Chapter 10 Plate Tectonics

Continental Drift Section 1

Theory First person to think the continents fit together – mapmaker, Abraham Ortelius First to propose the idea of Continental Drift was meteorologist – Alfred Wegener – Hypothesis that continents were connected but broke apart 200 million years ago and drifted to current position

Pangaea Wegener suggested the continents were once connected as one large landmass called Pangaea

Evidence for Continental Drift 1. Fossil clues – Fossils from the same animals were found on different continents Mesosaurus – Same plant fossils found on different continents Glossopteris

2. Climate clues – Fossils of tropical plants were found in very cold places – Glacier evidence such as … ? were found in tropical places

3. Rock clues – Similar rock structures are found on different continents Ex: Appalachian Mtns. = Greenland, western Europe

How did the continents move? Continental Drift idea – REJECTED

Continents Adrift Summary 1.Wegner found fossils of identical animals on different continents. He also found plant fossils that were identical on different continents. He knew that some of these plants could not have existed on these planets unless they were joined.

2. Scientists did not believe Wegner until they studied the ocean floor. (Sea Floor Spreading) 3. Lots of Earthquakes near the mid- ocean ridge because of plate tectonics.

4. The Earth appears to be cracked Into pieces called plate tectonics. (Like a cracked egg shell) 5. Some plates move in opposite directions (Divergent) Some plates collide (Convergent) Some plates slide past each other (transform boundary)

6. Plates carrying an ocean basin is heavier and will subduct under the continental plate. 7. Earthquakes will occur near boundaries. 8. Volcanoes occur near subduction zones.

Seafloor Spreading Section 2

Mapping the Ocean Bottom Sound waves – echo off the ocean floor, the longer it takes to return to the boat the deeper the water is.

Mid-ocean Ridges System of ridges, like a mountain range underwater, where new ocean floor forming from rising magma

Seafloor Spreading Harry Hess – Princeton University Sea floor spreads apart allowing the molten material to rise and cool forming new sea floor that is dense and thin Occurs at a mid-ocean ridge

Evidence for Spreading Glomar Challenger – research ship – Took samples of rock on ocean floor – Found rocks further away from the mid-ocean ridge were older

Organisms near mid-ocean ridges

Evidence for Spreading Paleomagnetism – The study of the change in the Earth’s magnetic field preserved in magnetic minerals through time – Iron-bearing minerals in the rock crystallize, line up, and point to magnetic North – Like little compass needles

Reverse Polarity – The Earth’s magnetic field can flip – Normal polarity- the magnetic field at present – Reverse polarity- the magnetic field flipped

Polar wander curve

Theory of Plate Tectonics Section 3 Theory that Earth’s crust and upper mantle are broken into plates that float and move around on a plastic-like layer of the mantle

Plates The crust and part of the upper mantle (lithosphere) that “floats” on the asthenosphere

Inner core: ~ solid ~ mostly iron Outer core: ~ liquid Mantle : ~ rocky ~ plastic like Crust: ~ rocky ~brittle

Lithosphere Crust and part of the upper mantle 100 km thick Less dense than material underneath

Asthenosphere Plastic-like layer below the lithosphere The lithosphere floats on the asthenosphere

Type of Plate Boundaries NOAA Learning Objective (video) NOAA Learning Objective (video)

1-Divergent Boundaries Two plates that are moving apart This is seafloor spreading Hot material rises to create new lithosphere Volcanoes and earthquakes common Rift valley – the crack Deep-sea trenches Ex: Mid-Atlantic Ridge

Mid-Atlantic Divergent Plate Boundary

Why doesn’t Earth’s surface keep expanding?

2-Convergent Plate Boundaries Two plates move together A: Continental plate vs. continental plate – Same densities, neither sinks below the other causing folded mtns. – Earthquakes common – EX: Appalachian Mtns., Himalayan Mtns., Alps

B: Continental plate vs. oceanic plate – Oceanic plate is more dense so sinks or subducts under continental plate – SUBDUCTION ZONE: WHERE ONE PLATE SINKS BELOW ANOTHER – High temps melt the subducting rock – Volcanoes and earthquakes common – EX: Cascade Mtns. in Washington, Andes Mtns.

Andes Mountains

C: Oceanic plate vs. oceanic plate The colder, older, denser plate will subduct under the other Subduction of two oceanic plates causes island arcs Volcanoes and earthquakes common EX: Philippines, Japanese Islands

3-Transform Plate Boundaries Where two plates slide past one another – Can move opposite directions – Same direction at different speeds – Earthquakes common

EX: San Andreas Fault California

Why do plates move? 2 theories the most accepted: convection currents cause plate movement Convection currents – hot, less dense mantle rising and cool, denser mantle sinks

Different plate movements form at the different plate boundaries

Faults Caused by plate tectonics when rocks break and move along surfaces

Normal Faults Faults that are caused by tension forces – Tension forces – forces that are pulled apart Rock layers above the fault move down compared with rock layers below the fault. Occurs at divergent plate boundaries

Reverse faults Faults that are caused by compression forces – Compression forces – squeeze objects together The rock layer above the fault move up compared to the rock layers below Occurs at convergent plate boundaries

Strike Slip faults Rocks on opp. sides of the fault move in opp. directions Rocks on opp. sides of fault move in same direction, different speeds – San Andreas Fault Occurs at transform boundaries

Review Convergent plate boundaries – Continental plates – continental plates = mountain ranges – Continental plates – oceanic plates = subduction zone – Oceanic plates – oceanic plates = subduction zone Creates island arcs Divergent plate boundaries – Rift valleys – “crack” where material slips downward – Deep – sea trenches Transform plate boundaries – Plates slide past each other

Review Normal faults – Occur at divergent boundaries – Side above the fault sinks Reverse Faults – Occur at convergent boundaries – Side above the fault is pushed up Strike-slip faults – Occur at transform boundaries – Two sides move in opposite directions or same direction different speed.

Calculating a Rate 60,000,000 cm ÷ 10,000,000 years = 6 cm/yr - The Theory of Plate Tectonics To calculate the rate of plate motion, divide the distance the plate moves by the time it takes to move that distance. Rate = distance/time For example, a plate takes two million years to move 156 km. Calculate its rate of motion. 156 km/2,000,000 years = 7.8 cm per year Practice Problem The Pacific plate is sliding past the North American plate. It has take ten million years for the plate to move 600 km. What is the Pacific plate’s rate of motion?

Plate Tectonic Testing Satellite Laser Ranging System – Scientists on the ground aim laser pulses at a satellite – Pulses reflect off satellite and determine the precise location on the ground