Geology facts for kids

Photo of the Ordovician–Silurian 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:

Geomorphology – the study of the shape (morphology) of the surface of the Earth.
Historical geology – the history of the events that shaped the Earth over the last 4.5 million years.
Hydrogeology – the study of water under the surface of the Earth
Palaeontology – the study of fossils
Petrology – the study of rocks how they form and where they are from.
Mineralogy – the study of minerals
Sedimentology – the study of sediments
Stratigraphy – the study of layered sedimentary rocks and how they were deposited.
Structural geology– the study of folds and faults and how mountains are formed by uplift. See Earth science
Volcanology – the study of volcanoes on land or under the ocean
Seismology – the study of earthquakes and strong ground-motion.
Engineering geology -the study of geologic hazards (such as landslides and earthquakes) applied to civil engineering.
Petroleum geology- the study of petroleum deposits in sedimentary rocks.

Types of rock
Principles of Stratigraphy
Gallery
Related pages
Images for kids
See also

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: 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

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.

Faults

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.

Soil

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

Today, handheld computers with GPS and geographic information systems software are often used in geological field work (digital geologic mapping).

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:

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.
Horizontal strata: 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 of sand or gravel, the sand or gravel must have come from an older rock.
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.
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.
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.

Gallery

Layers of sedimentary rock
A fault cutting through older sedimentary rocks
A conglomerate: sedimentary rock made from white pieces of older rock, broken up and mixed with red sand at the bottom of a river.
The Zimbabwe Great Dyke: A big band of igneous rock cuts between sedimentary rocks. This picture was taken from a satellite.
Beaches are interesting places to study Geology,
....so are caves.
This rock-face in France contains the mineral Mica.
A simple, important tool for the Geologist
The geologist, David Johnston, on the side of Mount St. Helens.
A young Butcher Bird is standing on a box of rock cores that have been drilled from underground.
Geologists look at some samples of rock, to find minerals for mining.
A map showing the geology of South East England. This area has been studied for hundreds of years.
A map showing the different soils and rocks under the Southern Ocean. Geologists are finding new information about this area.
This map shows the main "plates" of the Earth's surface and which way they are moving.
This diagram shows the chemical movement at a deep sea vent on the ocean floor.

Geography is the study of the Earth and its features, its inhabitants, and its phenomena.
Zoology is the study of animals
Botany is the study of plants and fungi

Images for kids

The rock cycle shows the relationship between igneous, sedimentary, and metamorphic rocks.
Native gold from Venezuela
Quartz from Tibet. Quartz makes up more than 10% of the Earth's crust by mass.
In this diagram based on seismic tomography, subducting slabs are in blue and continental margins and a few plate boundaries are in red. The blue blob in the cutaway section is the Farallon Plate, which is subducting beneath North America. The remnants of this plate on the surface of the Earth are the Juan de Fuca Plate and Explorer Plate, both in the northwestern United States and southwestern Canada and the Cocos Plate on the west coast of Mexico.
The Permian through Jurassic stratigraphy of the Colorado Plateau area of southeastern Utah is an example of both original horizontality and the law of superposition. These strata make up much of the famous prominent rock formations in widely spaced protected areas such as Capitol Reef National Park and Canyonlands National Park. From top to bottom: Rounded tan domes of the Navajo Sandstone, layered red Kayenta Formation, cliff-forming, vertically jointed, red Wingate Sandstone, slope-forming, purplish Chinle Formation, layered, lighter-red Moenkopi Formation, and white, layered Cutler Formation sandstone. Picture from Glen Canyon National Recreation Area, Utah.
The mineral zircon is often used in radiometric dating.
The San Andreas Fault in California
A diagram of folds, indicating an anticline and a syncline
Geological cross section of Kittatinny Mountain. This cross-section shows metamorphic rocks, overlain by younger sediments deposited after the metamorphic event. These rock units were later folded and faulted during the uplift of the mountain.
A standard Brunton Pocket Transit, commonly used by geologists for mapping and surveying
A typical USGS field mapping camp in the 1950s
A petrified log in Petrified Forest National Park, Arizona, U.S.A.
Folded rock strata
A diagram of an orogenic wedge. The wedge grows through faulting in the interior and along the main basal fault, called the décollement. It builds its shape into a critical taper, in which the angles within the wedge remain the same as failures inside the material balance failures along the décollement. It is analogous to a bulldozer pushing a pile of dirt, where the bulldozer is the overriding plate.
Different colors show the different minerals composing the mount Ritagli di Lecca seen from Fondachelli-Fantina, Sicily
Surface of Mars as photographed by the Viking 2 lander December 9, 1977
Man panning for gold on the Mokelumne. Harper's Weekly: How We Got Gold in California. 1860
Mud log in process, a common way to study the lithology when drilling oil wells
A child drinks water from a well built as part of a hydrogeological humanitarian project in Kenya
William Smith's geological map of England, Wales, and southern Scotland. Completed in 1815, it was the second national-scale geologic map, and by far the most accurate of its time.
Mikhail Lomonosov, Russian polymath, author of the first systematic treatise in scientific geology (1763)
James Hutton, Scottish geologist and father of modern geology
John Tuzo Wilson, Canadian geophysicist and father of plate tectonics
The volcanologist David A. Johnston 13 hours before his death at the 1980 eruption of Mount St. Helens