(Author: Fiona Kelly)

There are three basic branches of Geology: Minerals, Fossils, and Rocks. 

• Minerals:  A solid, natural substance, where the atoms are arranged in an orderly, repeating pattern. They are commonly referred to as crystals. Some examples are quartz, emerald, and onyx.  
• Fossil:  Any recognizable evidence of pre-existing life. These can take the form of a body fossil, or a trace. A body fossil is the classic dinosaur found in any natural history museum. It is a bone of a vertebrate. A trace fossil is an imprint left by a plant or animal that has since dying decomposed. Plants and invertebrates leave trace fossils. Fossils can also give the approximate geological time that a specimen lived in, giving key information about the Earth at that time, along with personal information about that specimen.  
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I'm going to describe rocks separately, because within this category are three more sub-categories. 

 

Rocks are a coherent, naturally occurring solid combination of minerals.  

• Igneous Rocks: Formed by freezing or cooling. Plutonic igneous rocks are formed by cooling slowly. Volcanic igneous rocks are formed by cooling quickly.  
• Metamorphic Rocks: Formed deep below the Earth's surface. Pre-existing rocks go through a solid state of change due to extreme temperature and pressure.  
• Sedimentary Rocks: Formed at or near the surface of Earth. Clastic sedimentary rocks are formed by the cementation of loose grains. Chemical sedimentary rocks are formed by the precipitation of mineral from water. Biogenic sedimentary rocks are shells, such as those found on oysters, clams, and mollusks.  

There is a bit of a debate surrounding the scientific definition of a word used to classify rocks. The term facies is a Latin word meaning aspect or appearance. However, this is where the agreement ends. Some scientists use the term to mean a body of rocks with specified characteristic. Others use it to mean sedimentary rocks that share appearances. In the environmental sense, the word is used to mean a type of depositional environment rocks are found in.  

These include but are not limited to: 

• Fluvial: Rivers or streams 
• Terrestrial: On land 
• Erg: Desert 
• Glacier: Ice