Geography quiz 30 multiple questions

CH. 2 – INTERNAL STRUCTURE OF THE EARTH

AND PLATE TECTONICS

Learning Objectives

• Describe the basic internal structure and processes of Earth.

• Summarize the various lines of evidence that support the theory of plate tectonics.

• Compare and contrast the different types of plate boundaries.

• Explain the mechanisms of plate tectonics.

• Outline how plate tectonics has changed the appearance of Earth’s surface over time.

• Compare and contrast the two fundamental processes that drive plate tectonics.

• Link plate tectonics processes to natural hazards.

Learning Objectives

• Describe the basic internal structure and processes of Earth.

• Summarize the various lines of evidence that support the theory of plate tectonics.

• Compare and contrast the different types of plate boundaries.

• Explain the mechanisms of plate tectonics.

• Outline how plate tectonics has changed the appearance of Earth’s surface over time.

• Compare and contrast the two fundamental processes that drive plate tectonics.

• Link plate tectonics processes to natural hazards.

Lithospheric Plates of the World

Earth and its Interior

Two Cities on a Plate Boundary

• California straddles the boundary between two tectonic plates • San Andreas fault: boundary

between North American and Pacific plates

• Los Angeles and San Francisco located on opposite sides of the fault

• Movement of San Andreas fault in 1906 • Caused major earthquake

• Earthquakes not understood at the time

• Scientific investigations led to identification of fault and new understanding of earthquakes

Plate Tectonics – Shift happens!

• Large-scale geologic processes that deform Earth’s

lithosphere

• Produce

landforms such

as ocean

basins,

continents, and

mountains.

• Processes are

driven by forces

within Earth

Movement of the Tectonic Plates

• Lithosphere is broken into pieces

• Lithospheric plates or tectonic plates

• Plate tectonics

• Plates move relative to one another

• Plates are created and destroyed

• Boundaries between lithospheric plates are geologically

active areas

• Responsible for several of the most devastating natural

hazards, such as earthquakes and volcanoes

Earth’s Plates

THREE types of plate boundaries:

1. Divergent

2. Convergent

3. Transform

Earth’s Plates

Location of volcanoes and earthquakes is not random!

Fit of the Continents

• Antonio Snider-Pelligrini (1858),

a geographer cut out a map of

Africa and South America

suggesting they were connected

at one time

• Other physical evidence based

on observation (maps, fossils

etc.) was used by Wegener

Continental Drift Hypothesis

• Alfred Wegener proposed the hypothesis of continental drift in 1915

• Theory based on congruity of the shape of the continents and similarity of fossils in South America + Africa

• Theory not accepted because could not explain how continents moved

Alfred Wegener Institute

Movement of the Tectonic Plates, cont.

• Seafloor spreading

• Explained mechanism for plate tectonics

• At mid-ocean ridges new crust is added to edges of

lithospheric plates

• Continents are carried along plates

• Crust is destroyed along other plate edges

• Subduction zones

• Earth remains constant, never growing or shrinking

Model of Plate Tectonics

The tectonic cycle – the “conveyor belt” model

1

2

3

The lithosphere moves laterally as if it were a conveyor belt

Direction of motion

The lithosphere moves laterally as if it were a conveyor belt

Direction of motion

Convection loop

Movement of the Tectonic Plates, cont.

• Sinking plates generate volcanoes and earthquakes

• Sinking ocean plates are wet and cold

• Plates come in contact with hot asthenosphere

• Plates melt to generate magma

• Magma rises to produce volcanoes

• Volcanic arcs

• Earthquakes occur along the path of the descending plate

• Wadati-Benioff zones

• Plate collides with another plate

• Denser plate dives under the

less-dense plate and is recycled

Denser plate: subducting plate

Less-dense plate:

overriding plate

Subduction Zone • Plate collides with another plate

• Denser plate dives under the

less-dense plate and is recycled

Plate Tectonics – Shift happens!

• Dark blue linear features = deep water

• Deep trenches

where plates

re-enter the

asthenosphere

• Surface

expression of

subduction

zones

Plate Tectonics – Shift happens!

• Light blue linear features = shallow water

• Along mid-

oceanic ridges • Bulge caused

by

asthenosphere

flowing

upwards

• Surface

expression of

spreading

centers

Movement of the Tectonic Plates, cont.

• Plate tectonics is a unifying theory

• Explains a variety of phenomena

• Evolutionary change

• How Earth works

• Direction of plate movement

• Distribution of earthquakes and volcanoes

• Similarities among fossils on different continents

• Changes in Earth’s magnetism

• Convection likely drives plate tectonics

Plate Movement

Hypothetical convection cells that may drive plate tectonics

Plate Movement

Internal Earth mostly solid, NOT liquid! Mantle is a

visco-elastic material

Types of Plate Boundaries

Defined by the relative movement of the plates on either

side of the boundary

Types of Plate Boundaries, cont.

