MAGMATISM

Lesson 8.2

MAGMATISM : HOW MAGMA IS FORMED

What is magma ?

• Magma is defined as molten rock material produced by partial melting of the mantle and crust. It contains liquids, gases, crystals and rock fragments.

Rock melts as a result of the addition of vosatiles .

Formation of Magma

• TEMPERATURE

Temperature increases with depth, which is called geothermal gradient. This increase will obviously induce melting .

Temperature increase with depth in Earth's internal structure.

Formation of Magma

• PRESSURE

Decrease in pressure causes adiabatic decompression. As pressure is decreased, melting temperatures of materials decrease.

Rocks starts to melt even with low temperature due to difference in pressures.

Formation of Magma

• VOLATILES

Volatiles are substances that evaporate easily and can exist in gaseous form in the surface of Earth. Examples of this kind of substances are water and carbon dioxide. When volatiles mix with hot mantle rock, magma forms.

Rocks melt as a result of the addition of volatile .

Formation of Magma

• VOLATILES

Magmas may reach the surface or stay inside magma chambers. These chambers have sizes ranging from one kilometer up to several tens of kilometers. Once magma reaches the surface of the Earth it is called lava .

• MAGMATISM

Magmatism occurs along plate boundaries or margins and sometimes within the plate. Cracks on Earth's crust are the result of these plate boundaries. In effect, hot mantle rock penetrates the crust and becomes magma. Plate boundaries can be classified as convergent, divergent, and transform.

The movements of plates produces convergent, divergent and transform boundaries .

CONVERGENT PLATE BOUNDARIES

- where two plates move towards each other. The plates converging can be oceanic crust to oceanic crust, oceanic crust to continental crust, or continental crust to continental crust. As these plates collide,magma formation, earthquakes or mountain building were formed.

DIVERGENT PLATE BOUNDARIES

-where plates move away from each other. As the plates move apart, thin, fractured rocks formed. These provide a means for the hot mantle to reach the surface. As a result, volcanic activity occurs.

TRANSFORM PLATE BOUNDARIES

-where plates slide past one another connecting the other two boundaries. As a result, earthquakes occur.

Classification of Magma

• Magmas are classified according to the ratio between the alkalis and silica content .
• Tholeiitic magmas or tholeiites, are produced by large degree of melting.
• Calc-alkaline basalts form along convergent plate boundaries above subduction zones.
• Alkaline basalts form from smaller degree of partial melting, which results in alkali-rich and silica depleted magma of diverse compositions.
• Carbonatites are the only magmas that are not produced by silicate source rock.

KEY POINTS :

• Magma is defined as molten rock material produced by partial melting of the mantle and crust. It contains liquids, gases, crystals and rock fragments.

• Temperature increases with depth, which is called geothermal gradient. Decrease in pressure causes adiabatic decompression. As pressure is decreased, melting temperatures of materials decrease .

• Volatiles are substances that evaporate easily and can exist in gaseous form in the surface of Earth.

• Magmatism occurs along plate boundaries or margins and sometimes within the plate.

• Magmas are classified according to the ratio between the alkalis and silica content