Earth's Materials, Structure, and Plate Tectonics
This domain examines minerals, the rock cycle, Earth's internal layers, and the evidence and mechanisms of plate tectonics. Questions often ask you to infer rock origin from texture and composition or to interpret a boundary setting from geologic features.
Core rules and concepts
- A mineral is naturally occurring, inorganic, crystalline, and has a definite chemical composition or compositional range.
- Silicate minerals are built from SiO4 tetrahedra; Bowen's reaction series predicts mineral crystallization order and weathering stability.
- Igneous texture records cooling history: coarse-grained = slow, intrusive cooling; fine-grained or glassy = rapid, extrusive cooling.
- Earth's compositional layers are crust, mantle, and core; mechanical layers are lithosphere, asthenosphere, mesosphere, outer core, and inner core.
- Divergent boundaries produce new crust and rift valleys; convergent boundaries produce subduction, trenches, arcs, and mountain belts; transform boundaries produce strike-slip faults.
- Seafloor magnetic stripes, age patterns, earthquakes, and hotspot chains support plate motion.
Worked example
A coarse-grained, dark rock contains abundant olivine, pyroxene, and calcium-rich plagioclase. The minerals are mafic, and the coarse texture indicates slow cooling beneath the surface. The rock is gabbro, the intrusive equivalent of basalt. If the same magma cooled rapidly at a mid-ocean ridge, the result would be fine-grained basalt; if gas-rich and frothy, scoria.
Traps that cost marks
- Confusing the asthenosphere (weak, partially molten mantle) with the lithosphere (rigid plates).
- Calling all volcanoes convergent; hotspots and divergent ridges also produce volcanism.
- Mistaking cleavage planes for crystal faces or fracture.
- Assuming subduction occurs at every convergent boundary; continental collision can lack subduction.