Some of the Earth's internal materials are directly accessible to geologists: shallow rocks, materials brought to the surface by geological events. To study the composition of deep rocks, geologists therefore resort to indirect methods.
I. Composition of directly accessible materials:
The envelopes of the Earth, accessible by sampling, have different chemical compositions that are determined from the study of representative rocks. These rocks are made up of minerals and/or glass.
A few definitions to start with:
Texture : relative arrangement and dimension of the elements of a rock.
Mineral : inorganic natural compound with a well-defined chemical composition (composes rocks).
Crystal : mineral with a perfectly ordered atomic structure (all minerals are crystals).
Glass : natural non-crystallized mineral substance.
Phenocrysts : large, geometric crystals.
Microlites : very small crystals, often needle-like.
Grainy texture : rock entirely formed of crystals resulting from slow cooling.
Microlite texture : crystals embedded in glass, phenocrysts visible to the naked eye and microlites under the microscope.
Related: USING MATHEMATICS TO FIND AN IMPORTANT ROCK TYPE
1. The three main types of rock on the earth's surface:
- Granites and granitoids:
They are the main representatives of the continental crust.
These rocks have a grainy structure , that is to say that all the crystals which constitute them are contiguous.
The main constituent minerals of granitoids are:
- quartz,
- black and white micas,
- orthoclase (potassium feldspars),
- the plagioclases.
The grainy structure of the granitoids testifies to a formation due to a slow cooling of the original magma.
- Basalts:
Basalts are generally dark in color and may have dark crystals.
They have a microlitic structure , that is to say that crystals of different sizes are "embedded" in an amorphous glass paste.
The larger crystals are pyroxene and olivine crystals while the smaller crystals (microlites) are plagioclase.
Gabbros have the same chemical composition as basalts because they come from the same magma injected at the axis of the ridge. The structure of the gabbros is grainy because their installation and therefore their cooling having been slower. The original magma of these rocks is made from the materials of the upper mantle.
Peridotites:
Their structure is grainy , proving that their crystallization was slow. The constituent crystals are pyroxene and olivine crystals.
2. The chemical composition of these rocks:
Chemical analyzes have shown that only a few elements make up the bulk of these rocks.
In the earth's crust: Si, O, Al, K, Na, Ca, Mg and Fe .
In the upper mantle: Si, O, Mg and Fe.
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| Section of the oceanic lithosphere |
II. Composition of inaccessible materials
Deep mantle and core materials are inaccessible. We can nevertheless, by models and reasoning that take into account the formation of the Earth from chondrites, specify their compositions.
The analysis of meteorites gives information about the composition of the Earth.
Indeed, the Earth was formed by accretion (growth of a geological object by successive contributions of matter) of asteroids attracting each other by gravitation and forming increasingly large pebbles, then embryos of planets.
By studying the composition of meteorites, we can therefore get an idea of the composition of the original materials of the Earth and by extension get an idea of the current internal composition of the Earth.
Two types of meteorites are studied:
- Chondrites , undifferentiated meteorites, composed of chondrules (sorts of drops of silicates (olivine, pyroxene, etc.) caught in a finely grained matrix of silicates and iron-nickel.
Chondrites give us information on the overall composition of the Earth before the differentiation of the different layers (so-called primitive magmatic Earth).
- Achondrites , differentiated stony meteorites. There are many types. They come from the outer layers of the planetoids), and they inform us about the composition of the Earth's surface layers.
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