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Earth Materials:

Intro to rocks & Igneous rocks

• Three main categories of rocks:

• Igneous

• Sedimentary

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• Sedimentary

• Metamorphic

• The most common minerals in rocks

(a.k.a. the “rock-forming” minerals)

• Igneous Rocks – description and classification

The three major categories of rocks Fig 3.1 Understanding Earth

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The three major categories of rocks Fig 3.1 Understanding Earth

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Composed of crystals and/or glass

May have layers if volcanic

Classified using composition and texture

Most are composed of fragments (e.g. sand); Some may be crystalline.

Always layered/stratified at some scale

Classified using composition and texture

Always crystalline

Some have aligned crystals (foliation), some do not

Classified using composition and texture

Relative abundance of rock types in the earth’s crust

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Fig 3.4 Understanding Earth

The “Rock-forming” minerals

The 8 most abundant mineral in the earth’s crust and the most common minerals in rocks are often called the “rock-forming”

minerals. All 8 can be found in igneous rocks, and all 8 are silicates.

Quartz (1)

Feldspars: Alkali feldspar (2)

Pl i l f ld

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Plagioclase feldspar (3)

Micas: Muscovite mica (4)

Biotite mica (5)

Pyroxene (6)

Amphibole (7)

Olivine (8)

Understanding Earth6

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Igneous rocks

• General categories based on where they form• Intrusive (plutonic)

• Extrusive (volcanic)

• Types of Intrusions

• Description and classification

• Texture

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Texture• crystal size(s)

• bubbles

• fragments

• Composition (minerals and color)

• Felsic

• Intermediate

• Mafic

• Origin of magma

Formation of igneous rocks: intrusive and extrusive rocks

Magma vs. Lava

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g

Magma is molten rock underground. Intrusive or Plutonic

Lava is molten rock which has reached the surface. Extrusive or VolcanicFig 3.2 Understanding Earth

Extrusive rocks:Volcanoes and Lava flows

Fig 4.8 Understanding Earth

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October 1915 eruption of Mt. Lassen (photo by Chester Mullen)

Fig 4.8 Understanding Earth

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Fig 4.2 Understanding Earth

Veins of granite cutting through metamorphosed sedimentary rock

Dike

Intrusive rocks:veins & dikes

Intrusive rocks: plutons & batholiths 11

Part of the Sierra-Nevada batholith in Yosemite National Park

A huge mass of intrusive rocks composed of many individual large intrusions (plutons)

Classification of igneous rocks by texture and composition

Igneous textures: crystal size

all large crystals

d d b l ti l l li d d d

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- produced by relatively slow cooling deep underground

-found in intrusive igneous rocks(also known as plutonic igneous rocks)

phaneritic texture - large crystals

pegmatitic texture - extremely large crystals

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Classification of igneous rocks by texture and composition

Igneous textures: crystal size

at least partly small crystals and/or glass

- produced by relatively fast cooling, usually at or near the earth’s surface

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- typical of extrusive (volcanic) rocks

- sometimes in intrusive rocks (e.g. shallow dikes)

glassy texture - no crystals

aphanitic texture - very small crystals

porphyritic texture - mixed sizes of crystals

Glassy texture: Obsidian, pumice, and volcanic ash

Fig 4.3 Understanding Earth

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Igneous texturesGeology 2nd ed Chernicoff

fine = aphanitic mixed = porphyritic15

Geology 2 ed. - Chernicoff

coarse = phaneritic extremely coarse = pegmatitic

Classification of igneous rocks by texture and composition

Igneous textures: other textures

bubbles: vesicular texture

Gas bubbles trapped in an igneous rock are called vesicles.

Examples: vesicular basalt and pumice

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Examples: vesicular basalt and pumice

broken fragments: pyroclastic texture

If there is enough gas, expanding bubbles may cause lava to explosively break into fragments.

Examples: volcanic ash and pumice

Pyroclastic deposits may have some of the same features as sedimentary rocks.

Vesicular textures:basalt with vesicles (left) and pumice (below)Understanding Earth

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Pyroclastic (fragmental) textures

Volcanic ash

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Tuff (this one contains both ash and pumice)

Pumice

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Classification of igneous rocks by texture and composition

Composition

observed as variations in:

• mineral abundance (quartz, feldspar, etc.),

• elemental abundance (silicon, oxygen, iron, etc.),

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• color (dark, medium, light)

Ultramafic i.e. peridotite

Mafic i.e. basalt and gabbro

Intermediate i.e. andesite and diorite

Felsic i.e. rhyolite and granite

Classification of igneous rocks: texture and composition

Mafic rocks: basalt (fine) and

gabbro (coarse)

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Fig 4.5 Understanding Earth

Extrusive/Volcanic(glassy, aphanitic, or porphyritic)

Intrusive (coarse or porphyritic)

Felsic rocks: rhyolite (fine) and

granite (coarse)

Ultramafic rocks

Understanding Earth

Classification of igneous rocks: texture and composition

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Example: Peridotite

Composition

Felsic Intermediate Mafic Ultramafic

Extrusive (glassy, aphanitic, porphyritic, Rhyolite Dacite Andesite Basalt Komatiite

(rare)re

Igneous rock classification22

fragmental, vesicular, etc.)

(rare)

Te

xtu

r

Intrusive (phaneritic, pegmatitic)

Granite Granodiorite Diorite Gabbro Peridotite

more silica; less iron & magnesium

less silica; more iron & magnesium

lighter darker

Igneous rockcompositions

More silica (SiO2), sodium, potassium

More iron, magnesium, calcium

Lighter color Darker color

g 4.

6 U

nder

stan

ding

Ear

th

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mod

ifie

d fr

om F

ig

Rhyolite or Granite

Dacite or Granodiorite

Andesite or Diorite

Basalt or Gabbro

Komatiite or Peridotite

How are magmas formed?

by partial melting of crust or mantle rocks

Three ways to melt rocks in the Earth’s mantle and crust:

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• increase temperature

• decrease pressure on already hot rock (decompression)

• add fluids (water) to already hot rock

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Magma formation at plate boundaries and hot spots

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Fig 4.8 Understanding Earth

Magma formation by addition of fluids at subduction zones

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Modified from Fig 4.19 Understanding Earth

Where and how are magmas formed?

Mid-ocean ridges – decompression

mafic (basaltic) magma

Continental rifts and other areas of extension – decompression

mostly mafic (basaltic) magma

S bd ti lti d t dditi f fl id

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Subduction zones – melting due to addition of fluids

mafic, intermediate, and felsic magmas - often water-rich

Mantle plumes (hot spots) – decompression melting

ocean basins: mafic magma

continents: mostly mafic magma, some intermediate and felsic magmas

Note: secondary melts may occur due to addition of heat from primary magma