Convergent Boundaries in Geology

Convergent Plate Boundaries

• Convergent plate boundaries are regions where two lithospheric plates move towards each other.
• The plates on either side can be of different types and ages, and can move at different speeds.
• Convergent boundaries have trenches, typically 5-8 km deep, but sometimes up to 11 km deep, and are associated with thick belts of earthquakes.
• Earthquakes start shallow at the trench and get progressively deeper (up to 600-700 km) on one side.
• Most convergent boundaries have a belt of active volcanoes that occur on the same side of the trench as the earthquakes.

Ocean-Ocean Convergence

• When two oceanic plates converge at a trench, the older, denser one descends (subducts) under the younger one.
• The subducting slab is heated by the surrounding mantle, causing volatile elements to escape and reduce the melting temperature of the asthenosphere.
• This generates buoyant andesitic magma that rises to feed surface volcanism.
• The extra water and gases in andesitic magma explain the explosive activity of subduction-zone volcanoes.
• Over millions of years, the erupted lava and volcanic debris pile up on the ocean floor until a chain of submarine volcanoes rises above sea level, forming a volcanic island arc.

Earthquakes and Subduction

• The descending plate pushes against the overriding plate, creating compressive stresses that cause frequent, and sometimes large, earthquakes.
• Earthquakes deepen away from the trench on an inclined plane known as a Wadati-Benioff zone.
• The largest earthquakes occur at shallow depths, where the descending slab is in direct contact with the overriding plate.
• As the slab continues to deepen, earthquakes occur not only at the surface of the subducting lithosphere but also within it, in response to stresses due to heating and gravity.

Flexural Bulge and Outer Rise Earthquakes

• The subducting slab flexes as it enters the subduction zone, creating a flexural bulge.
• The top part of the slab is in tension, inducing normal faulting on the flexural bulge and creating outer rise earthquakes.

Back-Arc Basins

• Behind many island-arc systems, small ocean basins with active spreading centers open up between the arc and the adjacent continent.
• These are called back-arc basins.

Volcanism at Divergent Boundaries

• Magma that erupts from divergent boundaries comes from the asthenosphere rising to re-establish isostatic equilibrium.
• The asthenosphere rises to fill the gap left by the thinning lithospheric plate.
• If the asthenosphere is hot and rises fast, it does not have time to cool, and its volatile elements free themselves from the crystalline lattice of the surrounding rock, inducing melting.
• This process is called decompression melting.
• Ridges typically consist of several eruption centers, each about 2-3 km long, separated by gaps of about 1 km.
• Beneath the spreading ridges, magma chambers link between and across the active segments.