Identify the Internet that are the most interesting. Plate Tectonics: A Scientific Revolution Unf

Identify the Internet that are the most interesting.

Plate Tectonics:
A Scientific
Revolution Unfolds

Chapter 5 Lecture

Natalie Bursztyn

Utah State University

Foundations of Earth Science

Eighth Edition

Summarize the view that most geologists held prior to the 1960s regarding the geographic positions of the ocean basins and continents.

Focus Question 5.1

Before 1960 geologists saw the positions of ocean basins and continents as fixed

Continental drift suggested but not agreeable

A new model of tectonic processes

A scientific revolution

Tectonic processes deform crust and create major structural features

From Continental Drift to Plate Tectonics

List and explain the evidence Wegener presented to support his continental drift hypothesis.

Focus Question 5.2

World maps in the 1600s suggested that South America and Africa fit together

In 1915, Alfred Wegener outlined the hypothesis of continental drift

Single supercontinent of all of Earth’s land: Pangaea

Fragmented ~200 mya and smaller landmasses drifted to their present positions

Continental Drift: An Idea Before Its Time

Similarity between coastlines on opposite sides of the Atlantic

Opponents argued that coastlines are modified through time by erosion and deposition

Continental shelf is a better approximation of the boundary of a continent

Evidence: The Continental Jigsaw Puzzle

Identical fossils found in South America and Africa

Paleontologists agree: land connection necessary to explain fossil record

Evidence: Fossils Matching Across the Seas

Mesosaurus

Small Permian aquatic freshwater reptile

Found in eastern South America and western Africa

Glossopteris

Seed fern

Africa, Australia, India, South America, and Antarctica

Opponents explain fossil patterns by rafting, oceanic land bridges, and island stepping stones

Evidence: Fossils Matching Across the Seas

Rock types and geologic features match up

2.2 billion-year-old igneous rocks in Brazil and Africa

Mountain belts end at coastlines and reappear across oceans

Evidence: Rock Types and Geologic Features

Evidence for glaciation on continents now at tropical latitudes

Can be explained by supercontinent located near the South Pole

Evidence: Ancient Climates

Wegener’s hypothesis of continental drift was met with criticism

Objections were based on lack of mechanism for continental drift

Wegener proposed that tidal forces moved continents and that sturdy continents broke through thin oceanic crust

The Great Debate

List the major differences between Earth’s lithosphere and asthenosphere.

Explain the importance of each in the plate tectonics theory.

Focus Questions 5.3

Oceanographic exploration increased dramatically following World War II

Discovery of global oceanic ridge system

Earthquakes at great depths in western Pacific ocean trenches

No oceanic crust older than 180 million years

Thin sediment accumulations in deep-ocean basins

Developments led to theory of plate tectonics

More encompassing theory than continental drift

The Theory of Plate Tectonics

Lithosphere is the crust and uppermost (coolest) mantle

Oceanic lithosphere varies in thickness

Thin at ridges, up to 100 km thick in deep-ocean basins

Mafic composition

More dense than continental lithosphere

Continental lithosphere 150–200 km thick

Felsic composition

Responds to forces by bending or breaking

Rigid Lithosphere Overlies Weak
Asthenosphere

Asthenosphere is the hotter, weaker mantle below the lithosphere

Rocks are nearly melted at this temperature and pressure

Responds to forces by flowing

Moves independently from lithosphere

Rigid Lithosphere Overlies Weak
Asthenosphere

Lithosphere is broken into irregular plates

Plates move as rigid units relative to other plates

7 major plates make up 94% of Earth

Earth’s Major Plates

Interaction between plates at plate boundaries

Divergent boundaries (constructive margins)

Two plates move apart

Upwelling of hot material from mantle creates new seafloor

Convergent boundaries (destructive margins)

Two plates move together

Oceanic lithosphere descends and is reabsorbed into mantle

Two continental blocks create a mountain belt

Transform plate boundaries (conservative margins)

Two plates slide past each other

No lithosphere is created or destroyed

Plate Movement

Sketch and describe the movement along a divergent plate boundary that results in the formation of new oceanic lithosphere.

