Earth, Molnar 2015 5x

Mid-ocean ridge orientation

Mostly trend north–south globally; segmented by transform faults and fracture zones

Island arc subduction structure

Arcuate volcanic islands + deep trench seaward + shallow/intermediate/deep earthquake zones (Benioff zone)

Outer trench morphology

Outer topographic rise forms seaward of trench due to lithospheric bending

Trench position rule

Trench always lies seaward; continental crust does not subduct (oceanic plate subducts)

Plate creation/destruction balance

Lithosphere created at ridges must equal lithosphere destroyed at subduction zones

Temperature control on subduction strength

Hot ridge lithosphere is weak; cold lithosphere is strong and bends/ fractures during subduction

Trench formation mechanism

Cold dense oceanic plate bends downward under gravity into asthenosphere forming trench

Flexure effects at trenches

Bending causes outer rise, normal faulting, and lithospheric stretching

Underthrusting evidence

Shallow earthquakes + fault plane solutions show oceanic plate slides beneath continent

Seismology and Earth structure

Seismic waves used to infer interior because they travel differently through solid vs hot/partially molten mantle

Seismic Q (attenuation)

High Q = low energy loss (solid lithosphere); low Q = high energy loss (hot/partly molten asthenosphere)

Benioff zone earthquakes

Deep and intermediate earthquakes occur within cold subducting slab (not just boundary)

Earthquake depth limits

Most subduction quakes <700 km; gap often occurs ~300–500 km

Stress in slabs (T and P axes)

T-axis = extension (upper slab); P-axis = compression (deeper slab) showing internal slab stress

Volcanism at subduction zones

Water from slab lowers mantle melting point → partial melt rises → volcanic arcs form