03 Rocks and the Rock Cycle
Learn how igneous, sedimentary, and metamorphic rocks form, what their features reveal, and how changing Earth processes connect them in the rock cycle.
How Rocks Reveal Their Origins
Rocks are naturally occurring aggregates of minerals or other materials. Geologists classify them by how they formed: igneous, sedimentary, or metamorphic. Their minerals, grain size, layering, and other textures act as evidence of the processes they experienced. A rock's present appearance may record more than one event in its history.
: Cooling and Crystallization
form when molten material cools and solidifies. Underground molten rock is called ; after it reaches Earth's surface, it is called .
Intrusive rocks cool underground. Slow cooling gives larger crystals time to grow; granite is an example.
Extrusive rocks cool at or near the surface. Faster cooling usually produces small crystals, as in basalt. Extremely rapid cooling can form volcanic glass such as obsidian.
Crystal size and texture therefore provide clues about where and how quickly the molten rock cooled. Large visible crystals in granite suggest slow underground cooling, while basalt's fine grains indicate faster cooling near the surface.
: From Deposition to Stone
form at or near Earth's surface from accumulated sediment, substances that precipitate from water, or material from organisms.
Clastic rocks, such as sandstone, consist of fragments of older rocks. Weathering produces sediment, which agents such as water or wind transport and deposit. After burial, compaction presses grains together and cementation binds them. Together, these processes are called . Chemical form when dissolved substances precipitate, while biologic form from accumulated remains of organisms.
commonly form in layers called . Grain size and composition can indicate what was deposited and how. Fossils, when preserved, provide evidence of past life. Sandstone records the of sand-sized grains; fossil-bearing limestone may preserve material derived from organisms.
: Change Without Complete Melting
form when existing rocks are altered by heat, pressure, and sometimes chemically active fluids—without melting completely. Heat from nearby can change surrounding rock through contact metamorphism.
Directed pressure may align minerals, producing , a layered or banded texture. Some are non-foliated. Shale can be transformed into slate as it is buried and compressed, while limestone can recrystallize into marble. Minerals and textures in the resulting rock help reveal the conditions it experienced. If the rock melts, it becomes rather than remaining metamorphic rock.
The : Connected, Flexible Pathways
The connects , igneous rock, sediment, sedimentary rock, and metamorphic rock through a range of geological processes:
Cooling and crystallization form igneous rock from or .
Weathering and erosion produce sediment. Deposition and burial, followed by compaction and cementation, can form sedimentary rock.
Heat and pressure can transform existing rock into metamorphic rock.
Melting produces , and uplift can expose buried rocks to weathering again.
These are possible routes, not steps every rock must follow in a fixed order. For example, an igneous rock can be buried and metamorphosed, then uplifted and eroded into sediment that is later lithified into sedimentary rock. Processes can repeat, and a rock can take different pathways.
Putting the Evidence Together
Geologists build an interpretation by combining several observations rather than relying on one feature alone:
Crystal size and texture can indicate cooling rate and setting.
Sediment grains, layers, and fossils can reveal what was deposited and provide clues about past environments and life.
and recrystallized minerals can indicate metamorphism and the conditions that changed the rock.
A rock's features may preserve evidence of several processes, so its history can be more complex than a single origin event. The key is to connect observable features with plausible formation and change processes.