GEO CO5

Plate Tectonics

Plate Tectonics

Earth’s surface is composed of a few large, thick plates that move

slowly and change in size

Plate boundaries

plates move away, toward, or past each other through intense

geologic activity

Alfred Wegener

noted South

America, Africa, India, Antarctica, and

Australia have almost identical late

Paleozoic rocks and fossils in early 1900s

Mesosaurus

This fossil was found in brazil only

Pangaea

supercontinent proposed by

Wegener

Laurasia

northern supercontinent

containing North America and Asia

(excluding India)

Gondwanaland

southern supercontinent

containing South America, Africa, India,

Antarctica, and Australia

(the name is kinda stupid idk)

Late Paleozoic period

Glaciation patterns were evident on the

southern continents (Gondwanaland)

• Coal beds deposited in the northern

continents from swampy, probably warm

environments (Laurasia)

Paleoclimate belts

suggested polar

wandering as potential evidence for

continental drift

Polar Wandering

the apparent movement of

the poles

true

Wegener’s proposed mechanism was not accepted by most

geologists in the northern hemisphere (true/false)

Paleomagnetism

– the study of ancient

magnetic fields

curie point

The mineral magnetite becomes

magnetized in cooling lava once its

temperature drops below the ________

200

Pangaea split apart _______ million years ago but the

continents have been in motion for much longer (2

to 4 billion years)

Seafloor spreading

the concept

that the sea floor is moving like a

conveyor belt away from the crest

of the mid-oceanic ridge until it

disappears by plunging beneath a

continent or island arc. Proposed

in 1962 by Harry Hes

Deep Mantle Convection

circulation pattern driven by

rising of hot material (hot mantle

rock) and/or the sinking of cold

material (oceanic crust)

Mid-Oceanic Ridge

• Hot mantle rocks rises beneath.

• Decompression melting occurs.

• Circulation pattern diverges moving rock away from the ridge.

• Rift valley forms due to tensional forces at the ridge crest

Oceanic Trenches

• Rock has cooled and become denser.

• Crust Sinks beneath a continent or island arc back into the

mantle.

Tectonic plates

– composed of the

relatively rigid lithosphere

• Float upon ductile asthenosphere.

• Interact at their boundaries.

true

Lithospheric thickness and age of seafloor

increase with distance from mid-oceanic

ridge.

(true/false)

divergent boundaries

plates move apart

• Can occur in the

middle of the

ocean or within a

continent.

• Marked by rifting,

basaltic

volcanism, and

eventual ridge

uplift.

• Eventually creates

a new ocean

basin.

convergent boundaries

plates move together

transform boundaries

plates slide past one another

Two offset segments of mid-oceanic

ridge.

A mid-oceanic ridge and a trench.

Two trenches.

• Transform offsets of mid-oceanic

ridges allow series of straight-line

segments to approximate curved

boundaries required by spheroidal

Earth

Marine magnetic anomalies

alternating positive and negative

magnetic anomalies that form a stripelike pattern parallel to the mid-oceanic

ridges

Vine-Matthews Hypothesis

New basaltic magma continually

extrudes at the ridge crest and cools to

record the earth’s magnetism including

magnetic field reversals

• Matches pattern of reversals seen in

continental rocks which allows us to

measure the rate of movement and to

predict the age of the sea floor

false

seafloor age DECREASES with distance from mid-oceanic ridge

Transform fault

fracture zone segment between offset

ridge crests

Ocean-ocean plate convergence

marked by ocean trench, Benioff

zone, and volcanic island arc

Ocean-continent plate convergence

marked by ocean trench, Benioff

zone, volcanic arc, and mountain belt

Continent-continent plate convergence

marked by mountain belts

and thrust faults

true

plate boundaries move over time AND change size(true/false)

smaller

North American plate is increasing in size and the Nazca plate is getting __________

Mantle convection

may be the cause or an effect of circulation up by ridge-push and/or slab-pull

Ridge Push

as new plate moves away from the

divergent boundary it cools and thickens and subsides

Slab Pull

cold lithosphere sinking at a steep angle

through the hot mantle should pull the surface part of

the plate away from the ridge crest.

Trench Suction

if subducting plates fall into the

mantle at angles steeper than their dip then trenches

and the overlying plates are pulled horizontally seaward toward the subducting plate

Mantle Plume

narrow columns of hot

mantle rock that rise through the mantle

Stationary with respect to moving plates.

hot spot

Large mantle plumes may spread out and

tear apart the overlying plate forming a

_______________ at the Earth’s surface

volcanic chains

Mantle plume hot spots in the interior of a plate produce __________

true

• Deep interior of the Earth must be studied indirectly

(true/false)

Geophysics

the branch of geology that studies the interior of the earth

12km

deepest drilled hole reached about _________, and DID not reach the mantle

Seismic waves

vibrations from a large

earthquake will pass

through the entire Earth

Seismic reflection

the return

of some waves to the surface

after bouncing off a rock layer

boundary

• Sharp boundary between two

materials of different densities

will reflect seismic waves

Seismic refraction

bending of

seismic waves as they pass from

one material to another having

different seismic wave velocities

Crust

the outer layer of rock that

forms a thin skin on Earth’s surface

Mantle

a thick shell of dense rock

that separates the crust above from the

core below

Core

the metallic central zone of

the Earth

Continental Crust

• Average thickness of 30 to 50 m

(18.6 to 31 miles).