Divergent Boundaries

• Plates move apart during seafloor spreading

• Magma from asthenosphere rises

Divergent Boundaries

Plates are pulled apart

under tension at

divergent zones:

Reduction in

pressure on

superheated

asthenosphere rock

Liquifies and rises

Buildup of magma

and heat causes

expansion and

elevation of

overlying

lithosphere

Divergent Boundaries

Gravity pulls the dome

downward:

Creating downward

down-dropped rift

valleys

Faulting progresses,

magma rises up through

cracks to build

volcanoes

Rifting + volcanism

continues, seafloor

spreading takes over,

down-dropped linear rift

valley fills with ocean

New sea is born

Divergent Boundaries around the World

Convergent Plate Boundaries: Subduction Zones

What pulls the

plate down?

Convergent Plate

Boundaries:

Collision Zones

Gros Morne National Park, Newfoundland

500 million years ago, a large piece of oceanic lithosphere

was scraped off the downgoing plate as it was subducting

Tabelands – access

to complete thickness

of the oceanic

lithosphere from the

upper mantle to the

Earth’s surface

3D view of different tectonic environments

Continent-Continent Collision

Credit: USGS

Continental collision between the

Indian and Asian plates

Tectonic

map

showing

India

pushing

into Asia

Transform Plate Boundaries

Plates slide past each other

Divergent Boundaries & Transform Faults

Rates of Plate Motion

• Plate motion is fast (geologically) - Plates move a few centimeters per year

• Movement may not be smooth or steady

• What happens when the rough edges along the plate move quickly?

• Plates can displace by several meters during a great earthquake - Such as with San Andreas fault

North American Plate Boundary

Subduction in California

Plate tectonics – Supporting evidence

• Oceanography

- Magnetization patterns on seafloor

- Age of ocean basins

- Bathymetry

• Earthquake hypocenters and epicenters

• Matching of fossils and rock types across

continents

A Detailed look at Seafloor Spreading

• Mid-ocean ridges discovered by Harry H. Hess

• Validity of seafloor spreading established by:

1) Identification and mapping of oceanic ridges

2) Dating of volcanic rocks on the floor of the ocean

3) Understanding and mapping of the paleomagnetic

history of ocean basins

https://www.e-

education.psu.edu/earth520/node/1811

Ocean bottom is on average about 3.8 km deep,

with two main exceptions:

• Continuous mountain ranges

-- several thousand feet in elevation from ocean floor

-- extend more than 65,000 km along ocean floors

– Volcanic mountains that form at spreading centers,

where plates pull apart and magma rises to fill gaps

• Narrow trenches extend to depths of more than 11 km

– Where tops of subducting plates turn downward

to enter mantle

Evidence of Plate Tectonics: Seafloor Topography

Ocean bottom is on average about 3.8 km deep,

with two main exceptions:

• Continuous mountain ranges -- several thousand feet in elevation from ocean floor

-- extend more than 65,000 km along ocean floors

– Volcanic mountains that form at spreading centers, where plates pull apart and magma rises to fill gaps

• Narrow trenches extend to depths of more than 11 km

– Where tops of subducting plates turn downward

to enter mantle

Evidence of Plate Tectonics: Seafloor Topography

Paleomagnetism

• Earth’s magnetic field can be represented by a dipole • Forces extend from North to

South Poles

• Magnetic poles do not coincide exactly with geographic poles

• Movements of iron-rich fluid generate a magnetic field around Earth - Which layer of Earth is

responsible for this?

Paleomagnetism

• Earth’s internal magnetic field NOT constant through time

• Over a few years: magnetic poles wander around the geographic poles

• Polarity reversal ~ every few 100,000 yrs - North magnetic pole south

magnetic pole

Normal polarity

(eg. Today)

Paleomagnetism, cont.

• Magnetic field has permanently magnetized some surface rocks at the time of their formation

- Iron-bearing minerals orient themselves parallel to the magnetic field at the critical temperature known as Curie Point

- Thermoremnant magnetization

Paleomagnetism is the study of magnetism of rocks at the time their magnetic signature is formed

Paleomagnetism, cont.

• Some volcanic rocks show

magnetism in opposite

direction as today

• Earth’s magnetic field has

reversed

• Cause is not well known

- Reversals are random

- Occur on average every few

hundred thousand years

Paleomagnetism, cont.

• Magnetic stripes

- Geologists towed

magnetometers along ocean

floor to measure magnetic

properties of rocks

- When mapped, the ocean

floor had stripes

• Areas of “regular” and “irregular”

magnetic fields

• Stripes were parallel to oceanic

ridges

• Sequences of stripe width

patterns matched the

sequences established by

geologists on land

Magnetic Anomalies on the Seafloor

• Using the magnetic anomalies, geologists can infer

ages for the ocean rocks

- Seafloor is no older than 200 million years old

• Spreading at the mid-ocean ridges can explain stripe

patterns

- Rising magma at ridges is extruded

oCooling rocks are normally magnetized

oField is reversed with new rocks that push old rocks away

Magnetic Reversals and Seafloor Spreading

Notice symmetry on either side of the ridge!

Magnetism and Age of the Seafloor

Map of magnetically striped Pacific

Ocean floor off Vancouver Island:

- Black areas are normally

magnetized

- Yellow areas point to reverse

polarity

Systematic increases in seafloor depth

• Ocean floor depths increase systematically with seafloor

age, moving away from mid-ocean ridges

• Why? As oceanic crust gets older, it cools and becomes

denser, therefore sinking lower into mantle.

