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Theory of Continental Drift Alfred Wegener,
German meteorologist
Proposed in 1912 in articles
Single large continent (Pangaea – “all earth”) started breaking apart about 225-200 MYA
Evidence for Continental Drift
Fit of African and South American continents (1858 drawing by geographer Antonio Snider-Pellegrini)
Weakness of Theory
What kind of forces could be strong enough to move such large masses of solid rock over such great distances?
Wegener suggested that the continents simply plowed through the ocean floor, but Harold Jeffreys, a noted English geophysicist, argued correctly that it was physically impossible for a large mass of solid rock to plow through the ocean floor without breaking up.
Earth’s “Inner Furnace”
To scale
Crust 0-100 km thick
CrustMantle
Outer core
Inner coreNot to scale
2,900 km
5,100 km
6,378 km
Lithosphere (crust & uppermost solid mantle)
Mantle
Core
After World War II, new evidence from ocean floor exploration and other
studies rekindled interest in Wegener's theory, ultimately leading to the development of the
theory of plate tectonics.
Evidence for Plate Tectonics
Ruggedness & youth of ocean floor Confirmation of repeated reversals of
Earth’s magnetic field in geologic past Emergence of the seafloor-spreading
hypothesis and associated recycling of oceanic crust
Documentation that the world's earthquake and volcanic activity is concentrated along oceanic trenches and submarine mountain ranges.
What is the nature of the forces propelling the plates?
Conceptual drawing of assumed
convection cells in the mantle.
Below a depth of about 700 km, the descending slab begins to
soften and flow, losing its form.
Types of Crust Continental crust
Makes up landmasses
Granitic (felsic) Lighter Less dense Thicker
Average age 2.3 billion years, oldest is 3.96 billion years
Oceanic Crust Lies under oceans
& seas Basaltic (mafic)
Heavier Denser Thinner
Average age 55 million years, totally recycled every 150 million years
Three Types of Plate Boundaries
Divergent: new crust forms as plates pull away from each other
Convergent: crust destroyed as plates collide, one dives under the other
Transform: crust neither created nor destroyed, plates slide past one another
Convergent Plate Boundaries
Continent-ocean collisions (denser oceanic crust dives under)
Ocean-ocean collisions (older denser crust dives under younger crust, form island arcs)
Continent-continent collisions (form mountain ranges – Himalayas)
Ocean: Sea Floor Spreading Ocean plates are being pushed apart by convection currents and new ocean floor is made
Crustal Features: Rift Valleys on land and Mid-Ocean Ridge in ocean
Divergent Boundaries
Convergent Boundaries
•Continental-Continental: huge mountains
•Continental-Oceanic: Subduction! The more dense oceantic crust sinks under
the continental. Trenches, mountains, and volcanoes occur.
Convergent Boundaries
•Continental-Continental: huge mountains
•Continental-Oceanic: Subduction! The more dense oceantic crust sinks under
the continental. Trenches, mountains, and volcanoes occur.
•Oceanic-Oceanic: Subduction! Very deep trenches and volcanoes
Transform
A plate boundary where two plates move past each other in opposite directions
Makes faults and causes earthquakes
Boundaries