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Geology facts for kids

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Photo of the OrdovicianSilurian boundary. Due to the folding of the Caledonian mountain range, the layers have been inverted (turned right over). This left the earlier Ordovician limestone sitting on top of the later Silurian brownish mudstone.

Geology is the study of the nonliving things that the Earth is made of. Geology is the study of rocks in the Earth's crust. People who study geology are called geologists. Some geologists study minerals and the useful substances the rocks contain (such as natural gas and oil). Geologists also study the history of the Earth.

Some of the important events in the Earth's history are floods, volcanic eruptions, earthquakes, orogeny (mountain building), and plate tectonics (movement of continents).

Geology is divided into special subjects that study one part of geology. Some of these subjects are:

Types of rock

Rocks can be found in all sorts of shapes and colours. 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

Kimberlite (a type of crystallized igneous rock)

Igneous rock is rock that has been made by volcanic action. Igneous rock is made when the lava (melted rock on the surface of the Earth) or magma (melted rock below the surface of the Earth) cools down and becomes hard.

Sedimentary rock

Red sandstone, Hilbre Island
Red sandstone: Wirral, England

Sedimentary rock is rock that has been made from sediment. Sediment is all solid pieces of stuff that are moved by the wind, water, or glaciers. Sediment can be made from clay, sand, gravel and the bodies and shells of animals. The sediment gets dropped in a layer 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. Some sedimentary rocks are made of just one type of sediment, all about the same size, such as sand. Other sedimentary rocks will have large and small lumps, and pieces made of different types of rock. Well-known sedimentary rocks are sandstone and limestone.

Metamorphic rock

Migma ss 2006
Folded strata in a metamorphic rock from near [Geirangerfjord], Norway

Metamorphic rock is rock that has been changed. The word "metamorphosis" means "change". Sometimes an igneous, or a sedimentary rock can be heated, or squashed under the ground, so that it changes. Metamorphic rock is often harder than the rock that it was before it changed. One well-known metamorphic rock is marble which is valued for its different colours, and because it can be carved and polished. Slate is a metamorphic rock that is a useful building material.


Normal faults - Arganda del Rey, Madrid, Spain
Normal faults in Spain, between which rock layers have slipped downwards (at photo's centre)

All three kinds of rock can be changed by being heated and squeezed 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.


Loess field in Germany (loess is wind-blown dust that is mostly silt)

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, go to Rock (geology). To find out more about soil, go to Soil.

Principles of Stratigraphy

PDA Mapping
Today, handheld computers with GPS and geographic information systems software are often used in geological field work (digital geologic mapping).
Permian Silicified Sclerobionts
Silicified (replaced with silica) fossils from the Road Canyon Formation (Middle Permian of Texas).

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 of stratigraphy by people like Nicolaus Steno, James Hutton and William Smith:

  1. Understanding the past: Geologist James 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.
  2. Horizontal strata: The layers in a sedimentary rock must have been horizontal (flat) when they were deposited (laid down).
  3. 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.
  4. In sedimentary rocks that are made of sand or gravel, the sand or gravel must have come from an older rock.
  5. The age of faults: 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.
  6. The age of a rock which cuts through other rocks: If an igneous rock cuts across sedimentary layers, it must be younger than the sedimentary rock.
  7. The relative age of fossils: A fossil in one rock type must be about the same age as the same type of fossil in the same type of rock in a different place. Likewise, a fossil in a rock layer below must be earlier than one in a higher layer.


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Kids robot.svg In Spanish: Geología para niños

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