EXAM 2 Review

Continental Drift Hypothesis: Observation

shapes of continents match across the Atlantic Ocean

Continental Drift Hypothesis: Interpretation

continents fit back together because once joined and later moved apart

What did Alfred Wegener propose in 1912?

the continental drift hypothesis

What is Pangaea?

All landmasses were originally united into a supercontinent.

Pangaea consisted of what two landmasses?

• A northern landmass called Laurasia

• A southern landmass called Gondwana

What is the evidence for Continental Drift?

• Continental fit

• Rock sequences and mountain ranges

• Fossil evidence

Continental Fit

There are close fits between the continents at their margins off the coasts at depths of about 2000 m- like giant puzzle pieces.

Rock sequences and mountain ranges

• Ocean, land, and glacial rock sequences of the Pennsylvanian to Jurassic age are nearly identical on all the Gondwana continents.

• Mountain ranges stretch across continents

Fossil evidence: Observation

Fossils and similar rock types are found on continents no separated by oceans

Fossil evidence: Interpretation

Continents were joined so land creatures could walk from one place to another

Features of the Seafloor

• Continental shelves

• Continental slopes

• Continental rises

• Abyssal plains

• Oceanic ridges

• Submarine vents

• Oceanic trenches

Continental margins include…

• the continental shelf

• the continental slope

• a continental rise

Abyssal plains

• Are the flattest locations on Earth

• Cover large areas of the seafloor

• Deep sea sediments accumulate on their surfaces

Mid-ocean (oceanic) ridges

• Long, continuous submarine mountain ranges composed of volcanic rock

• Nearly encircle the world but are offset at intervals by large fracture zones

Submarine hydrothermal vents

• Seafloor vents at or very near mid-oceanic ridges where circulating water is heated to over 400*C (~750*F) and discharged into seawater

• As the hot water circulates through the oceanic crust it dissolves metals that discharge as plumes, called black smokers.

Ocean trenches

are the seafloor expressions of subduction zones

• they are long, narrow, features that reach the greatest oceanic depths, sometimes >1.1 km

• characterized by low heat flow and frequent seismic activity

What creates Earth’s magnetic field?

Heat transfer from inner solid core to the liquid outer core, causing convection in the liquid core.

What is required for a magnetic field?

• Conducting, convecting material

• The outer core is molten iron-nickel metal so it conducts electricity and magnetism

• Earth’s rotation speed is sufficient for convection

Three magnetic measurements can be taken from rocks:

• Inclination- ~latitude and ~distance to the pole

• Declination- ~direction to the pole

• Positive (normal) or negative (reversed)- depending on what Earth’s field is doing

Paleomagnetism

is the remnant magnetism in ancient rocks, recording the direction and intensity of Earth’s magnetic field at the time of the rock’s formation.

What did Henry Hess proposed in 1962?

the theory of seafloor spreading

What did he suggested about the seafloor spreading?

He suggested that the seafloor separates at oceanic ridges, where new crust is formed by upwelling magma.

As the magma cools, the newly formed oceanic crust moves laterally away from the ridge symmetrically.

How was seafloor spreading confirmed?

• the ages of fossils in ocean sediments

• radiometric dating of the oceanic crust

The Plates

According to plate tectonic theory, the rigid lithosphere is divided into different-sized plates

What are the three main types of plate boundaries?

• Divergent (move apart)

• Convergent (Collide)

• Transform (slide past each other)

Divergent boundaries

form when two plates move away from each other

Where do most divergent boundaries occur?

Along the crests of oceanic ridges

• Mid-Atlantic Ridge

• East Pacific Ridge

Convergent boundaries

where two plates converge

What are three types of convergent boundaries?

• Oceanic-oceanic boundary

• Oceanic-continental boundary

• Continental-continental boundary

Oceanic-oceanic boundary

one oceanic plate subducts beneath the other and a volcanic island arc forms on the non-subducted plate

Oceanic trench

forms parallel to the volcanic island arc where the subduction occurs

What are some examples of oceanic-oceanic boundaries

• Japan

• Aleutian Islands (Alaska)

Volcanos result from…

rising magma produced by the partial melting of the subducting plate

Oceanic-continental boundary

an oceanic plate and a continental plate converge, with the denser oceanic plate subducting under the less dense continental plate

What are some examples of oceanic-continental boundaries?

• Andes (South America)

• Cascades (NW US)

Continental-continental boundary

two continents converge, the ocean floor separating them subducts, the two continents collide. Neither plate will subduct because they are both low-density.

Subduction at…

convergent boundaries involving an oceanic plate will produce volcanos and earthquakes of all depths

Earthquakes occur all along the…

subducting plate, resulting in shallow, medium, and deep earthquakes

when two continents collide, they are welded together to form an…

Interior mountain chain

What are some examples of continental-continental boundaries

• the Himalayan Mts. Formed from Indian-Asia collision

Recognizing ancient convergent plate boundaries

intensely deformed rocks, andesite lavas, and ophiolites

Ophiolites

chunks of oceanic crust and underlying mantle that have been uplifted and are found on land.

