Photo of theOrdovician–Silurian boundary. Due to the folding of the Caledonianmountain range, the layers have been inverted (turned right over). This left the earlier Ordovicianlimestone sitting on top of the later Silurian brownishmudstone.
Geology is the study of the nonliving things that theEarth is made of.[1][2] Geology is the study ofrocks in theEarth's crust. People who study geology are calledgeologists.[3] Some geologists studyminerals (mineralogist) and the useful substances the rocks contain such asores andfossil fuels. Geologists also study thehistory of the Earth.
Rocks can be very different from each other. Some are very hard and some are soft. Some rocks are very common, while others are rare. However, all the different rocks belong to three categories or types, igneous, sedimentary and metamorphic.
Igneous rock is rock that has been made by volcanic action. Igneous rock is made when thelava (melted rock on the surface of the Earth) or magma (melted rock below the surface of the Earth) cools and becomes hard.
Sedimentary rock is rock that has been made fromsediment. Sediment is solid pieces of stuff that are moved by wind, water, orglaciers, and dropped somewhere. Sediment can be made from clay,sand,gravel and the bodies andshells of animals. The sediment gets dropped in a layer, usually in water at the bottom of a river or sea. As the sediment piles up, the lowers layers get squashed together. Slowly they set hard into rock.
Metamorphic rock is rock that has been changed. Sometimes an igneous or a sedimentary rock is heated or squashed under the ground, so that it changes. Metamorphic rock is often harder than the rock that it was before it changed.Marble andslate are among the metamorphic rocks that people use to make things.
All three kinds of rocks can be changed by beingheated andsqueezed by forces in the Earth. When this happens,faults (cracks) may appear in the rock. Geologists can learn a lot about the history of the rock by studying the patterns of the fault lines. Earthquakes are caused when a fault breaks suddenly.
Soil is the stuff on the ground made of lots of particles (or tiny pieces). The particles of soil come from rocks that have broken down, and from rotting leaves and animals bodies. Soil covers a lot of the surface of the Earth. Plants of all sorts grow in soil.
To find out more about types of rocks, see therock (geology) article.To find out more about soil, see thesoil article.
Geologists use some simple ideas which help them to understand the rocks they are studying. The following ideas were worked out in the early days ofstratigraphy by people likeNicolaus Steno,James Hutton andWilliam Smith:
Understanding the past: GeologistJames Hutton said "The present is the key to the past". He meant that the sort of changes that are happening to the Earth's surface now are the same sorts of things that happened in the past. Geologists can understand things that happened millions of years ago, by looking at the changes which are happening today.
Horizontalstrata: The layers in a sedimentary rock must have been horizontal (flat) when they were deposited (laid down).
The age of the strata: Layers at the bottom must be older than layers at the top, unless all the rocks have been turned over.
In sedimentary rocks that are made ofsand orgravel, the sand or gravel must have come from an older rock.
The age offaults: If there is a crack or fault in a rock, then the fault is younger than the rock. Rocks are in strata (lots of layers). A geologist can see if the faults go through all the layer, or only some. This helps to tell the age of the rocks.
The age of a rock which cuts through other rocks: If anigneous rock cuts across sedimentary layers, it must be younger than the sedimentary rock.
The relative age offossils: A fossil in one rock type must be about the same age as the same type of fossil in the same type of rockin a different place. Likewise, a fossil in a rock layer below must be earlier than one in a higher layer.
↑Neuendorf, K. K. (2005). Glossary of geology. Springer Science & Business Media.
↑Oldroyd, D. R. (1996). Thinking about the Earth: A history of ideas in geology. Harvard University Press.
↑Raab, T., & Frodeman, R. (2002). What is it like to be a geologist? A phenomenology of geology and its epistemological implications. Philosophy & Geography, 5(1), 69-81.
↑Chapman, R. E. (2000). Petroleum geology.Elsevier.
↑Selley, R. C. (1998). Elements of petroleum geology. Gulf Professional Publishing.
↑Levorsen, A. I., & Berry, F. A. (1967). Geology of petroleum (Vol. 724). San Francisco: WH Freeman.
