Geology of the lithosphere (option T5): the lithosphere and asthenosphere, the structure and formation of oceanic and continental crust, the geophysical evidence for the Earth's interior (seismic, gravity, magnetic, heat flow), and the processes of isostasy and crustal recycling.

Take each geophysical method and say what it reveals, because the marks reward distinct techniques.

Seismology is the main tool. Earthquake P and S waves travel at speeds that depend on the density and state of the rock, and they refract and reflect at boundaries. The pattern of arrivals, the P-wave shadow zone and the failure of S waves to pass through the outer core reveal the layered structure (crust, mantle, liquid outer core, solid inner core) and the depths of the boundaries.

Gravity surveys detect variations in density: a gravity anomaly shows where denser or less dense material lies beneath the surface, for example dense ocean crust or low-density mountain roots.

Magnetic surveys detect the magnetisation of rocks, including the symmetrical magnetic stripes of the ocean floor that record sea-floor spreading.

Heat flow measurements show how heat escapes, higher at ridges (hot, rising mantle) and lower over old, cold crust, constraining the thermal structure.

Together seismology, gravity, magnetism and heat flow build the picture of a layered, dynamic interior.

Markers reward seismic evidence (wave speeds, shadow zone, S-wave behaviour) for the layered structure, plus gravity, magnetic and heat-flow methods, each linked to what it reveals.

Define isostasy and then apply it to the two cases, because both application and definition are required.

Isostasy is the buoyant equilibrium of the lithosphere floating on the denser, ductile asthenosphere, so that thicker or less dense crust stands higher, like a block floating in water.

Mountains stand high because the crust is thickened beneath them, giving a deep, low-density root that buoys the range up; as the mountains are eroded, the root slowly rises (isostatic rebound) to maintain balance, so erosion is partly compensated by uplift.

After deglaciation, the removal of the weight of a former ice sheet lets the depressed crust rebound upward (isostatic or post-glacial rebound), which is still raising parts of Scotland and Scandinavia today.

So isostasy is the floating balance of the lithosphere on the asthenosphere, explaining both the deep roots of mountains and the slow uplift of land after ice or load is removed.

Markers reward isostasy as buoyant balance on the asthenosphere, mountain roots and rebound during erosion, and post-glacial rebound after ice removal.