Rocks are made of minerals
A rock is a solid mass of one or more minerals, which are naturally occurring crystalline solids with a particular chemical makeup. Granite, for example, is a mixture of several minerals, while a pure piece of rock salt is mostly one. Geologists sort rocks into three main groups according to how they formed: igneous, sedimentary and metamorphic.
Igneous rocks: formed from molten material
Igneous rocks form when molten rock cools and solidifies. Below ground, molten rock is called magma; once it erupts at the surface it is called lava.
- Intrusive (plutonic) rocks cool slowly deep underground. Slow cooling gives crystals time to grow, so the rock is coarse-grained. Granite is the familiar example.
- Extrusive (volcanic) rocks cool quickly at or near the surface. Crystals are small or, if cooling is extremely fast, absent. Basalt is a common fine-grained example, and obsidian is a natural volcanic glass.
Because melting and solidifying are changes between the states of matter, you can think of igneous rock as the result of liquid rock freezing. Much of the ocean floor is made of basalt, and the way molten rock reaches the surface is covered in the discussion of volcanic eruptions.
Sedimentary rocks: built from pieces and deposits
Rocks exposed at the surface are slowly broken down. Weathering breaks them into smaller pieces or dissolves parts of them, through physical processes such as freezing and thawing and through chemical reactions with water and air. Rainwater is slightly acidic because it dissolves some carbon dioxide, which helps break down certain minerals, especially in limestone.
Erosion then carries the fragments, called sediment, away by water, wind or ice, and deposition drops them somewhere, often in a river delta, lake or sea floor. Over time, layers build up. Sediment becomes rock through lithification: the weight of overlying layers compacts it, and dissolved minerals act as a natural cement that binds the grains.
Common examples include:
- Sandstone, from cemented sand grains.
- Shale, from compacted mud and clay.
- Limestone, often formed from the shells and skeletons of marine organisms or from minerals that precipitate out of water.
- Conglomerate, from cemented gravel and pebbles.
Sedimentary rocks are usually layered, and they are the main rocks that hold fossils, since the gentle burial of organisms in sediment can preserve their remains. This is why dinosaur fossils are found in such rocks rather than in granite or basalt. Both limestone and granite were used in building the Egyptian pyramids, for example, showing how long people have relied on these materials.
Metamorphic rocks: changed by heat and pressure
Metamorphic rocks form when an existing rock is changed by heat, pressure or hot, chemically active fluids, without melting completely. The minerals recrystallize or rearrange into new forms that are stable under the new conditions. The conditions are typically found deep in the crust, near intruding magma, or where tectonic forces squeeze rocks together.
Some examples of typical changes:
- Limestone becomes marble.
- Shale becomes slate, and with more heat and pressure, schist.
- Sandstone becomes quartzite.
- Granite can become gneiss, which shows distinct bands of minerals.
Rocks that have been squeezed in one direction often develop a layered or banded texture called foliation. Rocks changed mainly by heat, with little directed pressure, often lack it.
The rock cycle is not a simple loop
The idea of the rock cycle describes how any rock type can become another over very long timescales. Magma cools into igneous rock. Weathering and erosion turn it into sediment, which becomes sedimentary rock. Deep burial or collision can turn that into metamorphic rock, and, if heated enough, it can melt into magma again.
Diagrams often show a neat circle, but real pathways are many. An igneous rock can be metamorphosed directly without becoming sediment. A sedimentary rock can be uplifted and eroded again. Metamorphic rock can be weathered into new sediment. The cycle has no fixed direction or schedule, and the timescales are typically millions of years or longer.
What drives the cycle
Two broad kinds of forces keep it moving. Energy from inside the Earth, including heat left from the planet’s formation and heat produced by radioactive decay, moves material around and melts rock. The movement of the crust described under plate tectonics lifts mountains, buries rocks and brings them back to the surface. Energy from the Sun, through the weather and the flow of water, drives weathering, erosion and transport at the surface.
How we learn the age of rocks
Geologists use the order of layers, since lower layers in an undisturbed sequence are generally older. They also use radiometric dating, which relies on the steady decay of certain atomic isotopes into other elements, to estimate how long ago some minerals, usually in igneous rocks, crystallized. Combining these methods is how the history preserved in rock is pieced together, though dates always carry some uncertainty.