Probing a bit deeper, volcanologists have learned that the composition of the original magma, or mixing with surface water, are the principal reasons for similarities and differences. Volcanoes within each family may resemble one another but they differ in physical details and behavior. It is not easy to predict eruptive behavior until the behavior has been witnessed or determined by examining the resulting deposits. Depending upon latitude, seasonal winter snows whiten the slopes to the base, and the heat of summer melts all but the highest snows. On the upper slopes of some volcanoes, such as Mount Rainier, the perennial snows spawn glaciers that descend for many miles as rivers of ice.
Most composite cones occur alone or are separated by several tens of kilometers.
Composite volcanoes are constructed from multiple eruptions that occur over periods ranging from hundreds to hundreds of thousands of years. Andesite magma, the most common, but not the only magma type forming composite cones, is more explosive than basalt magma because it contains more silica, but also produces lava flows. The explosive eruptions provide abundant pyroclastic debris in the stratocones. But, not all of the magma rises to the summit. Composite cones can therefore grow to great heights before collapsing under the weight of the Earth's gravitational field, as happened at Mount St. Helens.