Internal and External Geodynamics: Earth Structure, Tectonics, Magmatism, Metamorphism and Deformation
This domain covers the two engines that shape the planet: internal heat (residual heat plus radioactive decay), which drives tectonics, magmatism, metamorphism and deformation; and solar energy at the surface, which drives weathering, erosion, transport and sedimentation. The exam asks you to link processes to structures and to read a geological cross-section or block diagram.
Earth's layered structure
- Crust: oceanic (~7 km, basaltic) and continental (30–70 km, granitic).
- Mantle (peridotite, solid but ductile over geological time), outer core (liquid Fe-Ni, source of the magnetic field through the geodynamo), inner core (solid).
- Discontinuities: Moho (crust/mantle), Gutenberg (mantle/outer core), Lehmann (outer/inner core).
- Lithosphere (crust plus rigid upper mantle) rides over the asthenosphere; isostasy explains vertical compensation.
Plate tectonics
- Divergent boundaries: ridges, normal faults, basaltic volcanism. Convergent: subduction (trench, arc magmatism, andesitic-granitic magmas) or collision (folding, thrusting, mountain belts). Transform: strike-slip.
- Evidence: sea-floor magnetic stripes, palaeomagnetism, heat flow, seismicity distribution, the Wilson cycle.
Magmatism and metamorphism
- Magma series: basaltic (mafic, fluid, effusive), andesitic, rhyolitic (felsic, viscous, explosive).
- Plutonic rocks are coarse-grained (phaneritic); volcanic rocks are fine-grained or porphyritic.
- Metamorphism types: regional/dynamothermal (foliated: slate, schist, gneiss), contact (non-foliated hornfels), dynamic (mylonite), burial and hydrothermal. Index minerals mark grade: chlorite → biotite → garnet → staurolite → sillimanite.
Deformation
- Brittle: joints and faults - normal (extension), reverse/thrust (compression), strike-slip (shear).
- Ductile: folds (anticline, syncline, limbs, axial plane), commonly with cleavage.
Worked example
A limestone bed is folded into an upright anticline and then cut by a fault whose hanging wall has moved down relative to the footwall. The fault cuts the fold, so the fault is younger. Hanging wall down means a normal fault, i.e. extension. Compression therefore folded the bed first, and extension faulted it afterwards. Each step is read from cross-cutting relationships (relative age) plus fault geometry (stress regime).
Traps that cost marks
- Confusing the Moho with the Gutenberg discontinuity.
- Describing normal faults as compressional.
- Saying a dyke is older than the strata it cuts.
- Forgetting that foliation records directed pressure, while hornfels records heat alone.