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

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

Two Cities on a Plate Boundary

Topography is shaped

by Plate Tectonics!

Two cities on a Plate Boundary, cont.

• San Andreas fault system

- Many moderate to large earthquakes in Los Angeles on this

fault

- Mountain topography in coastal California result of fault

- Earthquakes since 1906 have cost hundreds of lives and

billions of dollars in property damage

• Future of the fault

- Los Angeles and San Francisco will be side by side in 20

million years

- May be a shift in the plate boundary and a change in the

topography

Origin of the Sun and Planets - Solar Nebula

• The sun and planets were born from a rotating disk of cosmic gas and dust,

the solar nebula

• The flattened form of the disk constrains the planets:

- To move in the same direction as the disk

- To have their orbits in the same plane

Planetary accretion

Accretion stages:

1. Accretion into miniature planets (diameter < 1 km)

2. Collisions between miniature planets form a few large planets

• All planets formed at the same time (~4.6 billion years ago)

Earth’s Early History

Heat-generating processes during the formative years of the Earth

cause differentiation

Differentiation

Differentiation: process by which gravity causes denser

material to gradually migrate to the center of a planet

Density increasing

from surface to

center

www.phys.org

The Geoid

The shape that the surface of the oceans would

take under the influence of Earth’s gravity and

rotation alone

Surface of the Earth

Land

Model of the Earth

Sea

Geoid Ellipsoid

Differentiation of the Earth

Earth is differentiated

into layers based on:

- Density

- Strength

www.phys.org

Internal Structure of Earth

• Internal processes have incredibly important impacts

on the surface of the Earth

• Responsible for continents and ocean basins

• Oceans’ currents and distribution of heat carried by seawater

controlled by configuration of continents and ocean basins

• Responsible for regional landforms

• Earth is layered and dynamic

• Internal structure of Earth

• By composition and density

• By physical properties (strength)

Earth and its Interior

Layers based on density

Crust:

Silicon & Oxygen Mantle:

Iron & Magnesium

Outer core:

Liquid iron

Inner core:

Solid iron

Layers based on density

Crust:

Silicon & Oxygen Mantle:

Iron & Magnesium

Outer core:

Liquid iron

Inner core:

Solid iron

Less dense

Dense

Very dense

Internal Structure of Earth,

cont.

Earth’s structure:

• Outer core

- Liquid

- 2,000 km (1,243 mi.) in thickness

- Composition similar to inner core

- Density (10.7 g/cm3)

• Inner core

- Solid

- 1,300 km (808 mi.) in thickness

- High temperature

- Composed of iron (90 percent by

weight) and other elements (sulfur,

oxygen, and nickel)

The core is a

“heat battery”

• The Earth is cooling down

• Cooling of the liquid outer core

• The inner core is growing over

time as the outer core cools

and solidifies!

• Tremendous heat is given off

as the liquid outer core

solidifies and the inner core

cools. >10,000 Giga-watts!

Internal Structure of

Earth, cont. • Mantle

- Solid

- 3,000 km (1,864 mi) in thickness

- Composed of iron- and magnesium-rich silicate rocks

- Average density 4.5 g/cm3

• Crust - Outer rock layer of Earth

- Density 2.8 g/cm3

- Moho discontinuity

- Separates lighter crustal rocks from more dense mantle

Layers based on density

Thin crust rich in silicon

and oxygen

Magnesium- and iron-

rich mantle

Iron-rich metallic core

Continental crust is

thicker and less dense

than oceanic crust

Continental vs. Oceanic Crust

Continental Crust

• Average thickness:

35-70 km

• Less dense

• Older (up to 4 Ga)

• Typically composed of

granite

Oceanic Crust

• Average thickness: 6-

7 km

• More dense

• Younger (less than

200 Ma)

• Typically composed of

basalt

Material Deformation

• When materials are subjected to external forces, stress,

they deform or undergo strain

• Stress applied perpendicular => stretching under tension,

or contraction under compression

• Shear stress =

parallel to surface

Material Deformation - responding to stress

Internal Structure of Earth, cont.

