Plate TectonicsChapter 7

Earth Science

Bellwork Ch7 L.1

Pangaea: one supercontinent that all the continents were once part ofContinental drift: suggests that continents are in constant motion on the surface of Earth

Launch Lab p. 217● Carefully peel clementine (set fruit aside)

o DO NOT EAT● Try to refit the peel back together like a puzzle● Trade with your neighbor and try to reassemble each

other’s peel● Which peel was easier to reconstruct? Why?● Look at a map, do the continents seem to fit together?● What additional evidence would you need to convince

yourself that they were once together?● TURN IN ANSWERS. Now, you may eat your fruit.

Pangaea

● Theory proposed by Alfred Wegenero all continents are

constantly movingo all continents were

once part of a supercontinent

Evidence for continental drift● Climate clues: study

sediments deposited by glaciers on different continentso Glacial grooves= scratches in

rocks made where glaciers were moving found on neighboring

continents

Evidence for continental drift cont’d● Fossil clues

o Look at animals present on different continents, some are specific to a single continent (kangaroos in Australia)

o Similar fossils found on separate continents! Glossopteris- fossil of a

plant found on five different continents.

Evidence for continental drift cont’d

● Rock clueso Determine where

rocks formedo Rock formations on

different continents had similar origins

Missing evidence

● HOW did the continents move?o What were the

forces?● Continental drift not

accepted until decades after Wegener death.

Homework Ch7 L.1

● p. 222 #1-8● Study for quiz● OUTLINE OF THE CHAPTER IS NOW THE

EXTRA CREDIT ASSIGNMENT

Bellwork Ch7 L.2Mid-ocean ridge: mountain ranges in the middle of oceansSeafloor spreading: process by which new oceanic crust forms along a mid-ocean ridge and older oceanic crust moves away from the ridgeNormal polarity: a state in which magnetized objects, such as compass needles, will orient themselves to point north.Magnetic reversal: reverses the magnetic field directionReversed polarity: the state in which magnetized objects would reverse direction and orient themselves to point south

Mapping the ocean floor

● Echo sounder used to determine depth of ocean flooro Created topographical mapo Saw vast mountain rangeso Mid-ocean ridges longer

than any mountain range on land.

Seafloor spreading● Steps

o Seafloor spreads and mantle below melts forming magma

o Magma rises through cracks and erupts to become lava

o Lava cools on ocean floor to form basalt rounded structures called pillow lavas

● Seafloor spreads apart and crust moves away from ridgeo closer to the ridge means youngero IF sea floor spreads then continents

must be moving

Topography of the sea floor

● Sediment accumulates over crust (over time)

close to ridge have almost no sediment

far from ridge have thick layer of sediment

● smooth seafloor● Called abyssal plain

Moving continents around

● Continents move as the sea floor spreads along a mid-ocean ridgeo NOT through solid

mantle or the sea floor

Magnetism

● Iron-rich outer core= magnet that creates Earth’s magnetic fieldo Normal polarity=

compasses point northo Polarity changes every

few hundred thousand to a million years

Rocks and magnetism● Basalt on ocean floor

contains elements that are magnetico minerals act like little magnets

● Lava cools and permanently records direction and orientation of the magnetic field at the time of eruption

Evidence to support theory● Scientists used

magnetometer to measure and record magnetic signature of rocks from seafloor o Parallel magnetic strips

on either side of mid-ocean ridge confirm that crust is carried away from ridge in opposite directions

Homework Ch7 L.2

● p.230 #1-11● Study for quiz

Bellwork Ch7 L.3Plate tectonics: Earth’s surface is made of rigid slabs of rock, or plates, that move with respect to each other.Lithosphere: cold and rigid outermost rock layerDivergent plate boundary: forms where two plates separateTransform plate boundary: forms where two plates slide past each otherConvergent plate boundary: forms where two plates collideSubduction: where the denser plate sinks below the more buoyant plante in a collisionConvection: circulation of material caused by differences in temperature and densityRidge push: force that pushes plates away from each other. Caused by rising mantle material at mid-ocean ridge.Slab pull: force caused by a sinking slab that pulls on the rest of the plate

Surface area conundrum

● If crust continues to form and is never destroyed then the area of the Earth should keep getting bigger and biggero IT DOESN’T!

Plate tectonics theory● Two points

o Earth’s surface is divided into large plateso Each plate moves over Earth’s surface

● Plates separate in ocean = earthquake and mid-ocean ridge formation● Plates come together= (one plate dives under another one) earthquake

and chain of volcanoes form● Plates slide against one another= earthquake

Tectonic plates

● Plates float on hot mantle (called asthenosphere)o Pacific plate is the largesto Boundaries can mark locations of mid-ocean ridgeso Plates are large pieces of lithosphere

● Lithosphere: cold and rigid outermost rock layer of Eartho made of crust and upper mantleo thin below mid-ocean ridgeso thick below continents

Plate Boundaries

● Exists where plates meet● Three kinds:

o Divergento Transformo Convergent

Divergent plate boundary● Forms where two plates separate● Mid-ocean ridges located at divergent plate

boundaries● Rift valleys formed by divergent plates on

land

Transform plate boundaries

● When two plates slide past one anothero Can get stuck and stress breaks rock apart and then

keeps moving rapid movement results in earthquakes

Convergent plate boundaries● Form where two plates collide

o the denser plate sinks below the more buoyant plate (called subduction) the denser plate sinks into the subduction zone

● Oceanic x Continental collisiono Denser Oceanic subducts under continentalo Forms volcanoes above subduction plate

● Continental x Continental collisiono mountains form from uplifted rocks. No subduction.

Density lab p.233● Work in pairs

o Each pair gets a set of 4 test tubes with different liquids in themo Drop a bead of equal density into each test tubeo Observe what happens to each bead

Which liquid has the highest density?How did you determine which liquid has the highest density?What happens when rocks of opposite densities collide?

Evidence for plate tectonics● Scientists can now

measure how fast continents are movingo Wegener couldn’t do this

● Explains why earthquakes and volcanoes occur in certain positionso Where do you think they

occur?

Plate Motion● Explains the HOW of

Wegener’s theory● Convection Currents

o circulation of material caused by differences in temperature

o Convection in the mantle (radioactive elements heat mantle)

o Heated elements expand and contacts Earth’s crust transferring thermal energy

o Heated elements then cool and sink back down. This expansion and sinking creates a current.

Plate motion cont’d● Forces causing plate motion

o Basal Drag caused by convection currents, they drag lithosphere similar to a

conveyor belt moves groceries.o Ridge Push

is the force that moves the plates away from the ridge by using the potential from the rising mantle at the mid-ocean ridge. Pushes the lithosphere in opposite directions.

o Slab Pull (slab= denser plate that sinks in subduction zone during collision) as the slab sinks it pulls the rest of the plate with a force

A theory in progress

Unanswered questions:● Why is the Earth the only planet (in our

system) with plate tectonics?● Why do earthquakes and volcanoes occur

far away from plate boundaries?● What forces dominate plate motion?

Homework Ch7 L.3

● p.241 #1-8● Study for quiz● STUDY FOR TEST● Extra Credit: outline of Chapter 7