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Hitachi High-Tech

Special Feature — Part 12

How is soil formed?

Soil makes up the ground that covers our planet.

The surface of the earth actually moves slowly in a circular motion over a long period of time — rising to the surface,
sinking to the inside, and then rising to the surface again.
The soil under our feet was created in the middle of the earth's cycle.
Volcanic eruptions and earthquakes also take place during this cycle.

Soil Covers the Earth

Limestone

Sedimentary
rock

Metamorphic
rock

Igneous
rock

Magma

Magma

Becomes stones, sand and clay

Soil samples provided by Japan Conservation Engineers & Co., Ltd.

Limestone

Limestone is formed from dead coral and shells that have clumped together and hardened. Unlike other types of rocks created by the movement of the earth or the flow of water and wind, these rocks are created by the activities of living things.
The shape of coral skeletons, which are made of calcium carbonate, make up the microscopic structure of limestone.

Limestone
Rocks made from clumped and hardened coral and shells
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Sedimentary rock

Rock grains, volcanic ash, mud and the skeletons of living things are carried to rivers and to the bottom of lakes and seas.
As they clump together, the material that came first is pressed down by the weight of the material on top of it, forming a type of rock called sedimentary rock.
As the structure of the materials that make up the rock remain intact, an electron microscope can be used to see that microorganisms are present in the rock. These are called diatoms, which are a type of algae found in fresh water and seawater.

Sedimentary rock
Rocks made from sand, mud and dead living things clumped together
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Metamorphic rock

A metamorphic rock is an already existing rock—either igneous or sedimentary—that has been transformed by exposure to the high temperature of magma inside the earth or strong pressure from the movement of oceanic plates.
Similar to metamorphic rock is serpentinite rock, which has re-crystallized while under pressure in one direction. Electron microscope images show the crystals all arranged in the same direction.

Metamorphic rock
Rocks whose properties have changed as a result of high temperature or pressure
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Igneous rock

An igneous rock is a rock made from cooled and solidified magma (liquid rock). There are two types of igneous rock, depending on how the magma cooled down.
Magma that cools after being expelled to the earth's surface by a volcanic eruption (becoming lava) is called volcanic rock, while magma that slowly cools and solidifies underground is called intrusive rock.
One type of intrusive rock called granite contains a variety of mineral crystals (granules about 100 μm in size) that form when it cools and solidifies.

Igneous rock
Rocks made from cooled and solidified magma
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