The deep rock of the earth and diamonds

By far the largest part of the earth’s crust consists of deep rock, which was formed by the solidification of magma. Magma is caused by the partial melting of rocks in the lower parts of the earth’s crust, or directly below them in the mantle. Magma is often lighter than the rocks at that depth, causing it to move upwards. In doing so, it intrudes into the rock above. As soon as the difference in density is no longer large enough, the rising magma comes to a standstill and collects in magma chambers.

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In a magma chamber, magma differentiation occurs: the magma changes its chemical composition. There are a number of ways in which this can happen. For example, a magma body may, over time, mix with other magmas of a different composition, or parts of the surrounding rock may melt due to the heat of the intrusion and “pollute” the magma.

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By slowly cooling down the magma, a process called crystal fractionation also takes place. Because magma is a mixture of different chemical components, it does not have a single crystallization temperature, but a crystallization trajectory. Minerals with a high melting point will crystallise first, minerals with a lower melting point will only crystallise later, when the magma has cooled down further. The high melting point minerals are minerals that are low in silica (called mafia) and rich in the elements iron and magnesium. In a magma chamber, these relatively heavy minerals will first crystallize and sink to the bottom of the chamber. This changes the composition of the magma: it becomes lighter and more silicacious (phelsic).

Magma can penetrate further upwards from a magma chamber, this happens in vertical magmas called dykes, or horizontal magmas called sills or laccoliths. Because magma becomes lighter and lighter during crystallisation, it will become easier to ascend further when the magma chamber has already cooled down partially. As a result of further crystallisation, more and more silicacious magma’s are formed.

Volcanic rock and volcanism
1rightarrow blue.svg See volcanic rock and volcanism for the main articles on the subject.
If magma penetrates through the crust so far that it reaches the surface, volcanism takes place. Sometimes this means that magma flows from a volcano over the surface, it is called lava. When lava solidifies this happens quickly and it forms a coagulation rock with very small crystals, an effluent rock. Examples are Basalt, ryolite or dacite. If lava solidifies very quickly, the atoms do not have enough time to accept a crystal lattice, which results in the formation of volcanic glass.

For many volcanoes, however, most of the rock formed is not formed by the solidification of lava. This is because volcanic eruptions are often explosive in nature. Explosive volcanism is caused in two ways. When the magma rises, gases and liquids will disperse as a result of the loss of pressure. The release of these gases causes the magma to expand and if this happens quickly enough, this can lead to explosions. Another way is for the magma to come into contact with groundwater or surface water as it rises. The high temperature of the magma causes the water to evaporate immediately and if there is enough water, this will result in large explosions.

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With explosive volcanism, large amounts of material can be blown into the air. These particles are called tefra and form a large eruption column above the volcano. Fast-moving glowing clouds or pyroclastic currents of tefra can also flow sideways from the volcano. When the volcano comes to rest, the tefra rains on the surface of the earth and forms a layer of pyroclastic rock. The large amount of unconsolidated material that is deposited can easily be eroded, causing mud currents (lah resin) and other mass movements.

Most volcanoes alternate between pyroclastic rocks, runoff rocks and deposits caused by mudflows.

Sedimentation and sedimentary rocks

A river deposits horizontal layers of gravel and erodes them in some places later. The layers cut demonstrate the principle of original horizontality.
The material removed by erosion is called sediment or detritus. It will be transported by water, wind, ice, gravity or a combination of all these possibilities, to be deposited elsewhere. When sediment settles and accumulates, it is called a sedimentary rock or sedimentary rock.