building a broad, gently-sloping symmetrical cone, shaped like a shield. They are among the largest volcanoes in the world and include Mauna Loa in Hawaii. The planet Mars has a shield volcano 100 miles (161 km) across.
What are the prospects for future eruptions at Lassen?
Because geologically recent volcanic activity in an area is the best guide to forecasting future eruptions, scientists study the lava flows, ash, and other deposits from past eruptions. Volcanoes in the Lassen area tend to erupt infrequently, and may be inactive for periods lasting centuries or even millennia. The most recent eruptions in the Lassen area were the relatively small events that occurred at Lassen Peak between 1914 and 1917. The most recent large eruption produced Chaos Crags about 1,100 years ago. Such large eruptions in the Lassen area have an average recurrence interval of about 10,000 years. However, the geologic history of the Lassen area indicates that volcanism there is episodic, having periods of relatively frequent eruptions separated by long quiet intervals. For example, the last large event before the Chaos Crags eruption was the one that built Lassen Peak 27,000 years ago.
What are the warning signs of an eruption?
The most important sign of an impending volcanic eruption is seismic activity beneath the volcanic area. Seismologists can interpret subtle differences between earthquakes related to the rise of magma and the more familiar quakes caused by tectonic faulting. Other warning signs of magma rising into the shallow subsurface might include increased release of volcanic gases from small openings called fumaroles, such as those found in the Bumpass Hell area of Lassen Volcanic National Park, and changes in the gas composition. Deformation of the ground surface in the vicinity of a volcano may also indicate that magma is approaching the surface. Typically, these warning signs appear a few weeks to months before an eruption, but can last for decades or even centuries without leading to an eruption.