The basic idea behind the principle of uniformitarianism is that any of the earth processes that we have discovered and can observe today are very likely to have been working and active in the past, too. That means that it is possible to gain insights on, for example, past climate by examining the deeper layers of the crust on a given point, and compare the structure and composition of the rocks found to newly formed rock.Close.

Minerals are defined as material that is naturally formed by inorganic processes and is solid with a specific chemical composition and a characteristic crystal structure. That is man-made crystals are not considered minerals, and any material that contains traces of organic matter can not be a mineral. As crystaline structures are rigid the resulting material has to be solid. The definitions for every mineral in existence are very specific and allow for no variation thus. Close.

Luster refers to the "shineness" of a material. That is, basically, the way a mineral reflects light - some of the characteristics that help identify minerals are whether they have a metallic luster or not, whether they are vitreous (similar to how broken glass looks), admantine (similar to diamonds), resinous, silky or pearly. Some minerals can be described by terms like dull or earthly as well. See Berry (256) and Kirkaldy (102). Close.

Mineral hardness is a relative scale as established by Frederick Mohs in 1822. The scale ranks Talc (that can be easily scratched) as the least hard mineral and Diamonds as the hardest. In the list the higher ranked minerals are able to leave scratches in the respective lower ranked ones. In order from weakest to strongest: Talc, Gypsum, Fluorite, Apatite, Feldspar, Quartz, Topaz, Corundum and Diamond. See Berry (255-256), Kiraldy (101-102) and Skinner (81). Close.

Cleavage refers to how a mineral splits apart. Due to the underlying structure of the minerals they split in characteristic manners. That is, these splits follow the crystal structure, splitting weak bonds in the atomic structure; the break occurs in the same direction and manner every time. Mica, for example, can easily be broken apart into thin layers. See Skinner (79), Berry (200-202) and Kirklady (102). Close.

The mineral colour is only of minor usefulness to characterise minerals as it is influenced by impurities. Furthermore, some minerals refract and reflect light internally; they only appear to have a certain colour.

Streak, in contrast, is "the color of finely powdered mineral" (Berry, 198). With minerals of low hardness this can be easily discovered by drawing them over a piece of paper or similar; with the harder minerals some parts have to be crushed before the streak can become visible. The streak of a specific mineral always has the same colour. See Berry (196-198), Kiraldy (102) and Skinner (80-81). Close.

Each crystal has, as mentioned, a specific structure. This structure (resulting from how the elements are bonded) is reflected by the external structure. That is, the angles between each of the crystals faces is the same, even if the outer appearance might differ from specimen to specimen. Minerals only can form crystals when there is enough space for these to grow; analysing a minerals structure using x-rays can, however, reveal the internal crystalline structure of a mineral. See Kiraldy (96-101) and Skinner (77-78). Close.

Igneous rock is the direct descendent of magma. It is rock that forms when magma cools and is the hardest and most abundant of the three types, in general. The oceanic crust is almost completely made of Igneous rock and the continental crust to 95%. Depending on where in the crust igneous rock is created composition varies. See Skinner (100).

Magma that breaks through the earth surface (extrusive) cools under different circumstances then the magma creating rocks inside the crust (intrusive) - a result of dykes (vertical canals of magma that don't reach the earth surface) or sills (magma filling spaces between existing rocks), for example. In most cases intrusive magma has more time to cool down, and through that features minerals that can grow larger then the quicker cooling extrusive lava. Likewise rock that is formed under very high pressure will limit mineral size.

As Igneous rock is formed through crystallization the various igneous rock types display a wide variety of colours and differing textures (depending on the size/coarseness of the minerals contained). Due to difference in the speed of cooling down intrusive rock (phanerites [from Greek "phaner(ós)" meaning "manifest, visible"]) is likely to be more coarse grained. It's grains are easily visible and least 2 mm in size. The extrusive (aphanite ["aphanes" "unseen, hidden, invisible"]) variation features minerals of less then 2mm size. Some lava cools so quickly that no crystallization can occur (obsidian); the surface looks glassy.

Other denominators include Pegmatites (mineral grains larger then 2 cm) and porphyry (a mixture of small and large crystals with at least 50% being coarse). See Skinner (101). Close.

Sedimentary rock results from erosion of existing rocks on the surface of the planet. Erosion refers to any process that moves material from one place to another on the earth surface - be it through gravity, water, or wind. Before erosion can happen weathering has to occur: material is loosened through physical or chemical processes. A rock that is being washed out by, for example, wave action would still undergo both processes (the wave action loosens material [weathering] but also carries it away afterward [erosion]). Weathering can also happen due to animal or human activity, exposing protected rock to the chemical/physical elements.

As the materials settle down again they are sorted - heavier and denser material is not transported as far or as quickly as the smaller particles. The result are layers of fairly homogenous material. These layers in turn are likely to be covered by more material later.

Sediments can be separated into three different types - clastic, chemical or biogenic. Clastic sediments are mostly fine particles - like sand grains - that have been loosened by physical means. Chemical sediments are those that have been effected by dissolution (that is dissolving), hydrolysis (decompositioned by water, usually when hydrogen ions bind part of the rock structure, and create a new different element), leaching (which is similar to dissolution, only that only part of the rock is affected) or oxidation (basically the principle of rusting.). See Skinner (153-155). Most of the time a mixture of these effects cause chemical weathering. Biogenic sediment is the result of organic matter dieing in the ocean and sinking to the ground where it slowly decomposes. This affects marine life more given that, unlike with surface animals there are few lifeforms that could feed on, or "recycle" material at that depth.

Once sediments have been deposited the weight of further material in the higher layers presses the material together and compaction occurs. This means that water is forced from the sediments as they are pressed together. As pressure increases further the elements of various sediment particles start to form bonds with each other and cementation occurs. Some unstable minerals might re-crystalize, on a similar principle: As they lack electrons they bond and form a more stable variant. Depending on whether oxygen is present or not the organic parts of the sediments are either broken apart (CO2 and H2O) or changed into carbon - a process referred to as chemical alteration. The result of these lithification processes - sedimentary rock - maintains a visibly layered structure. Sedimentary rock is the most common rock type on the surface of the earth. Close.

Metamorphic rock has its origin as either sedimentary or igneous rock. Similar to the process of lithification these rocks are transformed by pressure or heat and change their chemical composition from the source material. Close.

Magma is the molten material in the Astenosphere. Lava is magma that has reached the earth surface and is reacting with the environment. Close.