Transcript 8.2: Continents change position over time

Chapter 8: Plate Tectonics
8.1: Earth has several layers
8.2: Continents change position over time
8.3: Plates move apart
8.4: Plates converge or scrape past each other
8.2: Continents change position
over time
 Before, you learned:
 Earth’s main layers are the core, mantle, and crust
 The lithosphere and asthenosphere are the topmost
layers of Earth
 The lithosphere is made up of tectonic plates
 Now, you will learn:
 How the continental drift hypothesis was developed
 About evidence for plate movement from the sea floor
 How scientists devloped the theory of plate tectonics
Core, Mantle, Crust
Litho – “stone” or “rock”
Asthenes – “weak”
 Lithosphere: crust and very top of mantle – solid, most rigid
layer
 Asthenosphere: hotter, softer rock in the upper mantle (just
below the lithosphere) – can flow like hot tar
Less
dense
materials
rise
Denser
materials
sink
Continents join
together and
split apart
 As far back as the 1500’s, map makers noticed the western
coast of Africa and the eastern coast of South America
seemed to fit together
 1912, German scientist Alfred Wegener proposed the
hypothesis: continental drift
Evidence for Continental
Drift
 Fossils:
 Fossils of an ancient (270 million years ago) reptile: Mesosaurus
were found in South America AND western Africa, but no where
else in the world
 Explanation: the continents were once joined
 Climate:
 Greenland today is mostly covered in ice, yet tropical plant fossils
are found there
 South Africa is warm, but rocks were deeply scratched by ice sheets
 Geology:
 Kinds of rocks that make up the continents: those found in Brazil
match those in western Africa
 Limestone layers in the Appalachian Mountains (NA) exactly like
Scotland’s Highlands
Pangaea and
Continental
Drift
 Huge supercontinent
over 200 million years
ago
 Pangaea, “all lands”
 Centered over where
Africa lies today
 But how?
The theory of plate tectonics explains
how plates and their continents move
 Mid-1900s – scientists proved tectonic plates move
 Evidence from the sea floor:
 Huge underwater mountain ranges: mid-ocean ridges circling
earth like baseball seams
The theory of plate tectonics explains
how plates and their continents move
 Evidence from the Sea Floor:
 Sea-Floor Spreading: the ridges form along cracks in the
crust, melted rock rises through these cracks, cools, and
forms new oceanic crust
The theory of plate tectonics explains
how plates and their continents move
 Evidence from the sea floor:
 Age of the sea floor: youngest rocks cloest to the ridge,
oldest rocks farther away
 Oldest ocean floor is young – 160 to 180 my old;
continental crust much older: 4 billion yrs
 Ocean Trenches
 Sea floor spreads, then dense oceanic crust sinks into the
asthenosphere (upper mantle) into huge trenches (like
deep canyons)
 Old crust destroyed as new crust forms – Earth remains the
same size
Causes of Plate Movement
 Tectonic plates rest on the asthenosphere – layer of
soft, hot rock
 Moves by convection: heat transfer by the movement of
a material
 Hot soft rock rises, cools, and sinks, then is heated and rises
again: convection current – slow few cm/yr
 Slab pull: gravity pulls the edge of a cool, dense plate
into the asthenosphere – the entire plate is dragged
along
 Ridge push: material from a mid-ocean ridge slides
downhill from the ridge
Putting the Theory
Together
 Theory of plate tectonics: the Earth’s lithosphere is
made up of huge plates that move over the surface
of the Earth
 One plate could not shift without affecting the
others nearby
 Plates can move apart, push together, or scrape
past each other
 Most major earthquakes, volcanoes, and mountain
ranges appear where tectonic plates meet
 List of animations!
 http://www.wwnorton.com/college/geo/egeo/animat
ions/ch2.htm