Focus Question 5.4

Most divergent plate boundaries are along the crests of oceanic ridges

New ocean floor is generated when mantle fills narrow fractures in oceanic crust

Also called spreading centers

Divergent Plate Boundaries and Seafloor Spreading

[insert Figure 5.11 here]

Most divergent plate boundaries are associated with oceanic ridges

Elevated seafloor with high heat flow and volcanism

Longest topographic feature on Earth’s surface (covers 20% of surface)

Crest is 2 to 3 km higher than adjacent basin and can be 1000 to 4000 km wide

Rift valley is a deep canyon along the crest of a ridge resulting from tensional forces

Oceanic Ridges and Seafloor Spreading

Seafloor spreading is the process by which new seafloor is created along the ocean ridge system

Average spreading rate is ~5 cm/year

Up to 15 cm/year or as slow as 2 cm/year

New lithosphere is hot (less dense) but cools and subsides with age and distance from the ridge system

Thickness is dependent upon age

Oceanic Ridges and Seafloor Spreading

Continental rifting occurs when divergent boundaries develop within a continent

Tensional forces stretch and thin the lithosphere

Brittle crust breaks into large blocks

Eventually become ocean basins

East African Rift demonstrates initial stage

Continental Rifting

Divergent Plate Boundaries and Seafloor Spreading

Compare and contrast the three types of convergent plate boundaries.

Name a location where each type can be found.

Focus Questions 5.5

Convergent plate boundaries occur when two plates move toward each other

Convergence rate is equal to seafloor spreading

Characteristics vary depending on subducting crust

Subduction zones

Lithosphere descends into the mantle

Old oceanic crust is ~2% denser than asthenosphere

Continental crust less dense than asthenosphere

Convergent Plate Boundaries and Subduction

Deep ocean trenches

Long, linear depressions

Result of subduction

Angle of subduction varies

Nearly flat to nearly vertical

Depends on density of crust

Older crust is cooler and denser

Convergent Plate Boundaries and Subduction

Characteristics of convergent plate boundaries vary depending on type of crust being subducted

Oceanic + continental

Oceanic + oceanic

Continental + continental

Convergent Plate Boundaries and Subduction

Oceanic lithosphere + continental lithosphere = subduction of oceanic lithosphere

Continental lithosphere is less dense

Water from descending oceanic crust triggers partial melting of asthenosphere at ~100 km

Molten material is less dense and rises

Continental volcanic arcs

Oceanic-Continental Convergence

One slab subducts under another at
oceanic-oceanic convergent boundaries

Volcanism because of partial melting

Generates volcanic island arcs

Volcanic cones underlain by oceanic crust

Oceanic-Oceanic Convergence

Oceanic-Oceanic Continental Crust

Continental crust is buoyant

Neither plate subducts during continent-continent collisions

Folding and deformation of rocks

Mountain building

Continental-Continental Convergence

Describe the relative motion along a transform fault boundary.

Be able to locate several examples on a plate boundary map.

Focus Questions 5.6

Transform plate boundaries form when two plates slide horizontally past one another

Transform faults

No lithosphere is produced or destroyed

Connect spreading centers and offsets oceanic ridges

Linear breaks in the seafloor are fracture zones

Fracture zones are inactive

Active faults occur between offset ridge segments

Transform Plate Boundaries

Transport oceanic crust to destruction site

Transform Plate Boundaries

Few transform faults cut through continental crust

San Andreas Fault (California) and Alpine Fault (New Zealand) are exceptions

Transform Plate Boundaries

Explain why plates such as the African and Antarctic plates are increasing in size, while the Pacific plate is decreasing in size.

Focus Question 5.7

Total surface area of Earth is constant

Size and shape of individual plates changes

African and Antarctic Plates are growing

Surrounded by divergent boundaries

Pacific Plate is being consumed

Surrounded by convergent boundaries)

Plate boundaries move and change through time

How Do Plates and Plate Boundaries Change?

New ocean basins were created during the breakup of Pangaea

The Breakup of Pangaea

Present plate motions can be used to predict future continental positions

Plate Tectonics in the Future

List and explain the evidence used to support the plate tectonics theory.

