In plate tectonics, the process in which one plate plunges beneath another plate into the mantle is called:

Subduction is the process in which one tectonic plate, usually a denser oceanic plate, sinks beneath another plate and descends into the mantle. The area where this happens is called a subduction zone.

Why subduction happens:

• Oceanic crust is made of dense basaltic rock and becomes colder and heavier as it ages and moves away from the ridge where it formed.
• When it meets another plate, the denser plate sinks under the lighter one.
• The sinking edge of the plate pulls the rest of the plate along; this "slab pull" is one of the main forces that drive plate motion.

Features associated with subduction:

Deep ocean trenches form where one plate bends down beneath the other.

Strong earthquakes occur along the sinking plate, from shallow to great depths (up to about 700 km). The inclined zone of earthquakes is called the Wadati-Benioff zone.

Volcanoes form on the overriding plate, because the sinking plate releases water into the mantle above it, lowering its melting point and producing magma.

Types of subduction:

• Oceanic plate under oceanic plate: forms volcanic island arcs, such as Japan, the Aleutian Islands and the Mariana Islands.
• Oceanic plate under continental plate: forms coastal volcanic mountain ranges, such as the Andes in South America and the Cascade Range in the USA.

Continent-continent collision:

When two continental plates meet, neither sinks easily because continental crust is light. Instead, the crust crumples and thickens to form high mountains, such as the Himalayas, formed by the collision of the Indian Plate with the Eurasian Plate.

Effects:

Many of the world's strongest earthquakes, including the 2004 Sumatra-Andaman earthquake (magnitude 9.1) and the 2011 Tohoku earthquake in Japan (magnitude 9.0), happened in subduction zones.

Why not the other options?

• ✗Obduction is the opposite process, in which oceanic crust is pushed up onto a continent.
• ✗Accretion is the addition of material to a plate's edge.
• ✗Rifting is the pulling apart of the crust.