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Rocks: Materials of the Solid Earth

Chapter 2 Lecture

Natalie Bursztyn

Utah State University

Foundations of Earth Science

Eighth Edition

Sketch, label, and explain the rock cycle.

Focus Question 2.1

The rock cycle describes the interactions between the components of the Earth system

Origin of igneous, sedimentary, and metamorphic rocks and how they are connected

Any rock can be transformed into any other rock type under the right conditions

Earth as a System: The Rock Cycle

The rock cycle begins with magma

Forms from melting in Earth’s crust and upper mantle

Less dense magma rises toward the surface

Erupts at surface as lava or cools within crust

Cooling is called crystallization or solidification

Igneous rocks are crystallized from

Magma (within the crust)

Or lava (at Earth’s surface)

The Basic Cycle

Igneous rocks exposed at Earth's surface undergo weathering

Atmosphere decomposes rock

Generates loose material or dissolves it

Loose material is called sediment

Transported by gravity, running water, glaciers, wind, waves, etc.

Most sediment is transported to the ocean, but some is deposited in other environments

The Basic Cycle

Deposited sediment undergoes lithification

“Conversion into rock” by

Compaction

Cementation

Deformed by great heat and pressure if deeply buried or incorporated into a mountain chain

Metamorphism

Eventually enough heat will melt the rock and generate magma

The Basic Cycle

Rocks are not stable unchanging masses over geologic time scales

Rock cycle happens over millions or billions of years

Different stages of the rock cycle are occurring today all over Earth’s surface

New igneous rocks are forming in Hawaii

The Colorado Rockies are eroding and material is being carried to the Gulf of Mexico

The Basic Cycle

Rocks do not always go through the rock cycle from igneous to sedimentary to metamorphic

Igneous rocks may remain deeply buried and then become metamorphosed

Sedimentary and metamorphic rocks may be uplifted and eroded into sediment instead of melted

The rock cycle is driven by Earth’s internal heat and external processes, including weathering and erosion

Alternative Paths

Describe the two criteria used to classify igneous rocks.

Explain how the rate of cooling influences the crystal size of minerals.

Focus Questions 2.2

Igneous rocks form when magma or lava cools and crystallizes

Magma is generated most commonly by melting in the mantle, but some is generated by melting the crust

Rises because it is less dense than surrounding rock

Magma that reaches Earth’s surface is known as lava

Igneous Rocks: “Formed by Fire”

Solidification of lava at Earth’s surface creates extrusive or volcanic igneous rocks

Most volcanic eruptions are not violent

Abundant in the northwest (Cascades, Columbia Plateau)

Many oceanic islands are volcanic (Hawaii)

Igneous Rocks: “Formed by Fire”

Most magma never reaches the surface, and instead solidifies as intrusive or plutonic igneous rocks

Only exposed at the surface by uplift and erosion

Mount Washington (New Hampshire)

Stone Mountain (Georgia)

Mount Rushmore and the Black Hills (South Dakota)

Yosemite National Park (California)

Igneous Rocks: “Formed by Fire”

Magma contains ions including silicon and oxygen, gas (water vapor) confined by pressure, and some solid crystals

Crystallization occurs as mobile ions arrange into orderly patterns during cooling

As cooling continues, more ions are added to the crystals until all of the liquid becomes a solid mass of interlocking crystals

From Magma to Crystalline Rock

Rate of cooling strongly influences crystal size

Slow cooling results in fewer, larger crystals

Quick cooling results in a large number of small intergrown crystals

Instantaneous cooling (“quenching”) results in randomly distributed atoms, no crystal growth, and formation of volcanic glass

Volcanic ash is actually tiny shards of glass

Crystallization is also influenced by magma composition and dissolved gas

From Magma to Crystalline Rock

Igneous rocks are mainly composed of silicate minerals

Silicon and oxygen + Al, Ca, Na, K, Mg, and Fe make up 98% of most magmas

Also includes small amounts of trace elements

Titanium, manganese, gold, silver, uranium, etc.

