Igneous rocks

Earth Materials: Silicate Minerals & Igneous Rocks... Introduction to Rocks and Minerals Continued: Common rock-forming “silicate” minerals Chapter 5 Introduction to rocks & the rock c

Earth Materials: Silicate

Minerals & Igneous Rocks

Introduction to Rocks and Minerals

(Continued):

Common rock-forming

“silicate” minerals (Chapter 5)

Introduction to rocks & the rock cycle (Prelude A)

Igneous Rocks (Chapter 6)

Today’s Lecture:

Average composition of the Earth’s crust.

Percent of elements by WEIGHT

Question: What minerals would you expect to be most abundant on Earth?

Earth’s Crust

Primarily Si & O followed in abundance by

Fe, Mg, Ca, Na, K, etc

Dark-colored silicates (mantle and oceanic crust)

Olivine (Si, O, Fe, Mg)

Pyroxene (Si, O, Fe, Mg, Ca)

Amphibole (Si, O, Fe, Mg)

Light-colored silicates (crust, esp continental crust)

Quartz (SiO2) - Hard, transparent

Feldspar (Si, O, Al, K, Na, Ca) - Hard, white, gray, pink

Clay (Mostly come from weathering feldspar)

Calcite (CaCO3, shells) Limestone - Used for cement

The Common Rock-forming Minerals

1 silicon (Si) atom

4 oxygen (O) atoms

Basic Building Block of Silicate Minerals:

The Silicon-Oxygen Tetrahedron

An anion with charge of -4

Tetrahedra link up by forming covalent bonds between oxygen atoms:

Single silicon tetrahedron:

A silicon atom

covalently-bonded to four oxygens.

Two tetrahedra can join

by sharing an electron between adjacent oxygen atoms

Silicates: The Common Rock-forming Minerals

Basic Building Block:

The Silicon-Oxygen Tetrahedron

Oxygen atom

Silicon atom

The Common rock-forming minerals

Silicon-oxygen tetrahedra can be arranged into:

Single chains: Pyroxene Double chains: Amphibole Sheets: Micas

Silicate chains and sheets

Not electrically neutral! Unsatisfied

negative charges

on oxygen atoms located at the

edges of chains,

or between sheets, are neutralized by coordinating metallic ions at those sites.

Balancing Charges in Silicates: Role of Metal Cations

Ionic Substitution

Ions of similar size (ionic radius) and charge can substitute for one another in a mineral.

Definition of a rock:

A rock is:

Prelude Chapter: Rocks

2) Naturally occurring

1) Comprised of one or more minerals

There are three types of rocks:

Igneous (formed by cooling from magma)

Sedimentary (formed by the breakdown of other rocks)

Metamorphic (formed when preexisting rocks

are heated under pressure

Rocks and minerals

Some rocks composed entirely of one mineral

limestone (calcite)

Prelude Chapter: Rocks

Most rocks have more than one kind of mineral

granite

Some rocks contain non-mineral matter

coal (has organic debris) obsidian (volcanic glassy rock -> not crystalline)

Prelude Chapter: Rocks

collection of one or more

minerals

Prelude Chapter: Rocks

rock minerals mineral

Prelude Chapter: Rocks

So far we have:

rock

collection ofone or more

minerals

mineral

A collection

of one or moretypes of atoms minerals

Prelude Chapter: Rocks

The Rock Cycle

Rocks may be classified into three types:

Igneous:

Formed by the crystallization of

molten rock material called magma

Sedimentary:

Formed from pre-existing rocks by

weathering (chemical and physical breakup)

and erosion (transport).

Metamorphic:

Formed by textural and compositional changes

that occur when pre-existing rocks are buried

and subjected to increased temperatures and

pressures.

Rock Cycle (see accompanying slide/MOV):

Connects the three rock groups to each other by process The rock cycle is embedded within the hydrological and the plate cycles discussed previously,

Focus: Interlude A & Chapter 6 Igneous Rocks

Molten rock

When magma reaches the surface it

is called lava.

In the Earth is called magma.

Magma is buoyant, rises to surface,

& sometimes breaks through as volcanic eruptions

Igneous Rocks

Igneous rocks

Why care?

Igneous rocks make up bulk of Earth’s crust

Earth’s mantle is composed entirely of igneous rock!

Igneous rocks are important economically as

building stones and as host rocks for a variety of

mineral (ore) deposits

Volcanic activity is a well-known geological hazard, and the associated igneous rocks hold the secrets

for understanding both the nature of past volcanic

eruptions and the potential for future eruption hazards

Volcanic Igneous Rocks

Igneous rocks that form by the eruption

of magma at the surface are called

volcanic (or (or

In igneous rocks, texture is

controlled by the cooling rate

Plutonic Igneous Rocks

Igneous rocks that form deep below the surface are called

plutonic (intrusive) (intrusive)

igneous rocks.

To see them, they must be uplifted to surface

and the overlying rock eroded away.

As a magma cools, atoms arrange themselves

into orderly crystalline structures called

minerals This process is called:

Again, the rate of cooling controls the grain size of the rock formed the grain size of the rock formed.

Plutonic igneous rocks cool slowly at

depth and are therefore coarser grained!

Microscopic views of plutonic igneous rocks

Subsurface intrusion called

a dike

ALL ROCKS ARE CLASSIFIED ACCORDING TO THEIR:

TEXTURE AND MINERAL COMPOSITION

Types of Igneous Textures

Fine-grained Coarse-grained

Porphyritic Glassy

Fine-grained Coarse-grained

Porphyritic Glassy

fast cooling magma/lava

forms at or near surface

sometimes gas holes present

hard to see individual crystals

forms deep below the surface slow cooling

crystals are corase and intergrown

and intergrown

magma cooled slowly for a

while then erupted

minerals crystallized at

different temperatures and

or rates over a period of time

rapid cooling (quenching) at surface

amorphous:

atoms unable to form orderly

crystalline structures

Types of Igneous Textures

Devil’s Postpile

Plutonic or volcanic?