When minerals bond they form rocks, defined as an aggregation of solid matter composed of one or more minerals.
Hard granite (pink), smooth slate (grey) or soft coal (black) may seem different to the sight and touch, but they are all rocks.
Geologists divide them into three main categories according to where and how they formed:
igneous rocks - metamorphic rocks – sedimentary rocks.
Beneath the earth’s crust flow streams of magma, an incandescent mixture of rocks and liquid at high temperatures – above 1000° C- which rise upwards from the depths of our planet. When magma breaks through the earth’s surface and gushes out it is called lava. This substance cools and solidifies into igneous rocks. Most of the earth’s crust is made up of these kinds of rock, most of them buried by sediment, water, dirt or other rocks. Igneous rocks occur in two forms:
intrusive and effusive rocks
Intrusive rocks are formed from magma that cools within the earth; magma rises towards the crust and seeps into brittle rocks, but it does not always manage to emerge. The rock remains underground until erosion or tectonic movements bring it to the surface.
Effusive rocks, also called volcanic rocks, are formed as a result of lava cooling on the surface.
70% of the oceans’ floor is covered with basalt. This volcanic rock – effusive – pours out in a fluid state from large oceanic rifts that cross the oceans for a distance of more than 60,000 kilometers, and it solidifies on the sea floor.
Continents are formed mostly from magmatic rocks; there are less sedimentary and metamorphic rocks.
The crystals in igneous rocks are a precious clue to the formation of rocks: intrusive rocks cool slowly and form crystals that are visible to the naked eye; effusive rocks cool quickly and form small crystals, visible only by means of a microscope.
Granite is an intrusive rock formed from crystals composed of three minerals: quartz, feldspar and mica.
Basalt is an effusive rock made up of very small crystals of pyroxene and plagioclase.
In the depths of the earth, rocks change form and become
Some of them are pressed and folded by the vast weight of mountains, others are changed into new rocks by the heat of magma.
Extreme conditions occurring beneath the surface can transform all kinds of rocks into metamorphic rocks (igneous, sedimentary and even other metamorphic rocks).
One might think that in such a hostile environment these rocks would become more fragile; on the contrary, they become more resistant.
For example when limestone, which has very tiny holes, is compressed, it changes into compact marble.
A very fine sedimentary rock - clay – changes into smooth slate, used in Liguria (Italy) to make roofs and blackboards.
Metamorphic rocks give us some idea of the world existing below the earth’s crust, but in order to study them, rocks need to rise to the surface. This occurs for example when wind, water or other natural agents remove all the materials that cover them. As the less compact rocks are eroded, the more resistant metamorphic rocks rise to the surface creating mountain chains made up of burnt and twisted stone.
Atmospheric agents and erosion transform rocks into tiny fragments. Wind and water carry them into the beds of rivers, lakes or the sea, where they are deposited in layers. As they accumulate over the course of millions of years, these fragments, or sediments, seal together, thereby creating
Limestone, sandstone and clay are some of the most common sedimentary rocks. Salt (halite), oil – formed from the remains of microscopic plants and animals - and coal formed from the remains of plant life that covered the earth millions of years ago, are also sedimentary rocks.
Geologists use their knowledge of rocks to help identify deposits of these important energy resources.
Sedimentary rocks are all, therefore, rocks that have formed in freshwater or seawater, regardless of their original source or structure.
In these rocks, the remains or imprints of animals or fossil plants can almost always be found.
Even if we are not aware of it, the rocks which surround us are on a kind of carousel that moves at a very low speed. The forces of nature push them upwards in the form of mountains, throw them into the air in the form of molten rock, break them into tiny fragments, or cause them to sink into the subsurface. This movement of rocks slowly transforms our planet: peaks become valleys and ocean beds transform into mountains. It is only in this century that scientists have calculated when this carousel started: the journey began when the earth was formed, almost five billion years ago. It is a very slow process which, however, recurs unceasingly.
Last modified: Sabato 6 aprile 2013