During crystallization, these elements combine to form two major groups of silicate minerals

Igneous Compositions

Dark silicates are rich in iron and/or magnesium and relatively low in silica

Olivine, pyroxene, amphibole, biotite mica

Light silicates contain greater amounts of potassium, sodium and calcium and are richer in silica

Quartz, muscovite mica, feldspars

Feldspars are most abundant mineral group

40% of most igneous rocks

Igneous Compositions

Igneous rocks can be divided into broad groups according to proportions of light and dark minerals

Igneous Compositions

Granitic (felsic) rocks

Igneous rocks of granitic composition are made up almost entirely of light-colored silicates

Quartz and potassium feldspar

Felsic = feldspar + silica

Most contain ~10% dark silicate minerals

Biotite mica, amphibole

~70% silica

Major constituent of continental crust

Igneous Rocks: “Formed by Fire”

Basaltic (mafic) rocks

Contain at least 45% dark silicate minerals and Ca-rich plagioclase but no quartz

Mafic = magnesium + ferrum (iron)

Darker and more dense than granitic rocks because of iron content

Igneous Rocks: “Formed by Fire”

Andesitic (intermediate) rocks

Andesitic falls between granitic and basaltic composition

Mixture of both light- and dark-colored minerals

Contain at least 25% dark-silicate minerals

Amphibole and plagioclase feldspar

Associated with volcanic activity at continental margins

Igneous Rocks: “Formed by Fire”

Ultramafic rocks

Contain mostly dark-colored minerals

Olivine and pyroxene

For example, peridotite and dunite

Rare at Earth’s surface

Main constituent of upper mantle

Igneous Rocks: “Formed by Fire”

The texture of a rock is described based on the size, shape, and arrangement of mineral grains

Texture can be used to make inferences about a rock’s origin, for example:

Large crystals indicate slow cooling

Slow cooling is common in magma chambers deep in the crust

A rock with large crystals probably formed deep in the crust

What Can Igneous Textures Tell Us?

Fine-grained texture

Cooled rapidly at the surface or in small masses in the upper crust

Individual crystals are too small to see with the naked eye

Coarse-grained texture

Solidified at depth while insulated by surrounding rock

Masses of interlocking crystals roughly the same size (large enough to be seen by the naked eye)

What Can Igneous Textures Tell Us?

Porphyritic texture

Different minerals crystallize under different temperature and pressure conditions

One mineral can reach a large size before other minerals start to form

Large crystals (phenocrysts) in a matrix of smaller crystals (groundmass)

Vesicular texture

Exhibits voids left by gas bubbles that remained when lava solidified

Form in upper zone of a lava flow

What Can Igneous Textures Tell Us?

Glassy texture

Develops when rocks cool rapidly

Ions freeze in place before they can arrange themselves in an orderly crystalline structure

Pyroclastic (fragmental) texture

Composed of individual rock fragments ejected during explosive volcanic eruptions

Particles could be very fine ash, molten blobs, or large angular blocks

What Can Igneous Textures Tell Us?

Igneous rocks are classified by texture and mineral composition

Texture results from cooling history

Mineral composition derives from parent magma and environment of crystallization

Common Igneous Rocks

Granite

Coarse-grained

Forms when magma solidified slowly at depth

Uplifted during mountain building

Common Igneous Rocks

Rhyolite

Extrusive fine-grained equivalent of granite

Light-colored silicates, usually buff, pink, or light grey

Frequently contains voids and fragments of volcanic glass

Cooled rapidly at Earth’s surface

Common Igneous Rocks

Obsidian

Natural volcanic glass

Dark in color (from metallic ions), but felsic composition

Common Igneous Rocks

Pumice

Vesicular volcanic glass

Gas escape from molten lava forms a frothy, gray rock

Many pieces float in water because of vesicles

Common Igneous Rocks

Andesite

Medium-gray extrusive igneous rock

Fine-grained or porphyritic with phenocrysts of plagioclase feldspar or amphibole

Major constituent of volcanos along the Pacific Rim

Andes Mountains

Cascade Range

Diorite

Coarse-grained intrusive equivalent of andesite

Few or no visible quartz crystals

Common Igneous Rocks

Basalt

Most common extrusive igneous rock

Dark green to black, fine-grained

Contains pyroxene, olivine, and plagioclase feldspar

Relatively common at Earth’s surface

Volcanic islands (e.g., Hawaii, Iceland)

