Plate Tectonics

Drifting ContinentsChapter 17.1

Vocabulary

• Continental Drift – the continents were joined as a single landmass that broke apart 200 mya– Still drifting

• Pangaea – ancient landmass made up of all the continents

• Alfred Wegener – found evidence to support the theory of continental drift

http://www.divediscover.whoi.edu/tectonics/pangea-animation.html

Evidence of the Drift

• Rock formations– Large geologic structures, such as mountain

ranges fractured as the continents split– There should be similar rock types on opposite

sides of the Atlantic• Rocks on the Appalachians are identical to rocks in

Greenland and Europe

Evidence of the Drift

• Fossil formations– Similar fossils of several different animals and

plants that lived on or near land had been found on several different continents

• Land dwelling animals could not possibly have swum the great distances between continents

• Trilobites • Ages of fossils predated the breakup of Pangaea

Evidence of the Drift

• Climatic evidence– Fossils of plants indicating the same type of

climate have been found• On different continents• In current climates where they wouldn’t have survived

Evidence of the Drift

• Coal deposits – In Antarctica show that the land must have been

at one time closer to the equator• Glacial deposits

– 290 million year old deposits found in warm climates

– Land must have at one time been located near the south pole

Evidence of the Drift

• Wegener’s idea was generally rejected– Most scientists believed in the early 1900’s that

the continents were fixed– 2 flaws in the theory

• What force was strong enough to move the continents?• How could the continents move through solids?

Plate Tectonics

Seafloor Spreading17.2

Vocabulary

• Sonar – use of sound waves to detect and measure objects under water

• Magnetic reversal – when Earth’s magnetic field changes polarity between normal and reversed

Magnetic field demo

• Magnetometer – used to map the ocean floor by detecting small changes in magnetic fields

• Sometimes the magnetic field of the earth completely flips. – The north and the south poles swap places. – Such reversals, recorded in the magnetism of

ancient rocks, are unpredictable.– They come at irregular intervals averaging about

300,000 years; • the last one was 780,000 years ago. Are we overdue for

another? No one knows.

• Isochron – imaginary line on a map that shows points of the same age; formed at the same time

• Seafloor spreading – ocean crust is formed at mid-ocean ridges and destroyed at deep-sea trenches– Continuous cycle of magma intrusion and

spreading

Ridges and Trenches

• Mid-Atlantic Range – chain of underwater mountains that run throughout the ocean basins– Lenth of 65,000 km– Contains active and extinct volcanoes

• Topography – change in elevation in the crust

Tectonic PlatesPlate Boundaries

17-3

Tectonic plates

• Huge pieces of crust and upper mantle that fit together at their edges to cover Earth’s surface– 12 major plates and several minor plates– Move slowly

• In different directions and at different speeds in relation to each other

• Edges are called boundaries

Types of Boundaries

• Divergent (divide) Boundary• Convergent (collision) Boundary• Transform Boundary

Divergent Boundaries

• Definition – place where two of Earth’s tectonic plates are moving apart– Associated with volcanism, earthquakes, and heat

flow– Found primarily in the seafloor

Divergent Plate Boundaries

• Rift valley – long, narrow depression that forms when continental crust begins to separate at divergent boundaries

Convergent Boundaries

• Three types– Oceanic – Oceanic– Oceanic – Continental – Continental – Continental

• Subduction – process by which one tectonic plate slips beneath another tectonic plate

Oceanic – OceanicConvergent Boundary

Oceanic – Continental Convergent Boundary

Continental – ContinentalConvergent BoundarySubduction zone

Transform Boundaries – most likely to cause earthquakes

Transform Boundaries

Convection Chapter 17.4

Back to Wegener

• Remember the two flaws to his theory of continental drift?– What type of force could possibly move the

continents?– How do the continents move through solids?

Convection is the answer!!!

• Large scale motion in the mantle• Transfer of thermal energy by the movement

of heated material

Let’s talk about States of Matter

• As matter cools– It contracts– Becomes denser– Cooled matter than drops due to gravity

• Warmer matter – Is displaced– And then rise

So how’s it work in the Earth?!?

• Up and down flow produces a pattern of motion called a convection current

• Convection currents aid in the transfer of thermal energy– From warmer to cooler regions

• Earth’s mantle is composed of partially molten material– Radioactive decay heats up the molten material in

the mantle– Causes enormous convection currents to move

material throughout the mantle

Convection in the Mantle

• Driving mechanism of plate movements– Stiff part of mantle attached to the crust (cool)– Farther below, the mantle is hot and pliable

– So…– The cool drops and is heated– The warm rises and cools– And the cycle continues

• So… how does it all get started?– Set in motion by subduction– Move just a few centimeters per year

• So how are convergent and divergent movement related to mantle convection?– Rising material spreads out as it reaches the upper

mantle– Causes both upward and sideways forces– Downward part of convection occurs where

sinking force pulls tectonic plates downward