Continental vs. Oceanic Plate Convergence
In a contest between a dense oceanic plate and a less dense, buoyant continental plate, guess which one will sink? The dense, leading edge of the oceanic plate actually pulls the rest of the plate into the flowing asthenosphere and a subduction zone is born! Where the two plates intersect, a deep trench forms.
Geologists aren't sure how deep the oceanic plate sinks before it completely melts, but we do know that it remains solid far beyond depths of 100 km beneath the Earth's surface.
When the subducting oceanic plate sinks deeper than 100 kilometers,
huge temperature and pressure increases make the plate sweat'. Well,
not exactly, but the uncomfortable conditions force minerals
in the subducting plate to release trapped water and other
gasses. The gaseous sweat works
its way upward, causing a chain of chemical reactions that
melt the mantle above the subducting plate.
This hot, freshly melted liquid rock (magma) makes its way toward the surface. Most of the molten rock cools and solidifies in huge sponge-like magma chambers far below the Earth's surface. Large intrusive rock bodies that form the backbones of great mountain ranges such as the Sierra Nevada form by this process.
Some molten rock may break through the Earth's surface, instantly releasing the huge pressure built up in the gas-rich magma chambers below. Gasses, lava and ash explode out from the breached surface. Over time, layer upon layer of erupting lava and ash build volcanic mountain ranges above the simmering cauldrons below.
An example of this kind of convergence produces the spectacular volcanic landscape of the Northwest. Off the coast of Oregon, Washington, Alaska and Canada a small plate, the Juan de Fuca, dives beneath North America. This type of convergent plate boundary, called a subduction zone, is known for producing historic earthquakes of great magnitudes.
Look for curved volcanic mountain ranges with deep trenches alongside and it's a safe bet that you're looking at a subduction zone.