13) The newly generated molten mantle (magma) is less dense than the
surrounding rock, so it rises toward the surface. Most of the magma
cools and solidifies as large bodies of plutonic (intrusive) rocks far
below the Earth’s surface. These large bodies, when later exposed
by erosion, commonly form cores of many great mountain ranges
[such as the Sierra Nevada (California) or the Andes (South America)]
that are created along the subduction zones where the plates
converge.
14) Some of the molten rock may reach the Earth’s surface to erupt as
the pent-up gas pressure in the magma is suddenly released, forming
volcanic (extrusive) rocks. Over time, lava and ash erupted each time
magma reaches the surface will accumulate—layer upon layer—to
construct volcanic mountain ranges and plateaus, such as the
Cascade Range and the Columbia River Plateau (Pacific Northwest,
U.S.A.).