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Two volcanoes: why do they look so different?


Dear Dr. Rock,
When I drove US 97 in the Bend, Oregon area, I saw two kinds of volcanoes. One kind, The Sisters, were high, steep and snowy. The others, like Lava Butte just south of town, or Pilot Butte, right in town, are low heaps of loose rock fragments. You can drive right to the top. Why do these volcanoes look so different?  --Bill Shaffer, Fresno, California

Dear Bill,
Volcanoes form where magma reaches the surface. Magma is a red hot mix of molten rock, gases and solid mineral crystals. Magma that erupts at the surface is ‘lava’. There are several types of volcano structures, and there shape depends on the runniness, or viscosity, of the lava they erupt.

The Sisters are stratovolcanoes (sometimes called ‘composite volcanoes). So are the other big volcanoes in the Cascades: Hood, Rainier, Baker, etc. These are made of successive layers of lava that has erupted tens or hundreds of times over thousands of years from the crater at the top of stratovolcanoes. The magma that erupts at these volcanoes is thick and pasty when it erupts, and is called ‘andesite’. Because it is so pasty, andesite lava doesn’t flow for more than a couple of miles, and usually a lot less than that. It piles up near the crater to make a high cone. The smaller cones you saw are called ‘cinder cones’ after the slag-like, sharp edged black lava fragments they erupt. (These are also called ‘scoria’). This magma is particular-well charged with gas, so that lava is blown upward into the air as glowing hot blobs of molten rock. These ‘cinders’ rain down around the volcano’s vent to built a mound of loose fragments.

Cinder cones might also erupt basalt lava, which is much thinner and runnier than andesite, so these lava flows might flow quite a way. Lava Butte lava flows spread outward from the cinder cone for over 6 miles. Cinder cones are a flash-in-the-pan—they usually only erupt for a few years and then their magma supply is exhausted. Here is a US Geological Survey website where you can learn more about the different types of volcanoes: http://pubs.usgs.gov/gip/volc/types.html .

This sketch shows a cross section through a cinder cone. It is a heap of loose fragments- no lava flows are found in the upper cone. Image by USGS.

The Middle and South Sisters from the summit of the South Sister. These are stratovolcanoes.

Mount Baker (10,781') in the north Cascades of Washington. Erosion has revealed the layers of lava that built this active stratovolcano.Lava Butte, south of Bend Oregon, is a classic cinder cone, made of a heap of scoria and with a small crater on top. Note the lava flow in the foreground. Photo by USGS.



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