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Warm-up #43 Mar. 26
Brainstorming:Will California eventually slide into the ocean?
Have continents really drifted apart over the centuries?
PLATE TECTONICS
Unit 8
Continental Drift
Alfred Wegener – German scientist proposed the theory of continental drift.
This hypothesis stated that the continents had once been joined to form a single supercontinent
Pangaea – all land
Pangaea
Part of Wegener’s theory was that Pangaea started splitting into smaller continents 200 million years before
Wegener collected lots of evidence to prove this theory
The Continental Puzzle
The shorelines of the continents nearly fit together to make a large land mass.
HOWEVER The shorelines are constantly eroding and
changing.
Matching Fossils
Fossil evidence for continental drift includes several fossil organisms found on different landmasses.
At one point the theory was that there were land bridges, however there is no evidence of this.
Rock Types and Structures Several mountain belts that end at one
coastline reappear on a landmass across the ocean
Ancient Climates
Wegener found glacial deposits showing that between 220 million and 300 million years ago ice sheets covered large areas of the S. Hemisphere
Rejecting Wegener’s Hypothesis
Many scientists questioned Wegener’s theory
He could not describe a mechanism that was capable of moving the continents
There was not enough evidence
A New Theory
In 1968 a new theory was proposed –
PLATE TECTONICS
Plates
The lithosphere is broken up into segments called plates
The lithosphere is the uppermost layer of the mantle
Plates continuously move and change shape and size
Warm-up #44 Mar. 27 Mesosaurus provided evidence that South
America and Africa were once joined
On average, how much do you think the plates move a year?
5 centimeters
Plate Boundaries
A plate boundary is where two plates meet.
There are three main types of boundaries:DivergentConvergentTransform fault
Divergent
Also called spreading boundaries Occurs when two plates move apart Results in upwelling of material from the
mantle to create new seafloor
Convergent
Form where two plates move together Results in lithosphere plunging beneath
an overriding plate and descending into the mantle
Transform fault
Margins where two plates grind past each other without the production or destruction of lithosphere
What happens at boundaries?
Divergent BoundariesSeafloor spreading occurs along the
boundaryForms fractures on the ridge crestsFractures fill with molten materialWhen the boundary occurs on a continent,
rifts or rift valleys form
Convergent Boundaries
Oceanic-continental boundaryForms a subduction zone withinVolcanic arcs form
Oceanic-oceanic boundaryOften forms volcanoes on the ocean floorIsland arc forms as volcanoes emerge
Convergent Boundaries
Continental-continental boundariesNeither plate will subductCan produce mountains
Transform Fault Boundaries No new crust created No crust is destroyed Most are in oceanic crust Parallel the direction of
plate movement Aids movement of crustal
material
Boundary Brochures Needs to include:
3 types: Divergent, Convergent, TransformWhat each one is (use your notes)A picture for eachAn example for each
○ http://geology.com/plate-tectonics.shtmlNeeds to be colored
Warm-up #45 Mar.28Brainstorming: Do you think it is possible for Earth’s
magnetic poles to be reversed? If so, what do you think could happen to
the biosphere? Also, how might geologist use that to
support plate tectonics?
Evidence for Plate Tectonics Paleomagnetism
Most persuasive piece of evidence.Ancient magnetism preserved rocks.Paleomagnetic records show:
○ Polar wandering○ Earth’s magnetic field reversals
Evidence for Plate Tectonics Earthquake patterns
Associated with plate boundariesDeep-focus earthquakes along trenches
provide a method for tracking the plate’s descent
Evidence for Plate Tectonics Ocean Drilling
Deep Sea Drilling Project – Glomar Challenger
Age of deepest sediment○ Youngest are near the ridges○ Older are at a distance from the ridge
Ocean basins are geologically young
Evidence for Plate Tectonics Hot Spots
Rising plumes of mantle material.Volcanoes can form over
Driving Mechanism
No one model explains all plate motion Earth’s heat is the driving force Several models have been proposed
Convection currents in the mantleSlab-pull and slab-push model Hot plumes
Dynamic Earth http://www.learner.org/
interactives/dynamicearth/index.html
Ticket out the Door
1. What are the 3 main types of plate boundaries?
2. Name 2 pieces of evidence Wegener used to prove continental drift.
3. What is type of boundary is this?