Characteristics of magma

• Igneous rocks form as molten rock (magma) cools and solidifies

• Parent material of magma?

• How/where does melting occur?

• How does the magma move?

• Where/why does it stop?

Characteristics of magma

• General characteristic of magma

• Magma that reaches the surface is called lava

• Rocks formed from lava at the surface are classified as extrusive, or volcanic rocks

• Rocks formed from magma that crystallizes at depth are termed intrusive, or plutonic rocks

Characteristics of magma

• The nature of magma

• Consists of three components:

– A liquid portion, called melt, that is composed of mobile ions

– Solids, if any, are silicate minerals that have already crystallized from the melt

– Volatiles, which are gases dissolved in the melt, including water vapor (H₂O), carbon dioxide (CO₂), and sulfur dioxide (SO₂)

Characteristics of magma

• Crystallization of magma

• Cooling of magma results in the systematic arrangement of ions into orderly patterns - minerals

• The silicate minerals resulting from crystallization, form in a predictable order - return to this in a few minutes

Characteristics of magma

• Crystallization of magma

• Igneous rocks are typically classified by

– Texture

– Mineral composition

Igneous textures

• Texture is used to describe the overall appearance of a rock based on the size, shape, and arrangement of interlocking minerals

Igneous textures

• Factors affecting crystal size

• Rate of cooling

– Slow rate promotes the growth of fewer but larger crystals

– Fast rate forms many small crystals

– Very fast rate forms glass

• Amount of silica (SiO₂) present in magma

• Amount of dissolved gases

Igneous Textures

• Fine-grained

• Coarse-grained

• Multiple sizes (porphyritic)

• Glassy

• Explosive (pyroclastic)

• Really coarse-grained (pegmatitic)

Fine-grained texture

Coarse-grained texture

Porphyritic texture

Glassy texture

Igneous Compositions

• Igneous rocks are composed primarily of silicate minerals

• Dark (or ferromagnesian) silicates

Ferro = Iron

• Light (or nonferromagnesian) silicates

Igneous Compositions

• Granitic versus basaltic compositions

• Granitic composition

– Composed of dominantly light-colored silicates

– Designated as being felsic (feldspar and silica) in composition

– Contains high amounts of silica (SiO₂)

– Major constituents of continental crust

Igneous compositions

• Granitic versus basaltic compositions

• Basaltic composition

– Composed of dark silicates and calcium-rich feldspar

– Designated as being mafic (magnesium and ferrum, for iron) in composition

– More dense than granitic rocks

– Comprise the ocean floor as well as many volcanic islands

Igneous compositions

• Other compositional groups

• Intermediate (or andesitic) composition

– Contain at least 25 percent dark silicate minerals

– Associated with explosive volcanic activity

• Ultramafic composition

– Rare composition that is high in magnesium and iron

– Composed entirely of ferromagnesian silicates

Igneous compositions

• Silica content as an indicator of composition

• Silica content in crustal igneous rocks exhibits a considerable range

– A low of <45 percent in ultramafic rocks

– Over 70 percent in some felsic rocks

Igneous compositions

• Silica content influences a magma’s behavior

• Granitic magma

– High silica content

– Extremely viscous (sticky, hard to flow)

– Liquid exists at temperatures as low as 700°C

Igneous compositions

• Silica content influences a magma’s behavior

• Basaltic magma

– Much lower silica content

– Fluid-like behavior

– Crystallizes at higher temperatures

Granite

Rhyolite

Obsidian

Pumice

Andesite

Diorite

Basalt (Scoria)

Gabbro

Pyroclastic - Fragmental

Ash and pumice layers

Origin of Magma

• Highly debated topic

• Generating magma from solid rock

• Produced by partial melting of rocks in the crust and upper mantle

• Role of heat

– Temperature increases within Earth’s upper crust (called the geothermal gradient) average between 20°C to 30°C per kilometer

Origin of Magma

• Role of heat

– Rocks in the lower crust and upper mantle are near their melting points

– Any additional heat (from rocks descending into the mantle or rising heat from the mantle) may induce melting

Origin of Magma

• Role of pressure

– An increase in confining pressure causes an increase in a rock’s melting temperature or conversely, reducing the pressure lowers the melting temperature

– When confining pressures drop, decompression melting occurs

Origin of Magma

• Role of volatiles

– Volatiles (primarily water) cause rocks to melt at lower temperatures

– This is particularly important where wet oceanic lithosphere descends into the mantle

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