Rocks

• A rock is a naturally occurring solid object composed of one or more minerals.

• Rocks are formed in the lithosphere through a variety of natural processes.

• The lithosphere is Earth’s rigid outer layer, consisting of the crust and upper mantle.

• Rocks are grouped into three main categories based on how they are formed. These are igneous rocks, sedimentary rocks and metamorphic rocks.

• Igneous rocks are rocks formed when magma or lava cools and solidifies.

• Igneous rocks are composed of interlocking crystals. Consequently, they are generally hard and strong.

• Examples of igneous rocks include granite, basalt and andesite.

• Sedimentary rocks are rocks formed by the compaction and cementing together of sediments or the precipitation of dissolved minerals.

• Sedimentary rocks are composed of non-interlocking grains. Consequently, they are generally soft and break apart easily.

• Examples of sedimentary rocks include sandstone, limestone and gypsum rock.

• Metamorphic rocks are rocks formed when existing rocks undergo a transformation due to being subjected to high pressure and/or high temperature.

• Metamorphic rocks are composed of interlocking crystals that have been compressed under high pressure and temperature. Consequently, they are generally very hard and strong.

• Examples of metamorphic rocks include quartzite, marble and slate.

• Apart from their geological classification as igneous, sedimentary or metamorphic, rocks can also be classified based on their physical structure or their chemical composition.

• For example, siliceous rocks are those with a high percentage of the mineral silica. They include granite, chert and quartzite, which are igneous, sedimentary and metamorphic rocks.

The three main types of rocks are igneous rocks, sedimentary rocks and metamorphic rocks.

(Images: Eurico Zimbres, Wikimedia Commons; James St. John, Wikimedia Commons; James St. John, Wikimedia Commons)