Earth, Molnar 2015 6x

Continental crust properties

Thicker (25–75 km, avg ~35–40 km), less dense (~2.7 g/cm³), more buoyant than oceanic crust

Oceanic crust properties

Thin (~5–10 km, ~7 km typical), denser (~3.0 g/cm³), basaltic composition, younger and more recycled

Why continental crust resists subduction

Lower density + greater buoyancy means it is not easily dragged into mantle compared to dense oceanic lithosphere

Tectonics definition

Large-scale processes that deform Earth’s crust; plates behave as large, rigid to semi-rigid blocks

Earthquake distribution: oceanic margins

Narrow belts of earthquakes concentrated along plate boundaries; interiors mostly rigid with little deformation

Earthquake distribution: continental margins

Wide zones of deformation; earthquakes spread across broad regions rather than a single narrow line

Crust contribution to Earth structure

Crust is thin but forms the outer rigid layer; major differences in thickness control plate behaviour

Continental lithosphere deformation

More ductile/weak overall; deforms internally due to thicker crust and weaker mineral layers (e.g., quartz-rich crust)

Oceanic lithosphere strength

Stronger and more rigid at depth (~20–40 km) due to olivine-rich mantle lithosphere dominating structure

Mountain building at convergent margins

Continental crust can detach and stack into nappes, forming mountains like Alps and Himalayas

Isostasy (mountain formation principle)

Lithosphere rises or sinks depending on density/thickness balance between crust and mantle

Oceanic vs continental plate behaviour

Oceanic behaves rigidly; continental behaves less rigidly and undergoes distributed deformation

Reason for continental weakness at depth

Higher temperatures + quartz/feldspar crust minerals weaken with heat and allow ductile flow

Lithospheric strength layering

Crust and mantle have variable strength layers; weak zones allow detachment and thrusting in continents

Mountain building mechanism (continental collision)

Compression causes crustal shortening, thickening, and uplift when continental plates collide (no subduction of continent)