Bare 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 magnified image of sand, made up of tiny fragments of rock (note the one-millimetre scale bar, bottom right). The exact...Read More >>A magnified image of sand, made up of tiny fragments of rock (note the one-millimetre scale bar, bottom right). The exact composition of sand varies according to the local rocks, but the most common constituent of sand is silica (silicon dioxide), usually in the form of quartz. This sample of sand is made up mostly of quartz, chert, various igneous rocks and fragments of shell.
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.
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.
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