Landforms are produced by the endless struggle between internal earth forces that lift crustal masses upward and the external forces of erosion and denudation that wear them down. Where one finds high alpine mountains, internal earth forces dominate, while external earth forces dominate in the low plains of continental interiors. Sequential landforms are shaped by processes and agents of denudation. Volcanic Activity Remember that volcanism is a mountain-building process, representing massive accumulations of extruded of magma at the earth's surface. A volcano is a conical or dome-shaped initial landform built by the emission of lava and gases from a constrained vent in the earth's surface.
Magma rises through a narrow, pipelike conduit from a magma reservoir lying beneath.
The ejected fragments from a volcanic eruption are collectively called tephra . Two types of eruptions can occur from a volcano, an explosive one or a quiet one . The nature of the eruption depends on the type of magma that feeds the volcano. Remember that there are two types of igneous rocks, felsic and mafic . Magma derived from felsic rocks and minerals, such as rhyolite and andesite, have a high degree of viscosity, i.e. , the magma is think and gummy.