Bare rock faces in the mountainsBare rock faces in the mountains Rock is the hard material that makes up the Earth’s crust, its hard “shell”. Rock lies beneath all natural soil layers—and everything else: city streets, a sandy beach, even the ocean. Of course, bare rock lies at the surface itself in many places, such as at the tops of mountains or on sea cliffs. Rocks are stony masses made up from minerals. Minerals, in turn, consist of chemical elements, such as silicon, oxygen or magnesium.

A close up view of sandA close up view of sand

Quartz crystalsQuartz crystals


There are thousands of different minerals, but a very few kinds (feldspar, mica and quartz, for example) are found in nearly all the Earth’s rocks. Various combinations of minerals make up different types of rocks. For example, the rock sandstone consists mainly of grains of sand pressed and cemented together. Sand is made mainly of the minerals quartz and feldspar. Quartz consists of the chemical elements silicon and oxygen; feldspar is composed of a variety of elements.

Rock types

All rocks can be divided into three main groups depending on how they formed. Igneous rocks, such as granite and basalt, are formed when magma, molten rock from beneath the Earth’s crust, rises, cools and solidifies.A cross-section of the Earth's crustA cross-section of the Earth's crust

Sedimentary rocks, such as sandstone and mudstone, are made from sand, gravel, mud and other fragments of rock that result from erosion. These settle in layers in lakes, rivers and seas. As more layers settle on top of each other, the particles are compressed and cemented into sedimentary rock.

The third group, metamorphic rocks, such as marble and slate, are formed when rocks are subjected to such great pressure and heat that their mineral composition is altered.

The rock cycleThe rock cycle

Rock cycle

Rocks are constantly being changed. Weathering attacks all kinds of surface rocks—sedimentary, metamorphic or igneous rocks (1). The eroded fragments, called sediments, may be washed out to sea (2). Here the sediments may compact and harden to form layers of sedimentary rock (3). These layers may then later be cooked and crushed deep in the crust, forming metamorphic rocks (4). Alternatively they may eventually sink into the Earth's mantle in subduction zones where they melt and rise into the Earth's crust, sometimes erupting at the surface as volcanoes (5). This melted rock, known as magma, then cools to become
igneous rocks. The continual change from one rock
type to another is called the rock cycle.

 Ian Fairchild

See also in Science

See also in Earth

The oldest rocks on Earth are found in the Canadian Shield in the Northwest Territories. There, gneiss, a metamorphic rock which forms the core of an ancient mountain chain, has been dated to more than 4 billion years old.

Silicates are the most abundant mineral group in the Earth's crust, and feldspar is the most abundant mineral.

Most of the ocean floor is basalt, while most of the rock making up continents is granite.

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