You have seen that magmatic rocks are formed as much by the crystallization of magma within the earth as by lava released from volcanoes.
But the magmatic rocks - as well as the metamorphic ones - can be broken into small pieces or fragments that accumulate in sediment layers and eventually become, by compression, sedimentary rocks.
Finally, you also saw that sedimentary as well as magmatic rocks, under the action of high temperatures and pressure, can turn into metamorphic rocks.
But if a metamorphic rock is melted, it can again become a magmatic rock! These changes therefore form a cycle in which one rock over a long time can become another. It is the cycle of rocks.
How the soil formed
The layer of rocks on Earth's surface has been exposed for millions of years to changes in temperature and the action of rain, wind, river water and sea waves. All of this is slowly fragmenting the rocks and causing chemical changes. It was thus, by the action of weathering, that slowly the soil formed. And that's the same way it's continually reshaping itself.
Living things also contribute to this process of turning rocks into soil. Follow the scheme below:
- Rain and wind disintegrate the rocks.
- Pieces of lichen or seeds are blown to a lifeless region. The installation and reproduction of these organisms gradually modify the place. Lichens, for example, produce acids that help break down rocks. Plant roots that grow in rock crevices will contribute to this.
- As they die, these organisms enrich the soil in formation with organic matter, and when it decomposes, the soil becomes richer in mineral salts. Other plants, which need more nutrients to grow, can then settle on the spot. What is called ecological succession begins to occur: a number of organisms settle in until the typical soil and climate vegetation in the region is formed.
What is in the soil
There are many types of soil. Most of them are composed of sand and clay from the fragmentation of rocks and the remains of dead plants and animals (leaves, branches, roots, etc.). These remains are always being broken down by bacteria and fungi, which produce a dark organic matter called humus. As the decomposition continues, the humus is transformed into mineral salts and carbon dioxide. At the same time, however, more animals and vegetables deposit in the soil and more humus is formed.
THE decomposition transforms the organic substances of humus in mineral substances that will be used by the plants. In this way, matter is recycled: the matter that formed the body of living beings will eventually become part of them once decomposed.
We see, then, that the soil is formed by a mineral part, which originated from the breakdown of rocks, and an organic part, formed by the remains of dead organisms and the organic matter of the body of living beings that is undergoing decomposition. Several organisms still live in the soil, including bacteria and fungi, which are responsible for the decomposition of living organisms' organic matter.
In the spaces between the rock fragments, there is still water and air - both important for plant development.
Beneath the topsoil we find rock fragments. The greater the depth in relation to the ground, the larger the rock fragments.
Humans draw mineral resources from below-ground layers. Part of rainwater, for example, seeps into the soil, passing between the spaces of the clay and sand grains. Another part will also infiltrate sedimentary rocks and rock fractures, until it finds layers of waterproof rocks. Thus the so-called water sheets or water tables are formed, which supply the water wells.
Finally, in the deepest layer of the earth's crust, we find the rock that gave rise to the soil - the matrix rock.