Focus Question 5.8

Evidence from Deep Sea Drilling Project

Collect sediment and oceanic crust

Date fossils in sediment

Sediment age increases with distance from ridge

Sediment is thicker with increased distance from the ridge

Oldest seafloor is 180 million years old

Evidence: Ocean Drilling

Testing the Plate Tectonics Model

Volcanoes in the Hawaiian Island-Emperor Seamount Chain increase in age with distance from the Big Island of Hawaii

A cylinder of upwelling hot rock (mantle plume) is beneath Hawaii

A hot spot is an area of volcanism, high heat flow, and crustal uplift above a mantle plume

A hot-spot track formed as the Pacific Plate moved over the hot spot

Evidence: Mantle Plumes and Hot Spots

Testing the Plate Tectonics Model

Today North and South magnetic poles align approximately with geographic North and South poles

Iron-rich minerals influenced by magnetic pole

Basalt erupts above the curie temperature, so magnetite grains are nonmagnetic

Grains align to magnetic field during cooling

Rocks preserve a record of the direction of magnetic poles at the time of formation

Paleomagnetism or fossil magnetism

Position of paleomagnetic poles appears to change through time because of continental drift

Evidence: Paleomagnetism

Magnetic field reverses polarity during a magnetic reversal

Rocks with same magnetic field as today have normal polarity

Rocks with opposite magnetism have reverse polarity

Polarity of lava flows with radiometric ages was used to generate a magnetic time scale

Divided into chrons ~1 million years long

Finer-scale reversals within each chron

Vine and Matthews (1963) suggested stripes of normal and reverse polarity are evidence of seafloor spreading

Evidence: Paleomagnetism

Describe plate-mantle convection.

Explain two of the primary driving forces of plate motion.

Focus Questions 5.9

Mantle is solid, but hot and weak enough to flow

Convection occurs as hot, less dense material rises and surface material cools and sinks

During slab pull, cold, dense oceanic crust sinks because it is denser than the asthenosphere

During ridge push, gravity causes lithospheric slabs to slide down the ridge

Drag in the mantle also affects plate motion

Forces That Drive Plate Motion

Mantle convective flow drives plate motion

Subducting plates drive downward component of convection

Upwelling of hot rock at oceanic ridges drives upward component of convection

Convective flow is the heat transfer mechanism from Earth’s interior

Models of Plate-Mantle Convection

Multiple models for convective flow:

Whole-mantle convection

Cold oceanic lithosphere sinks and stirs entire mantle

Subducting slabs sink to core-mantle boundary

Balanced by buoyant mantle plumes

Layer cake model

Thin, dynamic layer in upper mantle

Thick, larger, sluggish layer below

Subducting slabs do not sink past 1000 km

Models of Plate-Mantle Convection

Restless Earth:
Earthquakes and
Mountain Building

Chapter 6 Lecture

Natalie Bursztyn
Utah State University

Foundations of
Earth Science
Eighth Edition

• Sketch and describe the mechanism that generates
most earthquakes.

Focus Question 6.1

• The sudden movement of one block of rock
slipping past another along a fault

• Most faults are locked until a sudden slip
• Seismic waves radiate out from the focus

(hypocenter), where slip begins
– Earth’s surface directly above the hypocenter is the

epicenter

What Is an Earthquake?

• Weak earthquakes can be generated by
– Volcanoes, landslides, and meteorite impacts

• Destructive earthquakes occur because tectonic
motion builds up stress
– Friction keeps the fault from slipping
– Slip initiates when stress overcomes friction
– Elastic rebound causes deformed rock to spring back

to undeformed position

Discovering the Causes of Earthquakes

• Convergent plate boundaries generate
compressional forces
– Mountain building and faulting associated with large

earthquakes
– Subducting plates form a megathrust fault

• Produce the most powerful earthquakes
• Vertical motion underneath the ocean generates tsunamis

Faults and Large Earthquakes

• Transform faults have many branches and
fractures
– Offset occurs in distinct segments

• Some segments displace slowly with little shaking during
fault creep

• Some segments produce numerous small earthquakes
• Some segments are locked for hundreds of years and

rupture in large earthquakes
– Earthquakes occur in repetitive cycles
– Rupture propagates in a discrete time period

Faults and Large Earthquakes

What Is an Earthquake?

• Compare and contrast the types of seismic waves.
• Describe the principle of the seismograph.

