Geology facts for kids

Photo of the Ordovician–Silurian boundary. These rock layers have been turned upside down! This means older limestone is now on top of younger mudstone.

Geology is the study of our amazing Earth. It's all about the nonliving things the Earth is made of, like rocks and minerals. People who study geology are called geologists.

Geologists explore many things. They look at the useful stuff hidden inside rocks, such as natural gas and oil. They also study the long history of the Earth. This includes big events like floods, volcanic eruptions, earthquakes, and how mountains are built. They also study plate tectonics, which is how continents move around.

Geology has many special areas of study:

Geomorphology – This is about the shapes and features of the Earth's surface, like valleys and mountains.
Historical geology – This looks at all the events that shaped Earth over billions of years.
Hydrogeology – This is the study of water found deep under the Earth's surface.
Palaeontology – This is the study of fossils, which are remains of ancient life.
Petrology – This focuses on rocks, how they form, and where they come from.
Mineralogy – This is the study of minerals, which are the building blocks of rocks.
Sedimentology – This studies sediments, which are tiny bits of rock, sand, and dirt.
Stratigraphy – This looks at layered sedimentary rocks and how they were laid down.
Structural geology – This studies how rocks bend and break, and how mountains are formed.
Volcanology – This is the study of volcanoes, both on land and under the ocean.
Seismology – This focuses on earthquakes and how they shake the ground.
Engineering geology – This uses geology to help with building projects, like understanding landslides.
Petroleum geology – This helps find petroleum (oil and gas) deposits in rocks.

Types of rock
Understanding Earth's History
Gallery
Related pages
Images for kids
See also

Types of rock

Rocks come in all sorts of shapes, sizes, and colors. Some are super hard, while others are soft. Some rocks are very common, but others are rare. However, all rocks fit into three main types: igneous, sedimentary, and metamorphic.

Igneous rock

Kimberlite is a type of igneous rock that has cooled and hardened.

Igneous rock forms from melted rock. When lava (melted rock on the Earth's surface) or magma (melted rock deep underground) cools down and hardens, it becomes igneous rock. Think of it like melted chocolate cooling into a solid bar!

Sedimentary rock

Red sandstone found in Wirral, England.

Sedimentary rock is made from tiny pieces of other materials called sediment. Sediment can be bits of clay, sand, gravel, or even parts of animal shells. Wind, water, or glaciers carry these sediments and drop them in layers, often at the bottom of rivers or seas.

As more and more sediment piles up, the lower layers get squashed together. Over a long time, they harden into rock. Some sedimentary rocks, like sandstone, are made of just one type of sediment. Others, like limestone, can have different sized pieces mixed together.

Metamorphic rock

Folded rock layers in a metamorphic rock from Norway.

Metamorphic rock is rock that has been changed. The word "metamorphosis" means "change." This happens when an igneous or sedimentary rock is heated or squeezed deep underground. The intense heat and pressure change the rock's structure.

Metamorphic rock is often harder than the rock it was before it changed. A famous metamorphic rock is marble, which is prized for its beautiful colors and how easily it can be carved and polished. Slate is another metamorphic rock, often used as a strong building material.

Faults

Normal faults in Spain. Rock layers have slipped downwards between these cracks.

All three types of rock can be changed by the powerful forces inside the Earth. When rocks are heated and squeezed, cracks called faults can appear. Geologists study these fault lines to learn about the rock's history. Earthquakes happen when a fault suddenly breaks and moves.

Soil

This is a Loess field in Germany. Loess is wind-blown dust, mostly silt.

Soil is the loose material that covers much of the Earth's surface. It's made of tiny pieces of broken-down rocks, mixed with rotting leaves and animal remains. Soil is super important because most plants grow in it!

To learn more about rocks, check out Rock (geology). To learn more about soil, check out Soil.

Understanding Earth's History

Today, geologists often use handheld computers with GPS and GIS software for mapping rocks in the field.

Silicified (meaning replaced with silica) fossils from Texas.

Geologists use some simple but powerful ideas to understand the rocks they study. These ideas were developed by early geologists like Nicolaus Steno, James Hutton, and William Smith.

The Present is the Key to the Past: Geologist James Hutton said this. He meant that the way Earth changes today (like rivers eroding land or volcanoes erupting) is similar to how it changed millions of years ago. By studying today's changes, geologists can understand what happened long ago.
Flat Layers: When sedimentary rocks are first laid down, their layers are usually flat. If you see them tilted or folded, you know something happened to them later.
Age of Layers: In undisturbed sedimentary rocks, the layers at the bottom are always older than the layers at the top. It's like a stack of books – the first one you put down is at the bottom.
Older Rock Pieces: If a sedimentary rock is made of sand or gravel, those pieces must have come from an even older rock that broke apart.
Age of Faults: If a crack or fault cuts through a rock, the fault must be younger than the rock it cuts. Geologists can see if a fault goes through all layers or just some, which helps tell the age.
Cutting Rocks are Younger: If an igneous rock cuts across layers of sedimentary rock, the igneous rock is younger than the sedimentary layers it cuts through.
Age of fossils: If you find the same type of fossil in rocks from different places, those rocks are likely about the same age. Also, a fossil in a lower rock layer is older than a fossil 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, mixed with red sand.
The Zimbabwe Great Dyke: A large band of igneous rock cutting through 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 rock samples 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.
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 people, and its natural events.
Zoology is the study of animals.
Botany is the study of plants and fungi.

Images for kids

The rock cycle shows how igneous, sedimentary, and metamorphic rocks are related.
Native gold from Venezuela.
Quartz from Tibet. Quartz makes up a big part of the Earth's crust.
This diagram shows moving slabs of rock (blue) and plate boundaries (red). The blue blob is the Farallon Plate, moving under North America.
These rock layers in Utah show how older layers are at the bottom and younger ones are at the top.
The mineral zircon is often used to figure out the age of rocks.
The famous San Andreas Fault in California.
A diagram showing how rock layers can fold into an anticline (upward fold) and a syncline (downward fold).
A geological map of Kittatinny Mountain. It shows how rocks were folded and faulted when the mountain formed.
A standard Brunton Pocket Transit, a common tool for geologists to map and survey.
A typical USGS field mapping camp in the 1950s.
A petrified log (wood turned to stone) in Petrified Forest National Park, Arizona, U.S.A.
Folded rock layers.
A diagram of an orogenic wedge, which shows how mountains grow through faulting.
Different colors show the different minerals that make up Mount Ritagli di Lecca in Sicily.
The surface of Mars, photographed by the Viking 2 lander.
A man panning for gold on the Mokelumne River.
Mud logging in process, a common way to study the rock types when drilling oil wells.
A child drinks water from a well built as part of a water project in Kenya.
William Smith's geological map of England, Wales, and southern Scotland from 1815.
Mikhail Lomonosov, a Russian scientist who wrote an early book on geology.
James Hutton, a Scottish geologist often called the "father of modern geology".
John Tuzo Wilson, a Canadian scientist who helped develop the idea of plate tectonics.
The volcanologist David A. Johnston shortly before the 1980 eruption of Mount St. Helens.