Igneous rocks and processes

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Igneous Rocks & Processes
A- Magmas

1- What is a magma?
A melt (usually of silicates) + crystals + gases that forms and occurs beneath the surface of the earth. When a magma reaches the surface and begins to flow, it loses its gases and becomes a lava.

2- What is the chemical composition of a magma?
Magmas do not always have the same chemical composition. This is evidenced by the variety of igneous rocks that occur at the surface of the earth or which formed at depth, and the different types of volcanic eruptions. By carefully studying the chemistry of the different types of igneous rocks, and their associations with each other, petrologists were able to classify magmas into four main chemical groups:
1- Acidic: rich in SiO2, Na2O and K2O. Rocks produced from such magmas may have up to 77% by weight SiO2. "Granite" (see below) is an example of an acidic rock, and many acidic magmas are broadly known as "granitic".
2- Intermediate: rich in SiO2, Na2O, K2O as well as CaO and Al2O3. Rocks produced from such magmas have SiO2 values in the range 55 to 65% by weight.
3- Basic: rich in CaO, MgO and FeO. Rocks of this type have SiO2 values of 45 - 55% by weight. Basalt (see below) is an example of a basic rock, and many basic magmas are broadly known as "basaltic".
4- Ultrabasic: Are magmas poor in SiO2, but with large amounts of FeO and MgO. Ultrabasic rocks may have SiO2 values as low as 38% by weight. Table 1 lists the chemical compositions of some igneous rocks belonging to these four types.

Table 1: Average chemical compositions of selected igneous rock types
Oxide
Acidic (Granite)
Intermediate (Andesite)
Basic
(Basalt)
Ultrabasic (Peridotite)

SiO2
71.3
57.94
49.2
42.26

TiO2
0.31
0.87
1.84
0.63

Al2O3
14.32
17.02
15.74
4.23

Fe2O3
1.21
3.27
3.79
3.61

FeO
1.64
4.04
7.13
6.58

MnO
0.05
0.14
0.2
0.41

MgO
0.71
3.33
6.73
31.24

CaO
1.84
6.79
9.47
5.05

Na2O
3.68
3.48
2.91
0.49

K2O
4.07
1.62
1.1
0.34

H2O
0.77
1.17
0.95
3.91

CO2
0.05
0.05
0.11
0.30

P2O5
0.12
0.21
0.35
0.10

3- How does a magma form?
Most magmas are generated by partial melting in the asthenosphere, but the same process can occur in other layers of the upper mantle or even in the uppermost mantle and the lower crust (i.e deep parts of the lithosphere!). In order for us to understand this process, and the depths at which it occurs, we have to consider three things:
(i) change of T with depth (needed to melt the rocks) * geothermal gradient
(ii) how different rocks melt at different temperatures
(iii) how the melting of rocks depends on pressures and water content * melting curves

The temperature in the earth generally increases regularly with increasing depth, and the variation of temperature with depth at a specific time in the earth's history is known as the "geothermal gradient". If a rock is buried deep below the surface, it will first be metamorphosed, then at some higher temperatue, some of its constituent minerals will begin to melt. Because different minerals have different melting points, and because a rock is an aggregate of different minerals, melting will take place over a range of temperatures. Accordingly, this process is known as "partial melting", since only part of the rock melts at any given temperature. Figures 2a & b show the relationship between two different geothermal gradients (one beneath the continents; normal or average geothermal gradient, and the other beneath the oceans; a high geopthermal gradient), and the melting curves of an acidic rock (a granite, in the presence of H2O) and that of an ultrabasic rock (a dry peridotite).
Two things are clear from this figure:
(i) Acidic rocks, which are light in colour, melt at lower temperature compared to basic and ultrabasic rocks, and
(ii) an acidic melt can be generated at depths as low as 35 km, whereas a basic magma is generated in the mantle at depths of 300 km!

4- How does the magma move?
Because the magma is predominantly in the liquid state, it usually has a density lower than that of the overlying rocks. Therefore, most magmas will have a tendency to rise to shallower levels of the crust or even to the surface. The movement of magmas from deeper to shallower levels takes place either along fissures, cracks, or bedding planes, or by a process known as "stoping", where the magma interacts with some of the overlying rocks, first by engulfing them then perhaps melting them, a process known as assimilation. Assimilation will therefore lead to a change in the chemical composition of the melt, and will create new conduits for the continued movement of the magma upwards. In addition to density, viscosity (cf. Chernicoff, p. 74) plays an important role in magma movement.

5- Where does the magma occur or accumulate?
The volume or space occupied by a magma at depth is known as the magma chamber.

6- How does an igneous rock form from a magma?
When a magma rises to shallower levels, and begins to lose heat, minerals begin to crystallize. Because melting is the reverse of crystallization, understanding how a rock melts will help us understand how the same rock can form from a magma. In general, for the same composition of magma/rock, minerals that melt last will be the first to crystallize. If these early formed minerals are "left" inside the magma chamber and allowed to react with the cooling liquid, the final rock to form after all the magma has crystallized will be a basic rock similar in composition to the original "parent" magma from which it crystallized. On the other hand, if the early formed crystals are somehow prevented from reacting with the remaining magma, this magma will gradually change its composition, becoming more and more acidic with progressive crystallization. The process by which a magma forms two or more "bodies" of different chemical compositions, and as a result of which the magma itself changes its own chemical composition, is known as magmatic differentiation (we have already talked about the differentiation of the earth into a core, mantle, crust, hydrosphere and atmosphere in the first chapter; note how magmatic differentiation plays a role in this process by comparing the compositions of the crust and mantle!). The two most common processes involved in magmatic differentiation are:
(i) Fractional crystallization: where the crystals that form from a magma are separated from this melt by settling down to the bottom of the magma chamber (if they are denser than the magma), by floating on top of the magma (if they are lighter), or by filter pressing (subjecting the magma chamber to stress which "squeezes out" the molten magma leaving behind the crystals).
(ii) Assimilation: where the magma engulfs and melts some of the surrounding country rocks, thus changing its own chemical composition (by being "contaminated" by the country rocks).
(cf the section on mineralogy of igneous rocks below!)

7- Where do magmas crystallize? How do the forms or structures of their resulting rocks vary with depth of crystallization?
If the magma is allowed to cool slowly at considerable depths, the minerals have time to form large crystals, and the resulting rock becomes texturally ...