Earth: Origins and Geology – Magmatic and Metamorphic Rocks

Earth Science: Magmatic and Metamorphic Rocks

The rock material that makes up the Earth is a mixture of natural minerals. Some rocks are created from molten mass as it cools, others are formed by the high temperatures and extreme pressure conditions of Earth’s interior.

Rocks are composed of a variety of minerals, the presence and ratios of which depend largely on their origin and the environmental conditions during their formation. In contrast to minerals, the composition of rocks can be varied.

They will only ever have identical compositions to one another when they are made up entirely of one single type of mineral.

Igneous rocks

Magmatites or igneous rocks are formed by the cooling of liquid magma. Minerals crystallize and amalgamate into larger rigid structures. If the process of solidification occurs deep inside the Earth, then plutonic rock is created. Plutonic rock, such as granite or diorite, has particularly large crystals. They are able to grow large due to the slow cooling process. However, only small crystals are created when magma reaches the surface of the Earth during the eruption of a volcano. Now the magma is referred to as lava.

Typical vulcanites or extrusive rocks include basalt and rhyolite. Some vulcanites also have single larger crystals. This is how embedded crystals, such as diamonds, are carried up to the surface. It is also possible that no crystallization of minerals occurs at all if the lava cools down rapidly. In this case volcanic glass such as obsidian is formed instead. Loose volcanic rock that originates from bursting magma, during eruptions that involve large amounts of gas, is classified as sediment.

Metamorphic rocks

Metamorphose means transformation: metamorphic rocks form by the transition of other types of rock within the Earth’s crust. This can only happen when the temperature and pressure are sufficiently high. Under these conditions the original rock material begins to melt and change structure. For example, marble forms during the transformation of sedimentary rocks or pararock.

The transformation of magmatites results in so-called orthorocks such as mica schists. Metamorphic rocks show the foliated structure (schistosity) characteristic of this type of rock. These rocks can be split into thin plates along the schistosity plane.


Meteorites are fragments of rocky material that hit the Earth’s surface. If they disintegrate in the atmosphere they are meteors (shooting stars). These intruders from the early days of our solar system can be extremely large and result in giant impact craters when they hit planets.

How- ever, most are no bigger than pebbles and arrive on Earth without notice They mainly consist of silicate minerals or an alloy of iron and nickel. There are different categories of meteorites. such as stony, iron or stony-iron


PETROLOGY is the study of the history of rocks, particularly their origin and formation processes.

Rocks are classified by occurrence, mineral and chemical composition, texture, and physical characteristics.