Focus Questions 6.2

• The study of earthquake waves is known as
seismology

• Waves are measured by seismometers
– Inertia keeps a weighted arm from moving while

ground motion moves the instrument
– Amplifies ground motion
– Generates seismograms

Seismology: The Study of Earthquake
Waves

Instruments That Record Earthquakes

• Two main types of seismic waves generated
by earthquakes:
– Surface waves travel in rock layers just below Earth’s

surface
– Body waves travel through Earth’s interior

Seismic Waves

• Two types of body waves:
– Primary or P waves

• Push/pull rocks in direction that wave is traveling
• Temporarily change volume of material
• Travel through solids, liquids, and gasses

– Secondary or S waves
• Shake particles at right angles to direction that wave is

traveling
• Change shape of material
• Do not travel through liquids or gasses

Seismic Waves

• Two types of surface waves:
– Some travel along Earth’s surface like rolling ocean

waves
– Others move Earth materials from side to side

• Most damaging type of ground motion

Seismic Waves

• Speed of travel is very different for each type of
wave

Seismic Waves

• Locate Earth’s major earthquake belts on a world
map.

• Label the regions associated with the largest
earthquakes.

Focus Questions 6.3

• Circum-Pacific belt convergent boundaries
experience 95% of earthquakes
– Megathrust faults generate largest earthquakes

• Earthquakes along Alpine-Himalayan belt because
of continental collision

• Weak earthquakes along oceanic ridge system
because tension pulls plates apart

• Transform and strike-slip faults generate large,
cyclical earthquakes

Earthquake Associated with Plate
Boundaries

• Less frequent earthquakes in central and eastern
United States

• Produce large areas of structural damage
– Underlying bedrock is older and more rigid
– Waves travel greater distances with less attenuation

• Intraplate earthquakes occur away from plate
boundaries

Damaging Earthquakes East of the Rockies

• P waves travel faster than S waves
• Difference in arrival time is exaggerated by

distance
– Greater interval between P and S wave arrivals

indicates greater distance to epicenter

Locating the Source of an Earthquake

• Distinguish between intensity scales and
magnitude scales.

Focus Question 6.4

• Intensity measures the amount of ground
shaking based on property damage

• Magnitude is a quantitative measure of energy
released in an earthquake
– Developed more recently

Determining the Size of an Earthquake

• Used for historical records
• Modified Mercalli Intensity Scale developed in

California in 1902
• Community Internet Intensity Map

– Website developed by U.S.G.S.
– Generated by user input

Intensity Scales

• Richter scale is related to the amplitude of the
largest seismic wave
– Logarithmic scale

• 10-fold increase in wave amplitude corresponds to
increase of 1 on the scale

• Each unit of increase equates to a 32-fold increase in the
energy released

– Not adequate for describing large earthquakes

Magnitude Scales

• Moment magnitude measures total energy
released based on amount of slide, area of
rupture, and strength of faulted rock
– Better at estimating the relative size of very large

earthquakes

Magnitude Scales

• List and describe the major destructive forces that
earthquake vibrations can trigger.

Focus Question 6.5

• Magnitude and other factors determine degree of
destruction

• Area 2050 km surrounding the epicenter
experiences equal shaking

• Ground motion diminishes rapidly outside of 50 km
• Earthquakes in stable interiors are felt over a

larger area

Earthquake Destruction

• Earthquake damage depends on:
– Intensity
– Duration
– Nature of surface materials
– Nature of building materials
– Construction practices

• Flexible wood and steel-reinforced buildings
withstand vibrations better

• Blocks and bricks generally sustain the most
damage

Destruction from Seismic Vibrations

• Soft sediment amplifies vibrations
• Vibrations cause loosely packed, waterlogged

materials to behave like a fluid
– During liquefaction stable soil becomes mobile and

rises to the surface
• Vibrations can also cause landslides, ground

subsidence, and fires

Destruction from Seismic Vibrations

• Megathrust displacement lifts large slabs of
seafloor, displaces water, and generates a
tsunami
– Low amplitude wave travels at very high speed in open

ocean
– Amplitude can reach tens of meters in shallow coastal

waters
– Arrival on shore is preceded by a rapid withdrawal of

water from beaches, followed by what appears as a
rapid rise in sea level with a turbulent surface

Tsunamis

• Tsunami warning system
developed after 1946
Hawaiian tsunami
– Seismic observatories

report large earthquakes
– Deep-sea buoys detect

energy released by
earthquakes

– Tidal gauges measure
rise and fall in sea level

Tsunamis

• Explain how Earth acquired its layered structure.
• List and describe each of its major layers.

