Understanding basic rock types gives you more ideas on how each type of rock is formed and this is a good basic for understanding geology.
Rocks can be classified into three main types which are igneous rocks, sedimentary rocks and metamorphic rocks. Figure 1 shows the relationship between the three main rock types.
Figure 1 – Rock Cycle
(Ref Image: http://www.geolsoc.org.uk/ks3/webdav/site/GSL/shared/images/education_and_careers/RockCycle/Rock%20Cycle%20all%20labels.jpg)
Igneous rocks are the most abundant type of rock on the Earth because it makes up about 70% of all rocks. Minerals such as quartz, plagioclase feldspar, pyroxene and olivine are important types of igneous rocks. Figure 3 illustrates minerals found in common igneous rocks.
Figure 2 – Minerals in Common Igneous Rocks.
(Ref Image: https://upload.wikimedia.org/wikipedia/commons/thumb/f/f4/Mineralogy_igneous_rocks_EN.svg/640px-Mineralogy_igneous_rocks_EN.svg.png)
Igneous rocks are formed by the cooling and solidification of hot molten magma from the mantle and it can be categorized as intrusive (plutonic) or extrusive (volcanic). Intrusive igneous rocks are formed when magma from the mantle moves up through deep, narrow cracks in the Earth. Intrusive igneous rocks cool very slowly, so crystals have time to develop. Their crystal growth is large.
On the other hand, extrusive igneous rocks are formed when hot magma reaches the surface of the Earth and it is cooled down very fast. Extrusive igneous rocks have fine crystals because they don’t have time to develop the large structures like the intrusive igneous rocks. For instant, when lava from a volcano erupts and reaches the surface, it will be rapidly cooled down. With an extremely fast cool down process, the atoms in hot magma don’t have time to create crystalline structures and structures like glass or minaraloids are formed. The Figure 3 illustrates about intrusive and extrusive igneous rock formations.
Figure 3 – Intrusive and Extrusive Igneous Rock Sources
(Ref Image: http://static1.squarespace.com/static/51bbeba5e4b0510af19f26f7/t/51dc5285e4b083e539ef0117/1373393552981/extrusive-and-intrusive-rocks.jpg?format=500w)
Sedimentary rocks are formed when igneous, metamorphic or pre-existing sedimentary rocks are eroded by external forces such as wind, rain, snow and glaciations. Rock particles are always transferred from their original regions and deposited in low-laying areas. Crystals of sedimentary rocks tend to be a rounded shaped because of abrasion while being transported. Figure 4 demonstrates a rock sedimentation process.
It is possible to have rocks which are half-way between igneous and sedimentary rocks. This case happens when hot lava is thrown into the air and spread around the area instead of flowing from a volcano.
Biogenic chemical sediments are formed from the accumulation of plants and animals in sedimentary areas and organic material within biogenic sediment has a possibility to be transformed into petroleum if conditions and chemical conditions are right. This is the reason why a majority of petroleum fields are discovered in areas of sedimentary rocks.
Figure 4 – Rock sedimentation
Table 1 shows size of particle, sedimentary and rock type.
Table 1 – Clastic Particle Definitions
(Ref Image: http://cramster-image.s3.amazonaws.com/definitions/9780071471091-t0137-01.jpg)
Metamorphic rocks are formed when other rocks (igneous, sedimentary or pre-existing metamorphic rocks) are changed by pressure and heat due to hot molten rock intrusion or deep burial in the Earth.
This can be seen in subduction zones where rocks on the lithosphere plate sink into mantle underneath other plates. Pressure and temperate transform existing rocks into metamorphic rocks. Metamorphic rocks can be classified by type and grade of metamorphism. There are several factors contributing to the final result of metamorphism, for example temperature, pressure, chemical reactivity, stress across zone of metamorphism and time.
Metamorphism types are classified by the physical conditions during metamorphism. Regional metamorphism commonly occurs in the continental crust and it may happen over a large area. This type of metamorphism involves a high differential stress level, as well as a high magnitude of mechanical deformation and chemical recrystallisation.
Contact metamorphism happens in areas close to the intrusion of hot magma, which creates chemical recrystallization.Cataclastic, or dynamic, metamorphism can be found along faults and their movement of tectonic creates rock deformation and high differential stresses. The rock has a pulverized texture due to fractures as the faults move. Most of the time the Cataclastic metamorphism rocks act as a fluid barrier between rocks. This is the reason why faults can trap oil or gas.
Burial metamorphism normally happens in very deep section of sedimentary basin rocks where the temperature is very high (300 C). Water in sedimentary rock accelerates the chemical recrystallization process.Hydrothermal metamorphism typically occurs because of the reactions of chemicals between heated rocks and fluids. This is usually associated with mid-ocean ridges. Figure 5 is a diagram of metamorphic facies with a common tectonic setting.
Figure 5 – Metamorphic Facies
(Ref Image: https://upload.wikimedia.org/wikipedia/commons/f/fe/Metamorphic_Facies.jpg)
Note – for petroleum exploration and production, we will focus on sedimentary rocks.