3 Rocks and the Rock Cycle
Learn how igneous, sedimentary, and metamorphic rocks form and how processes at Earth's surface and underground connect them in a flexible rock cycle.
Three Main Rock Types
A rock is a naturally occurring aggregate of minerals. Geologists group rocks according to how they form: igneous, sedimentary, or metamorphic. These categories describe processes, not steps in a fixed sequence. A rock may change from one type to another by several different routes, and many transformations take millions of years.
How Form
An igneous rock forms when molten rock cools and solidifies. Molten rock below Earth's surface is called ; molten rock at the surface is called lava.
Cooling rate affects crystal size:
that cools slowly underground can form large mineral crystals, as in granite.
Lava that cools quickly at or near the surface forms small crystals, as in basalt.
Extremely rapid cooling can prevent crystals from forming, producing volcanic glass such as obsidian.
Takeaway: The size of crystals can provide clues about where and how quickly molten rock cooled.
How Form
A sedimentary rock forms from material deposited at Earth's surface or from minerals that precipitate from water. Sediment may include weathered rock fragments, mineral grains, or the remains of organisms.
Three common formation types are:
Clastic rocks form when sediment is buried, compacted, and cemented together. Sandstone forms from sand-sized grains.
Chemical rocks form when dissolved minerals precipitate from water, often as water evaporates.
Biologic rocks form from accumulated organism remains. Many limestones contain shells or other biological material.
The conversion of loose sediment into solid rock is called . Compaction presses sediment together, and cementation binds grains with minerals.
Takeaway: preserve material that was deposited, precipitated, or accumulated at Earth's surface.
How Form
A metamorphic rock forms when an existing rock is changed by heat, pressure, or chemically active fluids, usually while remaining solid. Its minerals may recrystallize or new minerals may form. Pressure can align mineral grains into layers or bands.
Examples show how an original rock can change:
Shale can become slate.
Limestone can recrystallize into marble.
When metamorphism occurs next to a hot body of igneous rock, it is called contact metamorphism.
Takeaway: Metamorphism changes a rock without requiring it to melt.
Processes and Pathways in the
The links rock types through processes that operate at Earth's surface and underground. One possible route is:
Weathering breaks rock into smaller pieces; erosion moves those particles.
Deposition occurs when particles settle. Burial, compaction, and cementation can then lithify sediment into sedimentary rock.
Burial and increasing heat and pressure can change igneous or sedimentary rock into metamorphic rock.
Melting turns rock into . If or lava cools and crystallizes, igneous rock forms.
Uplift can bring buried rock to the surface, where weathering may begin again.
For example, granite can weather into sand; the sand can become sandstone; burial can change sandstone into quartzite; and melting quartzite can produce that later crystallizes into igneous rock.
This route is only one example. Any rock type can be weathered into sediment, metamorphosed under suitable conditions, or melted if it becomes hot enough. The cycle therefore has no single starting point or required order.
Takeaway: Rock types are connected by many possible pathways, rather than by one repeating sequence.