Magmatism and Magmatic Rocks

Magmatic rocks are formed by the cooling and crystallization of magma.
Magma is molten rock, usually rich in silica, with dissolved gases and some crystals.
It originates from the partial melting of the lower portion of the crust or the upper portion of the mantle.
The formation of magmatic rocks is related to the mobility of the lithosphere and typically occurs at convergent and divergent boundaries of lithospheric plates.
The consolidation of magma forms intrusive rocks, or plutonites, and extrusive rocks, or vulcanites.

Basaltic magmas (poor in silica): solidify to form oceanic crust, predominantly expelled at rifts and hot spots, originating from mantle rocks (peridotite).
- Solidifying at depth, they form gabbros.
- Solidifying at or near the surface, they form basalts.
Andesitic magmas (intermediate composition): form in subduction zones and are associated with highly volcanic areas. The composition depends on the amount and type of subducted material.
- Solidifying at depth, they form diorites.
- Solidifying at or near the surface, they form andesites.
Rhyolitic magmas (rich in silica): form from the partial melting of the continental crust and tend to be very rich in gases, in zones of plate convergence.
- At depth, they form granites.
- At or near the surface, they form rhyolites.
Crystallization begins with the formation of very small crystals.
These crystals grow and form larger crystals.
- Euhedral: Crystals with well-developed faces. These only develop when minerals form in environments with free space, allowing their growth.
- Anhedral: Minerals very close to each other, restricting their growth and forming crystals with imperfect faces.
Most of the Earth's crust is composed of a small group of minerals belonging to the silicates.

Several phenomena increase the variety and diversity of minerals: isomorphism and polymorphism.
- Isomorphism: The characteristic of two or more minerals that have the same crystal structure but different chemical compositions.
- Polymorphism: The characteristic of two or more minerals that have the same chemical composition but different crystal structures.

Magmatic differentiation is the chemical evolution of magma during cooling, as different minerals crystallize.
A single type of magma can give rise to different types of rocks because:
- Magma is a complex mixture of mineral substances.
- The crystallization of these minerals occurs at different temperatures because their solidification points are different.
- The continuous crystallization process results in a residual magma of continuously altered chemical composition.

Bowen's Reaction Series illustrates the sequence in which minerals crystallize in a cooling magma.
According to Bowen, there are two series of reactions:
- Discontinuous series (ferromagnesian minerals)
- Continuous series (plagioclases)
In the discontinuous series, as cooling occurs, the previously formed mineral reacts with the residual magma, giving rise to a mineral with a different chemical composition and structure, stable under the new temperature conditions.
In the continuous series, there is a change in the ions of the plagioclase, without altering the internal structure of the minerals.
Bowen's reaction series illustrates fractional crystallization: the separation, by cooling, of a magma into different components, with successive formation and removal of minerals at lower temperatures.

Gravitational differentiation occurs when denser minerals settle to the bottom of the magma chamber due to gravity, leaving the residual magma above, which will form less dense minerals.

The last fractions of magma, consisting of water with volatiles and other substances in solution, constitute hydrothermal solutions and can fill cracks in the rocks, forming veins (filões).

Mixing of magmas: Under certain conditions, magmas of different compositions can mix, through the connection of different magma chambers, potentially forming an intermediate magma.
Assimilation: The ascending magma can incorporate new constituents, resulting from the melting of the rocks of the chamber walls or conduits. Some portions of the surrounding rock may remain in the solid state and form enclaves.