Chapter 13 – Plate Tectonics
13.1 Continental Drift
Evidence
- Pangaea
o Means “all land”
o Alfred Wegener theorized no coincidence for continents fitting
together
o Suggested they were together at one point in the past
o One large landmass
o Proposed “continental drift” theory
 Continents have moved slowly to their current locations
- fossil clues to support Pangaea
o reptile Mesosaurus fossil found on Africa and S. America
o fern Glossopteris fossil found in Africa, Australia, India, S.
America, and Antarctica\
o Also suggested different locations on Earth with different climates
- climate clues
o Glacial deposits in grooves in S. America, Africa, India, and
Australia indicates glaciers covered these continents
o Indicates possibly connected and located near poles at one time
- rock clues
o similar rock structures are found on different continents
o parts of Appalachian mtns are similar to those found in
Greenland and Europe
13.2 Seafloor Spreading
Clues on ocean floor
- discovery of mid-ocean ridges sparked investigations for new sea floor
formation
- Seafloor spreading
o Hot, less-dense material below Earth’s crust is forced upward
toward surface
o Turns once it reaches the surface and flows sideways
o As it spreads, magma moves upward and flows through cracks
o Solidifies, cools and forms new seafloor
o Becomes more dense than the material around it and therefore
begins to sink
o Age of rocks
 No rocks at ridge older than 180 my
 Continental rocks as old as 4 by
 Youngest rocks are located right at the ridge
 Further out from ridge, older the rocks get
o Magnetic clues
 Iron composed minerals (ex. Magnetite) are attracted to
Earth’s magnetic field
 Magnetic reversals
 Normal pattern is from south pole to north
 Reversal sends pattern from north to south
 Iron composed minerals show shift
 Creates parallel strips on ocean floor
 Reversals occur thousands or millions of years
13.3 Plate Tectonics
Plate tectonics – Earth’s crust and upper mantle are broken into sections
- move on the asthenosphere
o plasticlike layer below lithosphere
Plates
- Lithosphere
o composed of the crust and part of upper mantle
- interact 3 ways:
o converge
 plates colliding
 3 types:
 1 - Both plates forced up creating mountain ranges
 Subduction zone
o When one plate is forced under another
 2 - Deep sea trenches are formed
 3 - Volcanic mountains
o diverge
 moving apart _
 opposite directions
 seafloor spreading
 mid-ocean ridges
o transform
 sliding past each other
 earthquakes occur
Causes
- convections currents
o the cycle of heating, rising, cooling, sinking
o different densities get hot from Earths interior, becomes less
dense, rises, cools as it moves further away from heat source,
becomes more dense, then sinks again
o affects the asthenosphere therefore affecting the lithosphere
Effects
- faults
- rift valleys
- mountains, arcs, volcanoes
- ridges
- earthquakes