What makes Igneous Rocks so Diverse?

The majority of a class was devoted to talking about how diverse igneous rock can be. Depending on pressure and temperature different Rocks can be solidified. The first thing that we learn, from powerpoint and hands-on learning, was the composition and textures. The combination of composition and texture is the basis for classification and naming igneous rocks. Each rock has a aphanitic (fine-grained) or phaneritic (coarse-grained) texture, or mineral grain size. Igneous rocks are further distinguished by the relative abundance of their mineral constituents. Light-colored (felsic) rocks are rich in silica (SiO2) and contain mostly quartz and feldspar. Dark-colored mafic and ultramafic rocks have a lower silica content and are richer in Iron and Magnesium. This gives way to the formation of dark silicate minerals, such as olivine and pyroxene. Geologists can also divide magma and igenous-rock compositions into felsic, intermediate, mafic, or ultramafic categories.

Geologists studies show that the diversity of magmas results from four processes:

  1. Different types of rock can melt or partially melt by different amounts.
  2. Magmas of different composition can mix to produce new, in-between compositions.
  3. Rock surrounding magma may partially melt and mix with, or be assimilated by, the original magma.
  4. One magma composition can be derived from another during crystallization.

Melting different types of rock produces magmas of different composition. The extent to which the rocks melt also determines the composition.

Magma mixing- a mix of two or more magmas of different composition can make a very diverse rock. As new magma arrives into a magma chamber, it encounters resident magma, which may be a different composition.

Magma Assimilation- When magma rises toward the surface, it comes in contact with, and may incorporate pieces of, the surrounding rock. These surrounding rocks may partially or completely melt and mix into the magma, which changes the magma composition as well.

Deriving one magma composition from another- this deals with the crystallization process where the liquid crystallizes into mineral. If the first-formed mineral crystals separate from the melt, then the remaining liquid magma has a different composition from the original magma,  this is called fractional crystallization. This happens because each fraction of the magma that crystallizes leaves behind a melt of new composition. In some cases, early-formed crystals settle to the bottom of the magma chamber, in other cases crystallization on walls of magma chamber. In both cases the remaining liquid is depleted in elements that are removed from the crystals. No single mineral compositions is exactly that of the whole magma, so crystallization of a fraction of the magma changes the compostions of the remaining melt. If the liquid melt separates from the crystals , then a new magma composition results.

Source-Textbook

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