Focus Questions 6.6

• Earth has distinct layers:
– Heaviest material at the center, lightest at top
– Iron core, rocky mantle and crust, water ocean, gaseous

atmosphere
• Interior is dynamic

– Mantle and crust are in motion
– Material is recycled from surface to deep interior

Earth’s Interior

• Seismic waves are the only way to “see” inside
the interior
– Waves are reflected at boundaries
– Refracted through layers
– Diffracted around obstacles
– Velocity increases with depth as stiffness and

compressibility of rock change
• Can be used to interpret composition and temperature of

rock

Probing Earth’s Interior: “Seeing” Seismic
Waves

Earth’s Interior

• Temperature increased as material accumulated
to form Earth
– Iron and nickel melted and sank to the center to

produce iron-rich core
– Buoyant rock rose to the surface and formed crust rich

in O, Si, and Al (+ Ca, Na, K, Fe, and Mg)
– Chemical segregation led to iron-rich core, primitive

crust, and mantle

Earth’s Layered Structured

• Earth is divided into three compositionally distinct
layers:
– Crust
– Mantle
– Core

• Can be further subdivided into zones based on
physical properties
– Solid or liquid
– Strength

Earth’s Layered Structure

• Thin, rocky crust is divided into:
• Oceanic crust

– ~7 km thick
– Composed of basalt
– Density ~3.0 g/cm3

• Continental crust
– ~35 – 40 (up to 70) km thick
– Many rock types
– Average density ~2.7 g/cm3

Earth’s Layered Structure

• Mantle
– Solid layer extending to 2900 km
– 82% of Earth’s volume

• Chemical change at boundary between crust and
mantle

• Uppermost mantle (first 660 km) is peridotite
– Stiff top of upper mantle (+ crust) is lithosphere

• Cool, rigid outer shell
• ~100 km thick

• Weaker portion below is asthenosphere
– Upper asthenosphere is partially melted
– Lithosphere moves independently of asthenosphere

Earth’s Layered Structure

• Core is an ironnickel alloy
– Density ~10 g/cm3
– Outer core is liquid

• 2270 km thick
• Generates Earth’s magnetic field

– Inner core is solid sphere
• 1216 km radius

Earth’s Layered Structure

Earth’s Layers

• Compare and contrast brittle and ductile
deformation.

Focus Question 6.7

• Deformation
– All changes in shape, position, or orientation of a rock

mass
– Bending and breaking occurs when stress exceeds

strength

Rock Deformation

• Elastic deformation
– Stress is gradually applied
– Rocks return to original size and shape when stress is

removed
– Ductile or brittle deformation occurs when elastic limit is

surpassed
• Strength of a rock is influenced by temperature,

confining pressure, rock type, and time

Types of Rock Deformation

• Brittle deformation
– Results in fractures
– Common at/near surface

• Ductile deformation
– Solid-state flow at great depths
– Produces a change in the size and shape of a rock
– Some chemical bonds break and new ones form

Types of Rock Deformation

• List and describe the major types of folds.

Focus Question 6.8

• Folds are wavelike undulations that form when
rocks bend under compression

• Compressional forces result in shortening and
thickening of the crust

Folds: Structures Formed by Ductile
Deformation

• Anticlines
– Upfolded or arched layers

• Synclines
– Associated downfolds or troughs

• Symmetrical
– Limbs are mirror images

• Asymmetrical
– Limbs are different

Anticlines and Synclines

• Overturned
– One or both limbs are tilted beyond the vertical

• Recumbant
– Axis is horizontal

Anticlines and Synclines

• Circular or elongated
upwarping structures are
called domes
– Upwarps in basement

rocks deform overlying
sedimentary strata

• Downwarping structures
are called basins

Domes and Basins

• Monoclines are large,
step-like folds
– Large blocks of basement

rock displaced upwards
– Ductile sedimentary strata

above drape over the fault

Monoclines

• Sketch and describe the relative motion of rock
bodies located on opposite sides of normal,
reverse, and strike-slip faults.

