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