Upper layers of the oceanic crust

Central Oregon and Washington

Gabbro

Coarse-grained intrusive equivalent of basalt

Not commonly exposed at Earth’s surface

Significant component of oceanic crust

Common Igneous Rocks

Magma can evolve

Different rock types can be generated from the same melt

Bowen’s reaction series describes which minerals solidify at specific temperatures

First to crystallize is olivine, then pyroxene and plagioclase

Amphibole and biotite at intermediate temperatures

Muscovite and potassium feldspar during late cooling

Quartz is last to solidify

Minerals that form in the same temperature range tend to be associated in the same igneous rocks

How Different Igneous Rocks Form

Magmatic differentiation is the formation of one or more secondary magmas from a single parent magma

Explains diversity of igneous rocks

Magma composition continually changes during cooling

As crystals form, certain elements are selectively removed, resulting in a depleted magma

Crystal settling occurs when dense minerals sink to the bottom of a magma chamber

How Different Igneous Rocks Form

Igneous Rocks: “Formed by Fire”

Define weathering.

Distinguish between the two main categories of weathering.

Focus Questions 2.3

Weathering is the transformation of a rock to reach equilibrium with its environment

Natural response of materials to a new environment

Two basic categories: mechanical and chemical

Generally occur simultaneously

Erosion transports weathered rock

Weathering of Rocks to Form Sediment

Mechanical weathering is the process of breaking down rocks into smaller pieces

Each piece retains the same physical properties of the original material

Increases surface area available for chemical weathering

Mechanical Weathering

Frost wedging

Ice expands ~9% when it freezes

Traditional explanation: water fills cracks in rocks and expands

Recent research: lenses of ice grow within cracks and pore spaces of rock until rock is weakened and fractures

Mechanical Weathering

Salt Crystal Growth

Sea spray or salty groundwater evaporate in rock’s crevices and pore spaces

Salt crystals grow larger and weaken the rock by pushing apart surrounding grains or enlarging tiny cracks

Common on rocky shorelines and in arid regions

Mechanical Weathering

Sheeting occurs when concentric slabs of intrusive igneous rock break loose

Removal of overlying rock reduces pressure and outer layers expand and separate

Continued weathering results in exfoliation domes

Mechanical Weathering

Biological activity also breaks rocks apart

Plant roots grow into cracks and wedge the rock apart

Burrowing animals expose rock to increased weathering

Decaying organisms produce acids, which contribute to chemical weathering

Mechanical Weathering

Chemical weathering alters the internal structure of minerals

Elements are removed or added

Original rock is transformed into new stable material

Makes outer portions of some rocks more susceptible to mechanical weathering

Water is most important agent of chemical weathering

Oxygen dissolved in water causes oxidation

Carbon dioxide dissolved in water is carbonic acid

Feldspar minerals are broken down into clay minerals

Silica is carried away by ground water

Quartz is very resistant to chemical weathering

Chemical Weathering

Chemical weathering of a silicate rock by carbonic acid

Feldspar minerals are broken down into clay minerals

Silica is carried away by ground water

Quartz is very resistant to chemical weathering

Products of Chemical Weathering

List and describe the different categories of sedimentary rocks.

Discuss the processes that change sediment into sedimentary rock.

Focus Questions 2.4

Sedimentary rocks form after weathering breaks rocks down, gravity and erosional agents transport and deposit the sediment, and the sediment becomes lithified

Most sedimentary rock is deposited by solid material settling out of a fluid

Sedimentary rocks make up ~5% of Earth’s outer 10 miles, but account for 75% of all continental rock outcrops

Used to reconstruct details about Earth’s history

Economically important

Coal, petroleum and natural gas, metals, fertilizer, construction materials

Sedimentary Rocks: Compacted and Cemented Sediment

Sedimentary rocks are classified in two groups

Detrital sedimentary rocks form from solid particles weathered from other rocks

Chemical and biochemical sedimentary form from ions carried in solution

Types of Sedimentary Rocks

Detrital sedimentary rocks

Contain a wide variety of minerals and rock fragments

Clay and quartz are most common

Distinguished by particle size

Also useful for determining environment of deposition

Higher energy carries larger particles

Mineral composition is also used to classify detrital sedimentary rocks

Types of Sedimentary Rocks

Chemical sedimentary rocks

Water carries ions in solution

Solid material precipitates to form chemical sediments

E.g. salt left behind when saltwater evaporates

Materials precipitated by organisms are known as biochemical sediments

E.g. shells and hard parts

Limestone is composed of calcite (CaCO3)