Focus Question 6.9

• Faults
– Fractures in the crust with appreciable displacement
– Movement parallel to dip are dip-slip faults

• Rock surface above the fault is the hanging wall
block

• Surface below the fault is the foot-wall block
• Fault scarps

– Long, low cliffs produced by vertical displacement

Faults: Structures Formed by Brittle
Deformation

Dip-Slip Faults

• Normal faults
– Hanging wall moves down relative to footwall
– Accommodate extension of crust

• Fault-block mountains are associated with large
normal faults
– Uplifted blocks are elevated topography called horsts
– Down-dropped blocks are basins called grabens

Dip-Slip Faults

• Reverse faults
– Hanging wall moves up relative to footwall

• Thrust faults
– Reverse faults with a dip of less than 45º
– Result from compressional stress
– Accommodate crustal shortening

Dip-Slip Faults

• Strike-slip fault
– Exhibits horizontal displacement
– Parallel to strike

Strike-Slip Fault

• Joints
– Not a fault
– Fractures with no appreciable displacement
– Develop in response to regional upwarping and

downwarping

Joints

• Locate and identify Earth’s major mountain belts
on a world map.

Focus Question 6.10

• Orogenesis is the set of processes that forms a
mountain belt

• Older mountain chains are eroded and less
topographically prominent

• Compressional mountains
– Large quantities of preexisting sedimentary and

crystalline rocks that have been faulted and folded

Mountain Building

• Sketch an Andean-type mountain belt and
describe how each of its major features is
generated.

Focus Question 6.11

• Subduction of oceanic lithosphere is the driving
force of orogenesis
– Volcanic island arcs form where subduction occurs

beneath oceanic lithosphere
– Continental volcanic arcs form where subduction

occurs below a continental plate
• Also forms mountainous topography along continental

margin

Subduction and Mountain Building

• Volcanic island arcs build mountains by volcanic
activity, emplacement of plutons, and the
accumulation of sediment from the subducting
plate onto the upper plate

Island Arc-Type Mountain Building

• Continental volcanic arcs form at Andean-type
convergent zones
– Before subduction, sediment accumulates on a passive

continental margin
– Becomes an active continental margin when a

subduction zone forms and deformation begins
– An accretionary wedge is an accumulation of

sedimentary and metamorphic rocks scraped from the
subducting plate

Andean-Type Mountain Building

• Summarize the stages in the development of a
collisional mountain belt such as the Himalayas.

Focus Question 6.12

• Cordilleran-type mountain building occurs in
Pacific-like ocean basins
– Rapid seafloor spreading is balanced by rapid

subduction
– Island arcs and crustal fragments (terranes) collide

with a continental margin
• Some terranes may have been microcontinents
• Small terranes are subducted
• Larger terranes are thrust onto the continent

Collisional Mountain Belts

Cordilleran-Type Mountain Belts

• Alpine-type orogenies result from continental
collisions

• Himalayas
– Collision between Indian and Eurasian plates

Alpine-Type Mountain Belts

[insert Figure 6.47 here]

• Appalachians
– Generated by orogenies that lasted a few hundred

million years
• One of the stages in assembling Pangaea

– Resulted from three distinct stages of mountain-building

Alpine-Type Mountain Belts

Volcanoes and Other Igneous Activity

Chapter 7 Lecture

Natalie Bursztyn

Utah State University

Foundations of Earth Science

Eighth Edition

Compare and contrast the 1980 eruption of Mount St. Helens with the most recent eruption of Kilauea, which began in 1983.

Focus Question 7.1

Mount St. Helens

Largest historic eruption in North America

Lowered peak by more than 400 m

Destroyed all trees in a 400 km2 area

Mudflows 29 km down Toutle River

Ejected 1 km3 ash more than 18 km into stratosphere

Mount St. Helens Versus Kilauea

Kilauea

Quiet eruption of fluid basaltic lava

Occasional lava sprays

Eruption began in 1983 and has been ongoing for more than 20 years

Mount St. Helens Versus Kilauea

Explain why some volcanic eruptions are explosive and others are quiescent.

