What is the rock cycle, and how does it work?

Rocks look like the most permanent things on Earth, but they are always changing, just very slowly. A grain of sand on a beach may once have been part of a mountain, and one day it may be part of a mountain again. The rock cycle is the set of processes that moves Earth's material between the three main types of rocks.

Igneous rock forms when molten rock cools and turns solid. Underground, molten rock is called magma; when it reaches the surface it is called lava. Magma that cools slowly deep underground has time to grow big crystals, as in granite. Lava that cools quickly on the surface, as at volcanoes, forms tiny crystals, as in basalt. Lava that cools almost instantly makes glassy obsidian, and frothy lava full of gas bubbles makes pumice, which can float.

Sedimentary rock forms from pieces of other rocks. Wind, rain, ice and changes in temperature break rocks into small bits (weathering). Rivers, wind, glaciers and the sea carry the bits away (erosion and transport). When the water slows down, the bits settle in layers (deposition), often on the bed of a lake or sea. Over time the layers on top press down (compaction) and minerals in the water glue the grains together (cementation). Sandstone, mudstone, shale, limestone and chalk form this way. Sedimentary rocks often have layers and are where most fossils are found.

Metamorphic rock forms when existing rock is changed by great heat and pressure deep underground, without melting. The minerals rearrange into new crystals, often in bands. Limestone becomes marble, mudstone or shale becomes slate, sandstone becomes quartzite and granite can become gneiss.

If any rock gets hot enough to melt, it becomes magma again, and when that cools the cycle continues. Movements of the tectonic plates push rock down to where it melts or is squeezed, and lift buried rock up into mountains (uplift), where weathering can start on it again.

The cycle has no fixed order and no starting point. Igneous rock can be weathered into sediment, or changed into metamorphic rock, or melted again. The same is true for the other two types. Two energy sources drive it all: Earth's internal heat melts and squeezes rock and moves the plates, while the Sun's energy drives the weather, rain and rivers that wear rocks down, with gravity pulling the pieces downhill.

To model the rock cycle at home, use old wax crayons: grate them into shavings (weathering), press the shavings in foil (compaction, a "sedimentary" crayon), press them while warm (heat and pressure, "metamorphic"), and melt and cool them (an "igneous" crayon). Ask an adult to do the melting.

What is the rock cycle in simple words?#

It is Earth's way of recycling rock. Rocks are melted, broken into pieces, buried and squeezed, so over time each type of rock turns into another.

What does the rock cycle consist of?#

Three rock types (igneous, sedimentary and metamorphic) and the processes that link them: melting, cooling, weathering, erosion, deposition, compaction and cementation, heat and pressure, and uplift.

What is the rock cycle in order?#

There is no single order. A common route is igneous → sediment → sedimentary → metamorphic → magma → igneous, but any rock can take a shortcut, such as metamorphic rock being weathered straight into sediment.

How long does the rock cycle take?#

Usually millions of years. Some steps are fast (lava can harden in hours or days), but wearing down a mountain or burying and changing rock takes far longer.

How does the rock cycle start?#

It has no beginning or end. Scientists think the first rocks on Earth were igneous, formed as the molten young planet cooled, but today every type of rock keeps changing into the others.

Why is the rock cycle important?#

It makes the soil plants grow in, building stone such as granite and marble, and the sedimentary layers that hold fossils and tell us about Earth's past.