Composite volcanoes

 

California's Mt. Shasta is a stratovolcano.

          Some of the earth’s grandest mountains are composite volcanoes – sometimes called stratovolcanoes. They are typically steep – sided , symmetrical cones of large dimension built out of lava flows, volcanic ash, cinders, blocks, and bombs that pile up in layers, or strata, much like layers of cake and frosting  and may rise as much as 8000 feet above their bases. Some of the most conspicuous and beautiful mountains in the world are composite volcanoes, including Mount Fuji in Japan, Mount Cotopaxi in Ecuador, Mount Shasta in California, Mount Hood in Oregon, and Mount Rainer in Washington.  

 

          Most composite volcanoes have a crater at the summit which contains a central vent or a clustered group of vents. Lavas either flow through breaks in the crater wall or issue from fissures on the flanks of the cone. Lava, solidified within the fissures, forms dikes that act as ribs which greatly strengthen the cone.

          The essential feature of a composite volcano is a conduit system through which magma from a reservoir deep in the earth’s crust rises to the surface. The volcano is built up by the accumulation of material erupted through the conduit and increases in size as lava, cinders, ash, etc, are added to its slopes.

          When a composite volcano becomes dormant, erosion begins to destroy the cone. As the cone is stripped away, the hardened magma filling the conduit ( the volcanic plug ) and fissures ( the dikes ) becomes exposed, and it too is slowly reduced by erosion. Finally, all that remains is the plug and dike complex projecting above the land surface – a telltale remnant of the vanished volcano.  

          An interesting variation of a composite volcano can be seen at Crater Lake in Oregon. From what geologists can interpret of its past, a high volcano – called Mount Mazama – probably similar in appearance to present – day Mount Rainier was once located at this spot. Following a series of tremendous explosions about 6800 years ago, the volcano lost its top. Enormous volumes of volcanic ash and dust were expelled and swept down the slopes as ash flows and avalanches. These large – volume explosions rapidly drained the lava beneath the mountain and weakened the upper part. The top then collapsed to form a large depression, which later filled with water and is now completely occupied by beautiful Crater Lake. A last gasp of eruptions produce a small cinder cone, which rises above the water surface as Wizard Island near the rim of the lake. Depressions such as Crater Lake, formed by collapse of volcanoes, are known as calderas. They are usually large, steep – walled, basin – shaped depressions formed by collapse of a large area over, and around, a volcanic vent or vents. Calderas range in form and size from roughly circular depressions 1 to 15 miles in diameter to huge elongated depressions as much as 60 miles long.

                                                                      

  Crater Lake, Oregon; Wizard Island, a cinder cone, rises above the lake surface.