Focus Question 7.2

Magma

Molten rock containing crystals and dissolved gas

Lava

Erupted magma

Basaltic magma

Generated by partial melting in upper mantle

At oceanic crust, erupts as highly fluid lava

At continental crust, collects at crustmantle boundary

Partial melting of overlying continental crust generates dense, silica-rich magma

Magma: Source Material for Volcanic Eruptions

Viscosity

Resistance to flow

How to decrease magma viscosity

Increase temperature

Decrease silica content

Rhyolitic magma (>70% Si) forms short, thick flows

Basaltic magma (~50% Si) is fluid

Gas content also dictates nature of eruption

Directly related to composition

Most common gas is water vapor

Quiescent Versus Explosive Eruptions

The Nature of Volcanic Eruptions

Triggered by addition of magma to near-surface magma chamber

Inflation and fracture of volcano summit

Fluid basaltic lava

Ongoing eruption of Kilauea since 1983

Quiescent Hawaiian-Type Eruptions

Pressure decreases as magma rises

Dissolved gas forms expanding bubbles

Viscous magma expels fragmented lava and gas

Buoyant plumes of material (eruption columns)

Rapid ejection of magma

Reduces pressure in magma chamber

Causes further expansion and eruption

How Explosive Eruptions Are Triggered

List and describe the three categories of materials extruded during volcanic eruptions.

Focus Question 7.3

90% of lava is basaltic

Most is erupted on seafloor (submarine volcanism)

Flows in thin, broad sheets or ribbons

Flow rate ~10 to 300 m/hr

Up to 30 km/hr downhill

~9% is andesitic/intermediate

<1% is rhyolitic

Thick flows move imperceptibly slow

Don’t flow beyond a few km from vents

Lava Flows

Two types of basaltic lava flows:

Rough, jagged blocks with sharp edges

Cooler, more viscous basaltic flows

Pahoehoe

Smooth, ropy surfaces

Hotter, less viscous basaltic flows

Lava Flows

Lava tubes

Insulated pathways of a lava flow

Pillow lavas

Numerous tube-like structures stacked atop each other

Form on the ocean floor

Lava Flows

Gases in magma are volatiles

Dissolved into magma because of confining pressure

~1–8% of total magma volume

Most abundant gases (in decreasing order): water vapor, carbon dioxide, sulfur dioxide, lesser amounts of hydrogen sulfide, carbon monoxide, and helium

Contribute to atmosphere

Significant quantities can alter global climate

Gases

Pyroclastic materials or tephra

Particles erupted from a volcano (ash and dust)

Hot ash fuses to form welded tuff

Lapilli and cinders are the size of small beads to walnuts

Blocks are larger than 64 mm

Bombs are streamlined blocks ejected while still molten

Scoria is vesicular ejecta

Pumice is felsic equivalent

Pyroclastic Materials

Materials Extruded During an Eruption

Draw and label a diagram that illustrates the basic features of a typical volcanic cone.

Focus Question 7.4

Volcanic landforms are the result of many generations of volcanic activity

Eruptions start at a fissure

Flow is localized into a circular conduit

Conduit terminates at a vent

Successive eruptions form a volcanic cone

Anatomy of a Volcano

Crater

Funnel-shaped depression at the summit

Form by erosion during, or collapse following, eruptions

Calderas are craters >1 km

Flank eruptions generate parasitic cones

Secondary vents that emit gas only are called fumaroles

Anatomy of a Volcano

Summarize the characteristics of shield volcanoes.

Provide one example of this type of volcano.

Focus Questions 7.5

Broad domed structures built by accumulation of basaltic lava

Most begin as seamounts (submarine volcano)

e.g., Canary Islands, Hawaiian Islands, Galapagos, Easter Island, Newberry Volcano in Oregon

Shield Volcanoes

9 km high

Low angle slopes

Well-developed caldera from collapse of magma chamber following eruption

Mauna Loa: Earth’s Largest Shield Volcano

Kilauea is most active and studied volcano

50 eruptions since 1823

Most recent began in 1983

Magma chamber inflates and earthquake swarms indicate an impending eruption

Kilauea: Hawaii’s Most Active Volcano

Shield Volcanoes

Describe the formation, size, and composition of cinder cones.

Focus Question 7.6

Cinder cones (scoria cones)

Symmetrical

Steep-sided

Loose accumulations of ejected scoria

Commonly pea- to walnut-sized fragments

Basaltic composition, reddish-brown color

Some produce lava flows

Craters are relatively large and deep

Cinder Cones

Cinder cones form quickly

Many in less than one month

Generally in a single eruptive event

Small size (30–300 m)

Cinder Cones

List the characteristics of composite volcanoes.

