GY111 Physical Geology Lecture 4: Igneous Rocks - PowerPoint Presentation

Slide1

GY111 Physical Geology

Lecture 4: Igneous RocksSlide2

Types of Rocks

Rock: an aggregate of one or more minerals

Igneous Rocks: crystallize from a magma

Sedimentary Rocks

Clastic: formed by the erosion of pre-existing rocks

Chemical/Biochemical: precipitated from chemical reactions

Metamorphic Rocks: formed by exposure to extreme heat & pressure below the melting pointSlide3

Magma

Magma is generated in the interior earth by heat from radioactive minerals

Volcanic eruptions prove that magma exists near the surface of the earth

Laboratory studies verify that common rocks will melt at the T & P inside the earth

Coarse grained igneous rocks prove that magma must cool slowly, and the only way that that can happen is that the surrounding rocks must be almost as hot as the magma itselfSlide4

Intrusive Igneous Rocks

Cool slowly at depths > 1 km

Form coarse-grained textures

Surrounding rock is termed “country” rock

May contain portions of the country rock that “fall” into the original magma chamber forming a xenolithSlide5

Extrusive Igneous Rocks

Form on the Earth’s surface

Lava: flow of magma onto the Earth’s surface

Pahoehoe: ropy surface (low viscosity)

Aa: fragmental surface (high viscosity)

Pyroclastic rocks: form from the explosive eruption of volcanoes

Ash: particles of glass

Tuff: a rock composed of fragments of pre-existing rock in an ash matrix

Pumice: a rock so full of voids (vesicles) that it can float in water (S.G. < 1.0)

Obsidian: massive volcanic glassSlide6

Lava Flow Types

Pahoehoe: ropy

Aa: fragmentedSlide7

Igneous Textural Terms

Aphanitic: mineral grains in rock are too small to be identified with a hand lens (rock cooled from magma rapidly)

Phaneritic: minerals grains in rock are large enough to be identified with a hand lens (rock cooled relatively slowly)

Phenocrysts: crystals that are distinctly larger than surrounding mineral grains

Porphyritic: a texture where relatively large phenocryst mineral grains are surrounded by smaller grainsSlide8

View of Textural Types

Aphanitic

PhaneriticSlide9

Composition

Felsic: light colored igneous rock relatively rich in Si, Na and K.

Intermediate: rock made up of equal proportions light and dark minerals.

Mafic: dark colored rock relatively rich in Ca, Fe and Mg.

Ultramafic: dark colored rock relatively rich in Fe and Mg

Note: red is considered a felsic (light) color; green is considered a mafic (dark) colorSlide10

Where Different Igneous Textures FormSlide11

Common Igneous MineralsSlide12

Classification of Igneous Rocks

Based on Mineral Content & TextureSlide13

Magma Formation

Magma formation is favored by increasing temperature and decreasing pressure

Magma formation is favored by increasing H2O content because it effectively lowers the melting point of minerals in rocks

Several tectonic environments favor magma formation:

Divergent boundaries, Hot Spots: pressure reduction in upwelling mantle (Decompression melting)

Convergent boundaries: increasing temperature and water content in subducting slab; frictional heatingSlide14

Granite Melting Curves

Experimental results with actual granite rock displays effect of pressure and water

T Deg. C

P

Kbar

500

600

800

4

6

8

10

Dry melting

curve

solid

melt

solid

melt

Wet (H2O) melting

curve

35 km

20 km

decompression

Subduction

Divergent

ConvergentSlide15

Fractional Crystallization

Controlled by Bowen’s Reaction Series

Discontinuous

Series

Continuous

SeriesSlide16

Palisades Sill: Example of Fractional Crystallization

Early high-temp crystals settle to the base of the magma chamberSlide17

Palisades Sill cont.

The end result is a layered intrusion- different layers have different compositionsSlide18

Forms of Magma Intrusions

Batholith: discordant; >= 100 km

2

Stock: discordant; >= 1 and < 100 km

2

Pluton: discordant; < 1 km

2

Dike: discordant; tabular

Sill: concordant; tabular

Laccolith: concordant; shield shaped

Lopolith: concordant; saucer shapedSlide19

Intrusive Forms

Note: laccoliths and lopoliths are not shown in this schematicSlide20

Plate Boundary Associations: Divergent

Divergent Boundaries: production of ophiolite sequences

Ultramafic mantle partially melts to form basalt and gabbro (mafic rocks)

While in contact with ocean water the ocean crust is hydrated and altered chemically (seawater alteration)Slide21

Plate Boundary Associations: Convergent

Subducted ocean lithosphere partially melts to produce intermediate and felsic magma

The hydration of the ocean lithosphere dramatically lowers its melting point leading to abundant felsic to intermediate magma generationSlide22

Volcanic Landforms

Central Vent Eruptions

Shield Volcanoes: low viscosity lava flows

Volcanic domes: viscous lava extruded as a dome after major pyroclastic eruption

Cinder cones: small low viscosity eruptions that spatter small fragments of lava that solidify as cinders

Stratovolcanoes: high viscosity pyroclastic eruptions build a steep-sided cone

Craters/Calderas: explosive eruptions will blast a small crater at the summit of a volcano, or a large caldera for more violent eruptions

Diatremes: rapid intrusion of a very low viscosity carbonate-rich magma. Diamond bearing diatremes are termed “Kimberlites”Slide23

Volcanic Landforms cont.

Central vent eruptions

Shield

Lava dome

Cinder cone

Stratovolcano (Composite)

CalderaSlide24

Caldera Formation

Result from very large pyroclastic eruptions (Super Eruptions)

The Yellowstone Caldera is one exampleSlide25

Fissure Eruptions

Flood Basalts: large outpourings of low viscosity basaltic lava fills in low areas

Ash Flow deposits: result from the fissure eruption of felsic magma to produce extremely large pyroclastic flows (Yellowstone)Slide26

Columbia River Flood Basalts

An example of a fissure eruption of mafic lavaSlide27

Hydrothermal Vents

Water-rich liquid at high temperature

Under high pressure water may have a temperature of over 400 deg. C and still be a liquid phase

Geysers: interaction between groundwater and a volcanic magma chamber

Hydrothermal veins: important economic mineral sources; boil off from magma during fractional crystallizationSlide28

Global Patterns of Volcanism

Divergent: low viscosity mafic magma with little or no H

2

O; generate shield volcanoes (Iceland)

Convergent: high viscosity intermediate and felsic magma with abundant H

2

O; generate stratovolcanoes (Cascade Range)

Hot Spot: low viscosity dry mafic magma produces shield volcanoes under ocean lithosphere (Hawaii); high viscosity wet felsic magma under continental lithosphere (Yellowstone) Slide29

Exam Summary

Know intrusive geometry classes

Know textural terms (aphanitic, phaneritic, etc.)

Know common rock-forming silicates in felsic, intermediate, etc., compositions

Know the characteristics of Shield versus Composite Cone volcanoes.

Be able to diagram Bowen’s Reaction Series and describe the Palisades Sill as an example or fractional crystallization.

Be able to describe the conditions that lead to the formation of aa, pahoehoe, pumice, obsidian, welded tuff, scoria.

Be able to explain why some volcanoes extrude low-viscosity lava whereas others tend to erupt explosively. Relate low- versus high-viscosity magma to types of plate tectonic boundaries.