Earth's Structure and Internal Processes
Domain II covers Earth's internal layering and the evidence for it, plate tectonics and its surface expressions, and the rock cycle that links internal and surface environments. Items typically ask you to infer a boundary type from a map or profile, explain seismic evidence, or sequence rock-forming processes.
Layering and how we know
- Compositional layers: crust (oceanic basalt about 3.0 g/cm³; continental granite about 2.7), mantle (peridotite), core (iron-nickel).
- Mechanical layers: rigid lithosphere (crust plus uppermost mantle) riding on the plastic asthenosphere; mesosphere; liquid outer core; solid inner core.
- Key evidence: seismic refraction and reflection, the Moho, S-waves that cannot pass the outer core (producing the S-wave shadow zone and the P-wave shadow from 103° to 142°), Earth's mean density, and the geodynamo generated by outer-core convection.
Plate tectonics
- Divergent: mid-ocean ridges, seafloor spreading, symmetric magnetic stripes, rift valleys.
- Convergent: ocean-ocean gives an island arc; ocean-continent gives a trench plus an andesitic volcanic arc; continent-continent gives a collision belt such as the Himalaya.
- Transform: horizontal sliding, as on the San Andreas Fault. Earthquakes concentrate at all three; deep-focus quakes occur only in subducting slabs.
- Driving forces: slab pull dominates, then ridge push and mantle convection; hotspots such as Hawaii record plate motion independent of boundaries.
Rocks and the rock cycle
Igneous rocks are classified by texture and composition (intrusive coarse-grained, extrusive fine-grained); sedimentary rocks are clastic, chemical, or organic and form by lithification; metamorphic rocks are foliated or nonfoliated depending on pressure and directed stress.
Worked example
A map shows a deep trench offshore, a line of andesitic volcanoes inland, and earthquake foci that deepen inland. The trench marks subduction of oceanic lithosphere; andesitic magma comes from partial melting of the mantle wedge hydrated by slab fluids; the deepening Wadati-Benioff zone traces the descending slab's dip. Conclusion: an ocean-continent convergent margin.
Traps
Equating lithosphere with crust; saying S-waves travel through the outer core; assuming the mantle is molten (it is mostly solid and flows only on long timescales); thinking all mountains are compressional; and confusing hotspot volcanism with boundary volcanism.