Seafloor Topography and Age

Age of the Ocean Floor

The present ocean floors are no older than

200 million years, WHY??

Age of the Ocean Floor, cont.

• Subduction!

• Thick, buoyant, continental crust stable at Earth’s surface

• Continents form by:

• Accretion of sediments

• Addition of volcanic materials

• Collisions of tectonic plates carrying continental landmasses

• Pattern of magnetic stripes allows us to reconstruct how

plates and continents embedded in them have moved

throughout history

Paleomagnetism, cont.

• Hot spots

• Volcanic centers resulting from hot materials from deep in

the mantle

• Materials move up through mantle and overlying plates

• Found under both continental and oceanic crust

• Continental: Yellowstone National Park

• Oceanic: Hawaiian-Emperor Chain

• Plates move over hot spots creating a chain of island volcanoes

• Seamounts are submarine volcanoes

Hawaiian Hot

Spot

Pangaea and Present Continents

• Movement of plates is responsible for present shapes and locations of continents • 180 million years ago there was the break-up of Pangaea

• Supercontinent extending from pole to pole and halfway around Earth

• 50 Million years ago India crashed into China creating the Himalayas

• Reconstruction of Pangaea and recent continental drift clears up:

• Fossil data difficult to explain with separated continents

• Evidence of glaciation on several continents

Two

Hundred

Million

Years of

Plate

Tectonics 180 million years ago

135 million years ago

Two

Hundred

Million

Years of

Plate

Tectonics

65 million years ago

Paleontological Evidence for Pangaea

Ancient Mountain Ranges

The same sequence of rocks is found in

North America, Great Britain, and Norway.

The pattern does not make sense with the

continents in their current configuration.

Matching rock types and rock ages

Glacial

Evidence for

Pangaea

• Glaciers carve the rock as they

move.

• Scientists can determine the

direction of movement

Reconstructed ice sheet on

Gondwana.

How Plate Tectonics works: Putting it Together

• Two possible driving mechanisms for plate tectonics • Ridge Push and slab pull

• Ridge push is a gravitational push away from crest of mid-ocean ridges

• Slab pull occurs when cool, dense ocean plates sink into the hotter, less dense asthenosphere • Weight of the plate pulls the plate along

• Evidence suggests that slab pull is the more important process

Push and Pull in Moving Plates

Plate Tectonics and Hazards

• Divergent plate boundaries (Mid-Atlantic Ridge) exhibit earthquakes and volcanic eruptions

• Transform boundaries (San Andreas Fault) have great earthquake hazards

• Convergent subduction plate boundaries are home to explosive volcanoes and earthquake hazards

• Convergent collision plate boundaries have high topography (Tibetan Plateau) and earthquakes

• Internal structure of Earth can be divided into layers

or concentric shells, based on either composition or

physical properties.

• The uppermost physical layer of Earth is known as

the lithosphere and is relatively strong and rigid

compared with the soft asthenopshere underlying it.

• A convection cell is a temperature-driven circulation

pattern that is assumed to operate within Earth and

may be involved in driving plate tectonics.

Chapter 2 Summary

• The lithosphere is broken into large pieces called

tectonic plates that move relative to one another.

• The three types of plate boundaries are divergent,

convergent, and transform faults.

• Evidence supporting plate tectonics theory includes

seafloor spreading, continental drift, the configuration

of hot spots and chains of volcanoes, and Wadati-

Benioff zones.

Chapter 2 Summary, cont.

• Convection currents in Earth’s liquid outer core

generates a magnetic field that is sufficiently strong

to be recorded in rocks that contain magnetic

minerals.

• The seafloor spreading hypothesis proposed a

mechanism for continental drift.

• Seafloor spreading is confirmed using the

paleomagnetic signature of the seafloor centered

around the mid-ocean ridges.

Chapter 2 Summary, cont.

• The age of the seafloor is younger than 200 million years old, which is 20 times younger than the age of continents.

• Hot spots are plumes of hot rock that rise from deep in the mantle and cause volcanoes above them at Earth’s surface.

• Alfred Wegener’s continental drift hypothesis suggested that all of Earth’s continents were in the past assembled in a single enormous continent, known as Pangaea.

Chapter 2 Summary, cont.

• With the validation of the seafloor spreading,

continental drift was confirmed and several long-

standing geologic problems have been resolved.

• The driving forces in plate tectonics are ridge push

and slab pull.

• Plate tectonics is extremely important is determining

the occurrence and frequency of volcanic eruptions,

earthquakes, and other natural hazards.

Chapter 2 Summary, cont.

• Divergent plate boundaries are linked to earthquakes

and volcanic eruptions, but the risk is low because most

do not occur on land.

• Transform boundaries are linked to earthquakes and

represent and appreciable risk as these faults occur on

land and stretch for long distances, often through

populated regions.

• Convergent plate boundaries are zones of greatest risk

as these are linked to the largest recorded earthquakes,

explosive volcanic eruptions, and tsunamis.

Chapter 2 Summary, cont.