Magma Formation and Composition

Overview

This lecture covers the formation, composition, and generation of magma, highlighting key melting processes and differences between magma and lava.

Magma is molten or semi-molten material beneath Earth's surface, forming igneous rocks when solidified.
Lava is molten rock that reaches or erupts onto Earth's surface.
The main difference between magma and lava is their location (underground vs. surface).

Magmatism is the formation and movement of magma beneath Earth's crust.
Magma forms mainly in the asthenosphere, located between the crust and upper mantle.
The asthenosphere is a critical zone for magma generation and movement within Earth.

Magma consists of molten rock, minerals, and gases.
The most abundant elements in magma are oxygen (46.6%), silicon (27.7%), aluminum (8.1%), iron (5%), calcium (3.6%), sodium (2.8%), potassium (2.6%), and magnesium (2.1%).
Oxygen and silicon dominate magma composition, regardless of source rock.

Partial melting occurs when only some minerals in a rock melt due to their different melting points.
Most igneous rocks are derived from magma formed by partial melting of rocks in the upper mantle or crust.
Pure materials (like candle wax) melt completely at one temperature, while rocks melt partially due to varied mineral components.

Three main conditions cause rocks to melt: increase in temperature, decrease in pressure, and addition of volatiles.
Heat transfer (increase in temperature) occurs at convergent plate boundaries through the transfer of heat from molten rocks.
Decompression melting (decrease in pressure) happens at mid-ocean ridges as rocks rise and pressure drops.
Flux melting (addition of volatiles) takes place at subduction zones where water and carbon dioxide lower the melting point of rocks.

Magma — Molten or semi-molten rock beneath Earth's surface.
Lava — Molten rock on Earth's surface after a volcanic eruption.
Asthenosphere — Earth layer between the crust and upper mantle where magma forms.
Partial Melting — Process where only some minerals in a rock melt due to varying melting points.
Heat Transfer — Melting process from increased temperature at convergent boundaries.
Decompression Melting — Melting from decreased pressure, commonly at mid-ocean ridges.
Flux Melting — Melting due to addition of volatiles like water and CO₂, at subduction zones.

Review the differences between magma and lava.
Memorize the three main conditions for rock melting and their geologic settings.
Study the major elemental composition percentages of magma.