Plate Tectonics Chapter 7 Earth Science

Bellwork Ch7 L.1 Pangaea: one supercontinent that all the continents were once part of Continental drift: suggests that continents are in constant motion on the surface of Earth

Launch Lab p. 217 Carefully peel clementine (set fruit aside) 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 Wegener all continents are constantly moving all continents were once part of a supercontinent

Evidence for continental drift Climate clues: study sediments deposited by glaciers on different continents Glacial grooves= scratches in rocks made where glaciers were moving found on neighboring continents

Evidence for continental drift cont’d Fossil clues Look at animals present on different continents, some are specific to a single continent (kangaroos in Australia) Similar fossils found on separate continents! Glossopteris- fossil of a plant found on five different continents.

Evidence for continental drift cont’d Rock clues Determine where rocks formed Rock formations on different continents had similar origins

Missing evidence HOW did the continents move? 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.2 Mid-ocean ridge: mountain ranges in the middle of oceans Seafloor spreading: process by which new oceanic crust forms along a mid-ocean ridge and older oceanic crust moves away from the ridge Normal polarity: a state in which magnetized objects, such as compass needles, will orient themselves to point north. Magnetic reversal: reverses the magnetic field direction Reversed 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 floor Created topographical map Saw vast mountain ranges Mid-ocean ridges longer than any mountain range on land.

Seafloor spreading Steps Seafloor spreads and mantle below melts forming magma Magma rises through cracks and erupts to become lava Lava cools on ocean floor to form basalt rounded structures called pillow lavas Seafloor spreads apart and crust moves away from ridge closer to the ridge means younger 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 ridge NOT through solid mantle or the sea floor

Magnetism Iron-rich outer core= magnet that creates Earth’s magnetic field Normal polarity= compasses point north Polarity changes every few hundred thousand to a million years

Rocks and magnetism Basalt on ocean floor contains elements that are magnetic 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 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.3 Plate 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 layer Divergent plate boundary: forms where two plates separate Transform plate boundary: forms where two plates slide past each other Convergent plate boundary: forms where two plates collide Subduction: where the denser plate sinks below the more buoyant plante in a collision Convection: circulation of material caused by differences in temperature and density Ridge 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 bigger IT DOESN’T!

Plate tectonics theory Two points Earth’s surface is divided into large plates 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) Pacific plate is the largest Boundaries can mark locations of mid-ocean ridges Plates are large pieces of lithosphere Lithosphere: cold and rigid outermost rock layer of Earth made of crust and upper mantle thin below mid-ocean ridges thick below continents

Plate Boundaries Exists where plates meet Three kinds: Divergent Transform 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 another 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 the denser plate sinks below the more buoyant plate (called subduction) the denser plate sinks into the subduction zone Oceanic x Continental collision Denser Oceanic subducts under continental Forms volcanoes above subduction plate Continental x Continental collision mountains form from uplifted rocks. No subduction.

Density lab p.233 Work in pairs Which liquid has the highest density? Each pair gets a set of 4 test tubes with different liquids in them Drop a bead of equal density into each test tube 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 moving Wegener couldn’t do this Explains why earthquakes and volcanoes occur in certain positions Where do you think they occur?

Plate Motion Explains the HOW of Wegener’s theory Convection Currents circulation of material caused by differences in temperature Convection in the mantle (radioactive elements heat mantle) Heated elements expand and contacts Earth’s crust transferring thermal energy Heated elements then cool and sink back down. This expansion and sinking creates a current.

Plate motion cont’d Forces causing plate motion Basal Drag Ridge Push caused by convection currents, they drag lithosphere similar to a conveyor belt moves groceries. 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. 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