Core Knowledge: Earth Materials, Time, and Structure
What this domain tests
Core knowledge covers the vocabulary and organizing principles behind the rest of geology: mineral definition and identification, the three rock classes and the rock cycle, relative and absolute dating, the geologic time scale, and Earth's layered interior. Questions often ask you to identify a material or infer a process from a short description.
Minerals
A mineral is naturally occurring, inorganic, solid, has an ordered atomic structure, and a definite composition that may vary within limits. Silicates dominate the crust because silicon-oxygen tetrahedra bond in chains, sheets, or frameworks (olivine, pyroxene, amphibole, mica, feldspar, quartz). Identify minerals by hardness (Mohs), cleavage versus fracture, luster, streak, and special properties.
Rocks and the rock cycle
Igneous rocks form from cooling magma or lava; slow cooling makes coarse intrusive textures, fast cooling makes fine extrusive textures. Sedimentary rocks form by compaction and cementation of clasts, precipitation, or organic accumulation; bedding and fossils are diagnostic. Metamorphic rocks recrystallize in the solid state under heat and pressure; foliation develops when minerals align. The rock cycle links these classes through melting, weathering, burial, and uplift.
Worked example
A radioactive parent has a half-life of 1.3 billion years. A crystal contains 1/8 of the original parent. How old is it? Each half-life halves the parent. 1/8 remaining means three halvings: 1 to 1/2 to 1/4 to 1/8. Age = 3 x 1.3 = 3.9 billion years. The same logic works in reverse: after four half-lives, 1/16 of the parent remains.
Traps
- Cleavage is a planar break controlled by atomic structure; fracture is irregular.
- Intrusive igneous rock is coarse-grained because it cooled slowly, not because it is older.
- Relative dating orders events; absolute dating gives numerical ages.
- The geologic time scale is not drawn to scale, and Precambrian time dominates it.