Transform boundaries

these are boundaries along which plates slide laterally past each other along transform faults

What are some examples of transform boundaries

• Mid-ocean ridge segments

• San Andreas fault in California

Ring of Fire

is a large concentration of high magnitude earthquakes and volcanos surrounding the Pacific Ocean

Hot spot

Location on Earth’s surface where a stationary column of magma, originating deep within the Earth has slowly risen to the surface and caused volcanism

What is an example of a Hot spot?

Hawaii

Seamounts

extinct oceanic volcanos. They formed as the oceanic crust moved over a hot mantle plume

The resulting hot spot leaves a trail of…

extinct and progressively older volcanoes that record the movement of the plate

What do Hot Spots provide?

• may be used to determine motion of plates

• they provide an apparently fixed reference point from which the rate and direction of plate movements can be measured

What moves the plates?

a convective heat system is the basic process responsible for plate motion

Sources of heat

• Accretion

• Differentiation

• Radioactive decay

Mantle convection cell model

• Cold stuff sinks

• Hot stuff rises

Spreading centers (ridges)

hot ascending limbs of cells

Trenches

cooled part of convention cells descend

Slab-pull

involves pulling the plate behind a subducting cold slab of lithosphere

Ridge-push

involves gravity pushing the oceanic lithosphere away from the higher spreading ridges and toward the subduction trenches

Deformation

any change in the shape or volume of a rock, such as when a rock is folded (bent) or fractured (broken-joints/ faults)

Where does deformation occur?

in building large mountain ranges at convergent plate boundaries through:

• emplacement of plutons

• volcanism

• metamorphism

• continental accretion

How does deformation happen?

• Force is a push or pull that causes a change in motion

• Stress is a force of deformation applied over an area

How do rocks respond to stress?

• Displacement

• Rotation

• Strain

Displacement

a volume of rock can be moved, or displaced, from one place to another, such as along faults

Rotation

a volume of rock can be rotated or tilted in response to stress

Strain

any change in shape, size, or orientation of a volume of rock

Elastic deformation

occurs if rocks return to their original shape when the stress is released.

Plastic deformation

happens when rocks fold or fracture when stress is applied and do not recover their original shape

Whether a rock will fracture or bend elastically or plastically depends on…

• type of stress applied

• pressure and temperature

• rock type

• length of time

How does rock behavior respond to changes in stress, temperature, and fluids?

• At shallow depths and low temperatures, most rocks are brittle and break

• At greater depths, temperature and pressure are higher and rocks flow/bend as weak solids

How does mineral behavior respond to changes in stress, temperature, and fluids?

• At shallow depths and low temperatures, minerals may be unaffected

• Deeper and at higher temperatures, minerals may recrystallize or new minerals may grow

Compression

shortens the rock layers by folding or faulting

Compression: Shallow levels

rocks fracture

Compression: Deeper levels

rocks flow and fold

Tension

lengthens the rocks or pulls them apart; fractures and faults form

Tension: shallow levels

rocks fracture

Tension: Deeper levels

rocks flow and stretch

Shear

deformation occurs along closely spaced planes like the slip between cards in a deck

Shear: shallow levels

rocks fracture

Shear: Deeper levels

rocks flow and fold

Principle of original horizontality says…

that most rocks are originally laid down flat.

When we see rocks that are inclined (tilted), they have been deformed by folding and/or fracturing.

Principle of superposition says…

that rock layers on the bottom are older in an undisturbed stack of rock

Strike

intersection of a horizontal plane with an inclined plane

• side-to-side motion

Dip

the maximum angle of an inclined plane

• up or down motion

Geologic structures

are rocks that have been deformed (including fracturing and/or folding)

Anticline

rock layers generally warped down; oldest rocks in the center of fold- shaped like an A

Syncline

rock layers generally folded up; youngest rocks in center of fold- middle sinks down

Joints

fractures along which no movement has taken place parallel to the fracture surface, although movement may occur perpendicular to the surface. They are not faults.

• Occur in almost all surface rocks from a few different mechanisms

Faults

fractures along which the opposite sides have moved relative to one another and parallel to the fracture surface.

Hanging wall

block above the fault (Where miners would hang their lanterns)

Footwall

block below the fault (miners could walk on the footwall)

Dip-slit faults

all movement is in the direction of dip (up or down) along dip-slit faults.

Normal dip-slip fault (from tension)

• hanging wall moves down relative to the footwall

• horst and graben formations occur with this type of fault

Reverse dip-slip fault (from compression)

• The hanging wall moves up relative to the footwall

Strike-slip faults

move with a side-to-side motion, parallel to the strike of the fault - not up or down

Block on opposite side of the fault moves to left

left lateral

Block on opposite side moves to right

right lateral

Valles Marineris, Mars

giant normal fault system- extension from large amounts of magma rising to form giant volcanoes, including Olympus Mons, caused the crust to pull apart

Europa, Jupiter’s moon

is covered in an ice shell covering a water ocean. Jupiter’s gravity causes the ice to break and move around, resulting in strike-slip faults

Venus

faults related to magma emplacement

Enceladus, Saturn’s moon

small icy moon with “tiger stripes” near the south pole- these are normal faults caused by Saturn’s gravity pulling the moon apart. Water from a subsurface ocean comes out of the fault as geysers.

What is an earthquake?

occurs when energy stored in stocks is suddenly released.