• Seismic waves travel about 6 km/sec

through continental crust.

• Composed of felsic rocks (granite)

Oceanic Crust

• Average thickness of 7 km (4.3

miles).

• Seismic waves travel about 7km/sec

through oceanic crust.

• Composed of mafic rocks (basalt).

Seismic Shadow Zones

• Primary evidence for existence and nature of

  Earth’s core

• Specific areas on the opposite side of the Earth

  from large earthquakes do not receive seismic

  waves, resulting in _________________

S–wave shadow zone

(≥103° from epicenter)

suggests outer core is a liquid

P–wave shadow zone

(103–142° from

epicenter) explained by refraction of waves

encountering core-mantle boundary

true

Careful observations of P-wave refraction

patterns indicate inner core is LIQUID (true/false)

iron nickel alloy

composition of the earth’s core

D layer

___________- at the base of

the mantle, is marked

by great changes in

seismic velocity,

density and

temperature

Isostasy

equilibrium of

adjacent blocks of brittle

crust “floating” on upper

mantlew

Isostatic adjustment

rising

or sinking of crustal blocks to

achieve isostatic balance

• Crust will rise when large

mass is rapidly removed

from the surface, as at end

of ice ages.

crustal rebound

Rise of crust after ice sheet

removal is called ___________

Gravitational force

determined

by the mass and the distance

between objects

Gravity meters

detect tiny

changes in gravity at Earth’s

surface related to total mass

beneath any given point

true

Gravity is slightly higher over dense

materials and slightly lower over

less dense materials.

TLDR: Gravity attraction is higher when mass/density is higher

(true /false)

Magnetic reversals

– times when the poles of

Earth’s magnetic field switch

• Recorded in magnetic minerals

• Occurred many times; timing appears chaotic

Magnetic Anomalies

local increases or

decreases in the Earth’s magnetic field

strength

true

Positive and negative magnetic

anomalies represent larger and smaller

than average local magnetic field

strengths, respectively

(true/false)

magnetometers

instruments used to

measure local

magnetic field

strength

• Can detect metallic

ore deposits, igneous

rocks, and thick layers

of non–magnetic

sediments beneath

Earth’s surface

Geothermal Gradient

temperature increase with depth into the Earth

• Tapers off sharply beneath lithosphere

• Due to steady pressure increase with depth, increased temperatures

produce little melt except in the outer core

Heat flow

the gradual loss of heat through Earth’s surface

• Major heat sources include original heat and radioactive decay

• Locally higher where magma is near surface

• Same magnitude, but with different sources, in the oceanic and

continental crust

Geologic structures

dynamicallyproduced patterns or arrangements

of rock or sediment that result from,

and give information about, forces

within the Earth.

• Produced as rocks change shape

(folded or faulted) and orientation

in response to applied stress

Structural geology

– the study of

the shapes, arrangement, and

interrelationships of rock units

and the forces that cause them.

Stress

force per unit area.

Strain

a change in size or shape

in response to stress.

• Geologic structures are

indicative of the type of stress

and its rate of application, as

well as the physical properties

of the rocks or sediments

Elastic deformation

deformed materials

return to original shape after stress is

removed.

Elastic limit

the point beyond which the

rock will not return to its original shape and

is permanently deformed

Ductile deformation

material is bent and

will not return to its original shape once the

stress is removed

Brittle deformation

material is bent and

will not return to its original shape once the

stress is removed

outcrops

Rock structures are determined on the

ground by geologists observing rock

__________, places where bedrock is

exposed at the surface

Geologic Maps

a map which uses

standardized symbols and patterns to

represent rock types and structures.

Strike

– the compass direction

of a line formed by the

intersection of an inclined plane

with a horizontal plane

Dip

the direction and angle

from horizontal in which a plane

is oriented.

strike and dip

Tilted beds, joints, and faults

are planar features whose

orientation is described by their ____________-

Geologic Cross Sections

a vertical slice through a portion

of the Earth

Folds

wavelike bends in layered rock

that represent rock strained in a ductile

manner, usually under compression.

Anticlines

upward-arching folds

Synclines

downward-arching folds.

Axial plane

divides a fold into its two

limbs

Hinge line

surface trace of an axial

plane

Plunging Fold

fold in which the

hinge line is not horizontal

Domes

structures in which the

beds dip away from a central

point, sometimes called doubly

plunging anticlines.

Basins

structures in which the

beds dip toward a central point,

sometimes called doubly plunging

synclines

Open

folds in which

the limbs dip gently.

Isoclinal

– folds with

parallel limbs.

Overturned

have

limbs that dip in the

same directions

Recumbent

overturned to the point

of being horizontal

Joints

fractures bedrock along which

no movement has occurred.

Columnar jointing

caused by

contraction of a cooling lava flow

Sheet jointing

caused by expansion

due to pressure release by removal of

overburden.

Joint Sets

multiple parallel joints

Faults

fractures in bedrock

along which movement has

occurred

Dip-slip faults

movement

is parallel (vertical) to the dip

of the fault plane