VolcanoA volcano is generally a conical-shaped hill or mountain built by accumulations of lava flows， tephra， and volcanic About 95% of active volcanoes occur at the plate subduction zones and at the mid-oceanic ridges The other 5% occur in areas associated with lithospheric hot These hot spots have no direct relationships with areas of crustal creation or subduction It is believed that hot spots are caused by plumes of rising magma that have their origin within the Over the last 2 million years，volcanoes have been depositing lava， tephra， and ash in particular areas of the globe These areas occur at hot spots， rift zones， and along plate boundaries where tectonic subduction is taking Not all volcanoes are the Geologists have classified five different types of This classification is based on the geomorphic form， magma chemistry， and the explosiveness of the The least explosive type of volcano is called a basalt These volcanoes produce a very fluid basaltic magma with horizontal The form of these volcanoes is flat to gently sloping and they can occupy an area from 100，000 to 1，000，000 square Deposits of these volcanoes can be as thick as 1，800 Large basalt plateaus are found in the Columbia River Plateau， western India， northern Australia， Iceland， Brazil， Argentina， and A Some basaltic magmas can produce very large slightly sloping volcanoes， 6 to 12°， that have gently flowing magmas called shield Shield volcanoes can be up to 9，000 meters The volcanoes of the Hawaiian Islands are typical of this Extruded materials from this type of volcano mainly consist of low-viscosity basaltic lava flows A cinder cone is a small volcano， between 100 and 400 meters tall， made up of exploded rock blasted out of a central vent at a high velocity These volcanoes develop from magma of basaltic to intermediate composition They form when large amounts of gas accumulate within rising Examples of cinder cones include Little Lake Volcano in California and Paricutin Volcano in M Composite volcanoes are made from alternate layers of lava flows and exploded Their height ranges from 100 to 3，500 meters The chemistry of the magma of these volcanoes is quite variable ranging from basalt to Magmas that are more granitic tend to be very explosive because of their relatively higher water Water at high temperatures and pressures is extremely Examples of composite volcanoes include Italy's Vesuvius， Japan's Mount Fuji， and Washington State's Mount Rainier and Mount S Helens The most explosive type of volcano is the The cataclysmic explosion of these volcanoes leaves a huge circular depression at the Earth's This depression is usually less than 40 kilometers in These volcanoes form when "wet" granitic magma quickly rises to the surface of the E When it gets to within a few kilometers of the surface the top of the magma cools to form a Beneath this dome the gaseous water in the magma creates extreme pressures because of When the pressure becomes too great the dome and magma are sent into the Earth's atmosphere in a tremendous On the island of Krakatau， a caldera-type volcano exploded in 1883 ejecting 75 cubic kilometers of material in the air and left a depression in the ground some 7 kilometers in A potentially very destructive caldera covering an area of about 2，000 square kilometers exists under Yellowstone National Park in the United States Investigations have discovered that over the last 2 million years this volcano has exploded on a regular interval of about 700，000 The last eruption occurred 630，000 years ago and the next could take place When the Yellowstone caldera last erupted， it blasted 1，000 cubic kilometers of volcanic ash and rock into the The ash ejected into the atmosphere created climatic havoc on a global The ash would have blocked sunlight from being received at the ground surface for a few A reduction in the reception of solar radiation would have caused the globle climate to cool Over time this ash settled back to the Earth's surface covering more than half of North A