The outer surface of the Earth consists of several

lithospheric plates moving relative to each other as rigid

bodies on a fluid substratum called the asthenosphere

• Lithosphere

- Cool, strong outermost layer of Earth (crust and upper mantle)

- Crust embedded on top

• Asthenosphere

- Below lithosphere

- Hot, soft/ductile slowly flowing layer of weak rock

- Higher water content and hotter

Layers based on Strength

Gaseous atmosphere

Liquid hydrosphere

Rigid lithosphere

Soft plastic asthenosphere

Stiff plastic mesosphere

Liquid outer core

Solid inner core

Internal Structure of Earth, cont.

The boundary between lithosphere and asthenosphere not defined by

a difference in chemical compositions, but in mechanical properties

(i.e. the rigidity of the material, how the material deforms under stress).

CONTINENTAL

PLATE

OCEANIC

PLATE MANTLE LITHOSPHERE

CRUSTAL

LITHOSPHERE

ASTHENOSPHERE

Tectonic plates are lithospheric plates

Tectonic plates

are lithospheric

plates “floating”

on top of the

asthenosphere

Lithosphere –

asthenosphere

boundary at a

depth of ~100 km

Buoyancy • Earth can be described as a series of layers where less dense

material floats on top of denser material

- Low-density crust floats on top of the denser mantle

- Mantle floats on top of the very dense core

root

load

Isostasy Surface elevation represents a balance between forces:

- Gravity : pushes plate into mantle

- Buoyancy : pushes plate back to float higher on mantle

Isostatic equilibrium describes this balance.

Isostasy is compensated after a disturbance.

Adding weight pushes lithosphere down

Removing weight causes isostatic rebound

Compensation is slow, requiring asthenosphere to flow.

root

load

Isostasy in Canada

• ~18,000 years ago, Canada was buried under a continental glacier with ice thickness ~5 km around Hudson Bay

• The weight of the ice sheet caused the land to sink more than 1 km

• 10,000 years ago the ice sheet had melted and retreated

Heat Transfer

Heat can be transmitted through solids and fluids by

conduction, through fluids by convection, and by radiation.

Heat Transfer

On a planetary scale, the same processes are active!

- Heat from the interior of Earth flows

to the surface by conduction

- In the mesosphere and

asthenosphere, heat is redistributed

by flow of plastic solids

- Hot, less-dense materials rises

- Cold, denser material sinks creating

convection cells

Internal Structure of Earth, cont. • Convection

- Earth’s internal heat causes magma to heat up and become less dense

- Less dense magma rises

- Cool magma falls back downward

Internal Structure of Earth, cont. • Convection

- Earth’s internal heat causes magma to heat up and become less dense

- Less dense magma rises

- Cool magma falls back downward

How do we infer the structure of the Earth?

How do we infer the structure of the Earth, cont.

• Seismology!

- Study of earthquakes

- Information on wave movement

• Earthquakes cause seismic energy to move through Earth

(more later)

- Some waves move through solids, but not liquids

- Some waves are reflected

Incident ray

Reflected

ray

How do we infer the structure of the Earth, cont.

• Seismology!

- Study of earthquakes

- Information on wave movement

• Earthquakes cause seismic energy to move through Earth

(more later)

- Some waves move through solids, but not liquids

- Some waves are reflected

- Some waves are refracted

Global seismic observations

Quick intro. - Seismic waves

P waves (Primary waves): compressional motion, 6-8 km/s

S waves (Secondary waves): shear motion, 3-5 km/s. Do

not pass through liquids

Surface waves: travel along surface of earth, < 3-4 km/s

S-waves and the outer core

• S-waves do not propagate in a liquid

• Liquid cannot support shear motions

• This is how we infer that the outer core is liquid

• S-waves do not propagate through the outer core

Seismic Shadow Zones

Seismic tomography can also tell us the locations

of hot and cold regions in the mantle

(credit: Global Seismology Group / Berkeley Seismological Laboratory)