Ok so the whole of this blog is based on geology and the study of rocks but I am yet to explain what one actually is. Hopefully this will clear somethings up...
1. What is a rock?
According to both national geographic and my first year lectures notes, a rock is a naturally occurring aggregate of minerals fused together. For some context a mineral is defined as a naturally occurring homogenous (so all the same) solid with a definitive chemical composition and an ordered, repeating structure. An example of a basic mineral is quartz (SiO2) which can be found in many different types of rocks such a granite (contains quartz, potassium feldspar, plagioclase feldspar, micas and hornblende) or sandstones (mostly quartz but sometimes contains feldspars, silts or clays).
There are three main types of rocks: sedimentary, igneous and metamorphic. Minerals cycle between the different types of rocks in a process known as the rock cycle.
2. Sedimentary rocks
Sedimentary rocks are rocks that are formed through the compaction of sediments deposited by wind or water. They form at or near the surface of the Earth and are rocks where you can find fossils that have formed when dead organisms get squished in with the compacting sediments. This compression takes millions of years.
Examples: Sandstone, Limestone, Mudstone, Shale
A lovely example of a sandstone (the Otter Sandstone) seen in the field in Dorset. If you look closely you might be able to spot a fault in the outcrop.
3. Igneous rocks
Igneous rocks form when molten rock (magma) cools and crystallises into a solid. This can happen at the Earth's surface when magma has erupted from a volcano as lava and then cools (extrusive) or still within the Earth's interior (intrusive). The mineral make up/composition of the rock will depend on the composition of the magma it cooled from.
Examples: Granite, Gabbro, Basalt
Lovely example of a pink granite from https://www.geologypage.com/2019/05/granite-rocks.html because despite the many, many photos I take in both practicals and in the field I couldn't find a good one of an igneous rock :(
4. Metamorphic rocks
Metamorphic rocks form when a rock (igneous or sedimentary) undergoes pressure and/or temperature changes that cause changes in the structure or composition of a rock resulting in it becoming a new type of rock. This could mean that the minerals that the rock is made of become different minerals or that the arrangement of the minerals change. The word metamorphosis (where the term metamorphic comes from) means to change.
Examples: Marble, Gneiss, Schist, Quartzite
A gneiss from one of our first year petrology practicals.
5. The rock cycle
The rock cycle is one of the fundamental concepts to understand when it comes to geology. Much as matter is conserved in chemical reactions, minerals are conserved and cycled round between different types of rocks due to a variety of processes, shown in the image below.
The rock cycle https://en.wikipedia.org/wiki/File:Rock_cycle.gif
Definitions:
Diagenesis - any processes (physical and chemical) that happen after burial but before metamorphism
Deposition - The laying down/dropping of sediment by something such as air/water that has been moving it.
Transport - The movement of sediment by wind, water or mass transport.
Erosion - The breakdown and removal of sediment by natural forces such as wind or water.
Pressure and Temperature - Conditions that change eg due to burial depth and cause minerals and rocks to undergo changes. The deeper something is buried, the higher the pressure and temperature.
Melting - When temperature and/or pressure of a solid changes so it reaches the liquidus and starts to change state to a liquid.
Crystallization - When a mineral in a liquid state becomes a mineral in a solid state.
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