Describe how these volcanoes form.

Focus Questions 7.7

Composite cones or stratovolcanoes

Located around the Ring of Fire

Large, symmetrical cones

Built by layers of cinder and ash alternating with lava flows

Primarily silica-rich andesitic magma

Associated with explosive eruptions and abundant pyroclastic material

Steep summit and gradually sloping flanks

Composite Volcanoes

Describe the major geologic hazards associated with volcanoes.

Focus Question 7.8

70 volcanic eruptions expected each year

One large-volume eruption each decade

500 million people live near active volcanoes

Volcanic hazards include:

Pyroclastic flows

Lahars

Lava flows

Ash and volcanic gasses

Volcanic Hazards

Pyroclastic flow (nuee ardente)

Hot volcanic gas infused in incandescent ash and lava fragments

Gravity driven, can move up to 100 km/hr

Low-density cloud of hot gases and fine ash on top of layer of vesicular pyroclastic material

Caused by collapse of eruption columns

Pyroclastic Flow: A Deadly Force of Nature

Volcanic Hazards

Lahars

Fluid mudflows

Water-saturated volcanic debris move down steep volcanic slopes

Can occur on dormant/extinct volcanoes

Lahars: Mudflows on Active and Inactive Cones

Tsunamis

Caused by collapse of volcano flanks into the ocean

Ash

Can damage buildings, living things, aircraft engines

Sulfur dioxide

Affects air quality and creates acid rain

Atmospheric cooling

Ash and aerosols reflect solar energy

Other Volcanic Hazards

Volcanic Hazards

List volcanic landforms other than shield, cinder, and composite volcanoes.

Describe their formation.

Focus Questions 7.9

Caldera

Steep-sided crater less than 1 km in diameter

Formed by summit collapse following draining of the magma chamber

Other Volcanic Landforms

Fissure eruptions

Emit basaltic lavas from fissures (fractures)

Basalt plateaus

Flat, broad accumulations of basalt emitted from fissures

Flood basalts

Molten lava having flowed long distances within a basalt plateau

Other Volcanic Landforms

Volcanic necks (plugs)

Eroded volcanic cones expose the solidified magma inside the conduit

Other Volcanic Landforms

Compare and contrast these intrusive igneous structures:

Dikes.

Sills.

Batholiths.

Stocks.

Laccoliths.

Focus Question 7.10

Magma that crystallizes in Earth’s crust displacing host or country rock forms intrusions or plutons

Exposed by uplift and erosion

Classified according to shape

Tabular or massive

May cut across existing structures

Discordant

Or inject parallel to features

Concordant

Intrusive Igneous Activity

Tabular intrusive bodies

Magma is injected into a fracture or other zone of weakness

Dikes are discordant

Sills are concordant

Intrusive Igneous Activity

Columnar jointing occurs as a result of shrinkage fractures that develop when igneous rocks cool

Intrusive Igneous Activity

Large intrusive bodies include:

Batholiths

Linear masses of felsic rocks hundreds of km long

Stocks

Surface exposure <100 km2

Laccoliths

Lift the sedimentary strata that they penetrate

Intrusive Igneous Activity

Summarize the major processes that generate magma from solid rock.

Focus Question 7.11

Earth’s crust and mantle are composed primarily of solid rock

Rock is composed of a variety of minerals with different melting points

Rocks melt over a range of temperatures

Incomplete melting of rocks is partial melting

Partial Melting

Geothermal gradient averages ~25°C/km

Mantle is solid under normal conditions

Pressure increases melting temperature

Decompression melting is triggered when confining pressure decreases

Occurs at oceanic ridges

Generating Magma from Solid Rock

Adding water lowers melting temperature

Occurs at convergent boundaries

Generating Magma from Solid Rock

Mantle derived magma pools beneath crustal rocks

Heat from basaltic magma generates silica-rich magma via melting of continental crust

Also occurs during continental collisions

Generating Magma from Solid Rock

Explain how the geographic distribution of volcanic activity is related to plate tectonics.

Focus Question 7.12

Most volcanoes are found near:

Ring of Fire around the Pacific Ocean

Mid-ocean ridges

Few are randomly distributed

Plate Tectonics and Volcanism

Intraplate volcanism

A mantle plume of hot material ascends to the surface

Plate Tectonics and Volcanic Activity

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