Nearly 90% is formed by organisms

Types of Sedimentary Rocks

Examples of chemical sedimentary rocks:

Coquina: loosely cemented shell fragments

Chalk: hard parts of microscopic organisms

Travertine: inorganic limestone that forms in caves

Chert, flint, jasper, and agate: microcrystalline quartz

Salt and gypsum form in evaporite deposits

Coal consists mostly of organic matter

Types of Sedimentary Rocks

Lithification is the process by which sediment is transformed into sedimentary rock

Compaction occurs when grains are pressed closer together so that pore space is reduced

Weight of accumulated sediment

Most significant in fine-grained rocks

Cementation occurs when water containing dissolved minerals moves through pores

Cement precipitates, fills pores, and joins particles together

Calcite, silica, and iron oxide are common cements

Significant in coarse-grained rocks

Lithification of Sediment

Sedimentary rocks form in layers called strata or beds

Characteristic of sedimentary rocks

Thickness ranges from microscopic to tens of meters

Bedding planes ark the end of one episode of sedimentation and the beginning of another

Fossils are traces or remains of life found in some sedimentary rocks

Important clues of ancient environment

Can be used to match up rocks of the same age found in different places

Features of Sedimentary Rocks

Sedimentary rocks provide evidence for deciphering past environments

Features of Sedimentary Rocks

Define metamorphism.

Explain how metamorphic rocks form.

Describe the agents of metamorphism.

Focus Questions 2.5

Metamorphic rocks are produced when preexisting parent rock is transformed

Parent rock can be igneous, sedimentary, or metamorphic

Metamorphism occurs when parent rock is subjected to a different physical or chemical environment

Elevated temperature and pressure

Changes mineralogy, texture, and sometimes chemical composition

Equilibrium with new environment

Metamorphism progresses incrementally

Low-grade (slight changes) to high-grade (substantial changes)

Metamorphic Rocks: New Rock from Old

Most metamorphism occurs in one of two settings:

Contact metamorphism

Rock temperature increases because of intruding magma

Regional metamorphism

Pressure and high temperature during mountain building

Metamorphic Rocks: New Rock from Old

Agents of metamorphism

Heat (from intrusion of magma or burial)

Chemical reactions and recrystallization of new minerals

Confining pressure (equal in all directions because of burial)

Compaction and recrystallization of new minerals

What Drives Metamorphism?

Differential stress (greater in one direction because of mountain building)

Deformation and development of metamorphic textures

Rocks can react by breaking (brittle) or bending (ductile) depending on temperature

Chemically active fluids (hydrothermal fluid rich in ions)

Catalyze recrystallization reactions

Can dissolve a mineral from one area and precipitate it in another

Can change chemical composition of surrounding rock

What Drives Metamorphism?

What Drives Metamorphism

Metamorphism can change the texture of a rock

Low-grade metamorphism makes rocks compact and more dense

High-grade metamorphism causes recrystallization and growth of visible crystals

Metamorphic Textures

Foliation is the development of a flat arrangement of mineral grains or structural features

Metamorphic Textures

Foliation is characteristic of regional metamorphism

Driven by compressional stress

Causes mineral grains to develop parallel alignment

Includes:

Parallel alignment of micas

Parallel alignment of flattened pebbles

Separation of light and dark minerals

Development of rock cleavage

Metamorphic Textures

Nonfoliated rocks occur when deformation is minimal and parent rock is composed largely of stable minerals

Metamorphic Rocks Textures

Common Metamorphic Rocks

Common foliated metamorphic rocks:

Slate has characteristic rock cleavage

From metamorphism of shale or volcanic ash

Phyllite has larger mineral grains than slate, which give it a glossy sheen and wavy surface

Schist is formed by regional metamorphism of shale

Gneiss is a banded metamorphic rock that may have intricate folds

Common Metamorphic Rocks

Common nonfoliated metamorphic rocks:

Marble is a coarse crystalline rock

From metamorphism of limestone

Quartzite is very hard because of fused quartz grains

From metamorphosed quartz sandstone

Metamorphic Rocks: New Rock from Old