Minerals first, then rocks
A mineral is a naturally occurring solid with a specific chemical makeup and an orderly internal arrangement of atoms. Quartz, feldspar, mica and calcite are common examples. A rock is usually a collection of one or more minerals (or sometimes fragments of other rocks, or organic material such as the remains that form coal) held together.
Geologists group rocks into three families according to how they formed: igneous, sedimentary and metamorphic. The rock cycle is the idea that these are not fixed categories. Over long periods, any rock can be transformed into any of the other types, or into a new rock of the same type. There is no single starting point and no fixed order; the “cycle” is better thought of as a web of possible pathways.
Igneous rocks: from molten rock
Igneous rocks form when molten rock cools and solidifies. The place where this happens matters a great deal.
- Intrusive (plutonic) rocks cool slowly underground. The slow cooling gives mineral crystals time to grow, so they are usually coarse-grained and visible to the naked eye. Granite is a familiar example.
- Extrusive (volcanic) rocks cool quickly at or near the surface, after lava erupts. The crystals are tiny or absent, as in basalt, which is common in oceanic crust. Very fast cooling can produce volcanic glass.
How magma rises and erupts determines which of these forms, which is covered in more detail in explanations of volcanic eruptions.
Sedimentary rocks: layers from fragments and solutions
Sedimentary rocks form at or near Earth’s surface. Wind, water, ice and temperature changes break rock into pieces in a process called weathering, and erosion carries the pieces away. Eventually, particles settle in layers, in places such as river deltas, lake beds, deserts and the sea floor. Over time, the layers become rock through compaction (squeezing out water) and cementation (minerals precipitating between grains and binding them).
Sedimentary rocks come in a few main types:
- Clastic rocks, made of fragments, such as sandstone (sand-sized grains), siltstone and shale (very fine particles).
- Chemical rocks, formed when minerals precipitate from water, such as rock salt left behind when seawater or salty lakes evaporate.
- Organic or biological rocks, such as limestone built largely from the shells and skeletons of marine organisms, and coal from plant material.
Because of their layering and the way they form in water and air, sedimentary rocks are where most fossils are found. The process of fossil formation depends on rapid burial in sediment. Layers also record environments: ripple marks can suggest flowing water, and thick salt beds suggest evaporating seas. Exposed cliffs, like those in the Grand Canyon, show stacks of sedimentary layers that geologists read much like pages in a book.
Metamorphic rocks: changed without melting
Metamorphic rocks form when existing rocks are changed by heat, pressure or chemically active fluids, but without fully melting. The minerals recrystallize or rearrange into new forms that are stable under the new conditions, and the texture changes too.
Typical examples include the following.
- Limestone can become marble.
- Sandstone rich in quartz can become quartzite.
- Shale can become slate, and with more heat and pressure, schist and gneiss.
Metamorphism commonly occurs deep in the crust, near intrusions of hot magma, or where plates collide and build mountains. Pressure applied in one direction often aligns minerals into bands or sheets, giving many metamorphic rocks a layered or “foliated” appearance. If conditions become hot enough that the rock melts, the molten material becomes magma, and the cycle begins again.
How one type becomes another
The links between families are the core of the rock cycle. A few possible pathways follow.
- Igneous to sedimentary. A granite mountain weathers into sand and clay, which are carried by rivers and deposited, then buried and cemented into sandstone and shale.
- Sedimentary to metamorphic. Deep burial or the heat of nearby magma transforms those rocks into quartzite or slate.
- Metamorphic to igneous. With enough heat, the rock melts. When the magma cools, it forms igneous rock.
- Igneous to metamorphic. An existing igneous rock can be buried and changed by heat and pressure without ever being weathered.
- Any rock to sedimentary. Uplift brings rocks to the surface, where weathering begins again.
Because of these shortcuts, a rock does not need to visit every stage.
What drives the cycle
Two great engines run the cycle. Heat from Earth’s interior, and the movement of plates, drive melting, deep burial, metamorphism and the uplift of mountains. This is part of the larger picture of Earth structure and plate tectonics. At the surface, energy from the Sun powers the atmosphere and the water cycle, which drive weathering and erosion. Climate affects how fast rocks break down: heavy rain, freezing and thawing, and chemical reactions with water all speed up the process.
How long it takes
The timescales are enormous. Cooling of a magma body can take thousands to millions of years, and the formation of sedimentary rock and mountain uplift occurs over millions of years. The oldest known rocks and mineral grains are several billion years old, which shows that some material can survive for a very long time, while the oceanic crust is generally much younger because it is recycled at subduction zones.
Why it matters in practice
The rock cycle explains where building stone, metal ores, fuels and groundwater-bearing rocks come from, and why they occur in some regions and not others. It also helps explain landscapes and hazards, since the type of rock affects how a slope fails, how water drains, and how a region’s soil develops. Recognizing the three families can also make a walk in a quarry or a visit to a geology museum much more rewarding.