Earth Science: Geology and Weather

Minerals, Rocks, and the Rock Cycle

To build a strong foundation for the General Science subtest, you need to distinguish between minerals and rocks. A mineral is defined as a naturally occurring, inorganic solid that features both a definite chemical composition and an ordered crystalline structure. Geologists identify minerals through measurable physical properties. The Mohs hardness scale ranks minerals from 1 to 10 based on their scratch resistance, where talc is the softest at 1 and diamond is the hardest at 10. You can also test a mineral's streak, which is the specific color of powder left behind when it is scraped across an unglazed porcelain plate. A mineral may break along cleavage planes, fracture along other surfaces, or show both when broken in different directions. Cleavage is the tendency of a mineral to break smoothly along specific planes of weak atomic bonds, while fracture describes how a mineral breaks along surfaces other than cleavage planes, producing uneven or smoothly curved surfaces.

Rocks are aggregates formed from minerals and other materials, divided into three major categories based on how they form. Igneous rocks develop from the cooling and solidification of molten rock; this molten material is called magma beneath Earth's surface and lava once it erupts above the surface. Intrusive igneous rocks form below Earth's surface, where slow cooling gives mineral grains time to grow into large, visible crystals, as seen in granite. Extrusive igneous rocks cool rapidly on Earth's surface, creating fine-grained or glassy textures, such as basalt and obsidian.

The remaining two rock groups form through surface processes or intense environmental conditions. Sedimentary rocks form when deposited sediments undergo compaction and cementation, when dissolved minerals precipitate out of water, or when biological remains accumulate. Sedimentary rocks often form in relatively low-temperature environments where sediments accumulate, making them the rock type most likely to preserve fossils. Common sedimentary rocks include sandstone, shale, and limestone. Metamorphic rocks form when existing rocks are altered by intense heat, pressure, or chemically active fluids without melting. Foliated metamorphic rocks display a parallel alignment of minerals or visible bands, as seen in slate, schist, and gneiss. Under these intense conditions, existing rocks transform: sedimentary limestone recrystallizes into metamorphic marble, and sedimentary shale transforms into foliated slate. The rock cycle demonstrates that any rock type can transform into any other rock type through continuous geological processes over time.