Ridges and Trenches


Midocean Ridges

plates spread apart
new lithosphere forms

earthquakes on normal faults
the ocean crust is almost entirely mafic: extrusive basalts and intrusive gabbros
newly formed lithosphere is hot and buoyant

Transform Faults

plates move in side-by side motion
earthquakes on strike-slip (transform) faults

Fracture Zones

both sides are part of same plate (no earthquakes)
the younger side is higher

Subduction Zones

deep ocean trenches mark places where cold, dense oceanic lithosphere is returned to the mantle
occur at ocean-ocean and ocean-continent convergent plate boundaries
thrust and reverse fault earthquakes occur in the brittle crust near the trench
deep-focus earthquakes delineate the upper surface of the descending slab (to about 650 km depth)
water released by metamorphic reactions in the subducting crust generates melting in the overlying mantle
this yields a volcanic arc running parallel to the trench where the slab lies at about 100 km depth