Igneous Rocks

Origin of Magmas

Lecture Outline

· Origin of magmas

· Crystallization and igneous textures

· Evolution of magma

· Rock Classification

· Magma composition and Plate Tectonics

Origin of Magmas

· Igneous rocks form from molten rock

» Magma - molten rock below surface

» Lava - molten rock at surface

– Gases can escape lava more easily than magma

· Crystallization vs. precipitation

» Crystallization - crystals forming from magma or lava; occurs with cooling (“freezing”)

» Precipitation - crystals forming from aqueous (water) solution; due to supersaturation of ions

Types of Igneous Rocks

· Extrusive or Volcanic rocks (GEODe 360-365)

» Rock crystallizes from lava at Earth’s surface

· Intrusive or Plutonic rocks (GEODe 366-370)

» Rock crystallizes from magma within the crust

Intrusive Rocks (Yosemite)

http://www.stanford.edu/~wgupta/images/yosemite%20-%20yosemite%20falls%20from%20halfway%20up.jpg

Why do rocks melt?

· Role of Heat

» T increases with depth - Geothermal gradient:

30°C per kilometer

Temperatures of Local Groundwaters

Why and where do melts form?

· Role of Heat

» T increases with depth: Geothermal gradient

– 30°C per kilometer

» Near the Earth’s surface, igneous minerals melt at T’s of 700° to 1100°C

» At what depth does melting begin?

Convergent Plate Boundary

• If minerals melt at 700° to 1100°C, why isn’t the lower mantle molten?

Why and where do melts form?

· Role of Pressure (P)

» Melting T of minerals increases with increasing P

» Hot, rising material melts as pressure decreases (decompression melting)

» Example: Divergent boundary (ocean crust) and hot spots

· Mid-ocean ridge (MOR)

» Rising mantle rock undergoes decompression melting

» Less dense magma rises and collects in magma chamber

Why and where do melts form (cont.)?

· Role of Volatiles (especially water)

» Volatiles – compounds or elements readily vaporized to gas

– Ex: H₂O and CO₂

– Cause explosive volcanic eruptions

– Easily lost when magma is at surface

» Addition of water lowers melting temperatures – promotes melting (Fig. 4.19)

» Subducted crust carries water which enhances melting of minerals

» Ex: Convergent boundaries

Origin of Magmas
Melting

· Melting (not so simple)

» Different minerals have different melting temperatures

– Silica-rich minerals melt first (lower melting T), crystallize last

– Which silicate mineral is silica rich?

Origin of Magmas
Partial Melting

· Partial melting

» Only some of the minerals of a rock may melt

– Minerals with lowest melting T melt first (i.e., silica rich minerals)

– Partial melt rich in silica

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