Post on 16-Dec-2015
transcript
CONTINENTAL DRIFT, SEA-FLOOR
SPREADING, AND PLATE TECTONICS
An ancient, but amazingly accurate world map!
Early map of trade routes
More modern map of sea bottom features. Look at the jigsaw puzzle outlines of the continents!
Alfred Wegener• Most people thought the
complementary shapes were coincidence. Alfred Wegener was an exception who sought more evidence that the continents had been joined. He found it in the rocks & fossils and proposed the hypothesis of Continental Drift.
Continental Drift - A Hypothesis
Three important ideas changed our understanding of the earth:
• Continental drift - a hypothesis • Sea-floor spreading- a hypothesis• Plate Tectonics - a theory
What Is This Supercontinent? Pangaea
Who proposed the hypothesis of continental drift?
Alfred Wegener in 1912
When did Pangaea break up? About 180-200 million years ago
The most current theoryshows the developmentof modern continentalshape and location
What Kind of Evidence Supported Continental Drift?
Matching Mountain Belts • Appalachians and Caledonian Mountains • Cape Mountains, S. Africa and Sierra of Argentina
• Fossil Evidence How could fossils of land animals appear on widely separated continents?
(Evolved independently, On rafts, Swam there, Land bridges) • Wegener’s explanation: Continents were all one Past Climates • Now-tropical Africa, India, etc., had ice sheets 300 million years ago • Tillite
• Now-midlatitude North America and Europe, once had tropical swamps • Coal
Why Was Continental Drift Not Accepted?
How could vast continents plow through the ocean floor?
What was the driving mechanism? Rotation of earth Pull of the moon
Inadequate explanation - forces too weak
Wegner’s Problem
• The problem with the Continental Drift Hypothesis was that Wegener had no mechanism for moving the continents through the ocean floor.
• Powerful physical arguments prevented continental drift from acceptance by the majority of scientists, particularly those in the northern hemisphere.
• Wegener died in 1930 with his hypothesis largely ignored. But despite the problems and an overwhelming rejection of his ideas, not everyone forgot about his work.
• In the 1940’s and 1950’s geophysicists studying the record of Earth’s magnetic field began to revive some of the ideas to explain observed variations in their observations.
During WWII and with the initiation of the Cold War, much research was focused attention on the ocean floor.
– Researchers were surprised to find long, large mountain chains and deep trenches.
– They also found that the rocks which compose the ocean floor were young, ranging in age from recent to 200 Ma.
Seafloor Spreading
• Sea Floor Spreading is the hypothesis in which new ocean floor is created at the mid-ocean ridges and that plates move apart at the ridges and the continents move with them.
Hypothesis of Sea-Floor Spreading
Who proposed this hypothesis? Harry Hess
What Goes On At Mid-Ocean Ridges?
Sea-floor spreading Shallow earthquakes
High heat flow and volcanism Birthplace of new oceanic crust Youngest rocks of the sea floor
Magnetic stripes Thinnest sediment
What Was The Proof Of Sea-Floor Spreading?
Paleomagnetism of the sea floor Ancient magnetism records flip-flops of earth’s
magnetic field North orientation - normal South orientation - reverse
How Does the Age of the Sea Floor Support Sea-Floor Spreading?
Oceans are youngest at the Mid-Ocean ridges Oldest near the continents
Summary of Sea-Floor Spreading New sea floor is created at mid-ocean ridges. Sea floor is destroyed in subduction zones.
Seafloor Recycling
• Since new material is created at the mid-ocean ridges and Earth is not expanding, somewhere material must be removed from the surface.
• It turns out that old ocean floor is “subducted” into the mantle at subduction zones.
Plate Tectonics - What Is This Grand Unifying Theory?
The lithosphere is cracked into seven or more large plates with many smaller ones.
The lithosphere moves over a semi-molten asthenosphere. Most tectonic plates include both continental crust and
oceanic crust. Map of Tectonic Plates
What global features can now be related to the story of plate tectonics?
Deep Oceanic Trenches Mid-Ocean Ridges Volcanic Activity
Earthquake Epicenters
• By plate we really mean a “shell” on Earth’s surface that is about 100-200 km thick and includes all of the crust, and the uppermost part of the mantle.
• Tectonics is the term we use in geoscience to represent the formation of folds, fractures, faults, etc. in rocks.
Major Plates
Newest Design - 2002
Global Seismicity
New Volcanism
Earthquakes
• Most earthquakes occur along plate margins because plate margins are relatively weak, but about 10% of earthquakes occur within the interior of plates.
• We classify earthquakes depending on where they are located
• Interplate - between plates• Intraplate - within plates
Earthquakes Coincide With Plate Boundaries
Most geological activity occurs along plate boundaries
Divergent Plate Boundaries Convergent Plate Boundaries Transform Plate Boundaries
Divergent Plate Boundaries
Where are most divergent plate boundaries? In the middle of oceans
Fault Offset the Mid-Ocean Ridges Shallow earthquakes occur as plates move past
Volcanoes erupt along the rifts What Kind of Plate Boundary is Iceland?
A divergent plate boundary that forms an island Do Divergent Plate Boundaries Occur on Land?
East African Rift Valleys
Convergent Plate Boundaries
How many ways can plates converge? Ocean-Continent
Ocean-Ocean Continent-Continent
What Goes On At Ocean-Continent Convergent Plate Boundaries?
Earthquakes occur as the ocean plate subducts An ocean trench forms as the sea floor bends
Volcanoes form as magma erupts from the subduction zone
Mountains form as sediments are folded and faulted
What Kind Of Convergent Plate Boundary?
Ocean-Ocean Plate Boundary
Earthquakes occur as one ocean plate subducts An ocean trench forms as the sea floor bends
A volcanic island arc forms as magma rises above the subduction zone
Continent-Continent Convergent Plate Boundary
Mountain building occurs as the continental crust is folded and faulted
Earthquakes occur as two continents collide NO volcanic activity
India crashes into Asia Himalayan Mountains and Tibetan Plateau
What Kind Of Plate Boundary? San Andreas Transform Fault Plate Boundary
Earthquakes only
•What Moves the Plates? Convection currents in the earth's mantle
Slab-pull in the trenchesRidge-push at the ridges