Plate-Convergence Summary Notes
Oceanic–Oceanic Convergence
- Two oceanic plates collide; the denser (usually older) plate subducts.
- Plate density increases with age due to cooling, thickening, and hydrothermal metamorphism.
- Subduction generates melt; magma rises to form an island (volcanic) arc on the overriding plate.
- Creates deep trenches; e.g., Mariana Trench/Arc.
- Back-arc region may undergo spreading due to slab pull.
Mariana Trench & Back-Arc System
- Deepest point on Earth at \approx 10{,}957\,\text{m}.
- Pacific Plate (age \approx 170\,\text{Ma}) subducts beneath younger Philippine Plate (age \approx 60\,\text{Ma}).
- Rapid slab descent pulls overriding plate forward, generating tension and back-arc seafloor spreading.
- Produces arcuate volcanic chain above the slab.
Oceanic–Continental Convergence
- Oceanic crust is denser and subducts beneath continental crust.
- Generates continental magmatic arcs and shrinks oceans.
- Typical of the Pacific "Ring of Fire" (e.g., Cascadia Subduction Zone).
Benioff Zones & Earthquakes
- Earthquake foci trace the dipping slab to \sim 700\,\text{km} depth.
- Below 700\,\text{km} rocks deform plastically; seismicity wanes.
- Depth pattern aids imaging of slab geometry in both O–O and O–C settings.
Pacific Ring of Fire
- Encircles Pacific Plate with subduction on most margins.
- Hosts intense seismic and volcanic activity.
- Earthquake depth increases landward, marking the slab dip.
- Divergent/transform boundaries have shallower, less intense quakes.
Continental–Continental Convergence
- Continental crust buoyant; neither plate subducts.
- Colliding margins crumple, producing large orogenic belts (mountain ranges).
- Examples: Himalayas, Alps, Appalachians, Grenville.
Closing of the Mediterranean & Alpine Orogeny
- Mediterranean = remnant of Tethys Ocean; has been closing for \sim 150\,\text{Ma} via subduction.
- Remaining oceanic crust scarce; continental fragments (terranes) now collide, forming Alpine fold-thrust belt.
- Matterhorn: African crust thrust onto Eurasian Plate; foreground ophiolites are obducted oceanic slices.
Key Terms
- Subduction: descent of one plate beneath another into the mantle.
- Back-arc Spreading: extensional seafloor spreading behind a volcanic arc due to slab pull.
- Volcanic Arc: chain of volcanoes above a subducting slab (island arc on oceanic crust; magmatic arc on continental crust).
- Trench: deep, narrow depression marking the plate interface.
- Orogeny: mountain-building event from continental collisions.
- Terrane: crustal fragment with distinct geologic history accreted to a continent.
