Formation
of the
Earth
Previous Theories
 Continental Drift Theory
 Developed by Alfred Wegner (1900’s)
 Believed continents were once all
combined into one landmass he called
Pangaea meaning “All Earth”
 Continents seemed to fit together like a
jigsaw puzzle
 Explained why fossils of the same plants
and animals are found on the coast of
Africa and South America
Pangaea
More Theories
 Sea Floor Spreading (1950’s)
Cracks in the ocean floor spread
apart allowing magma to rise and
create new ocean crust
This helped explain underwater
mountain ridges that were found
at the bottom of oceans
Mapping the World’s Oceans
Plate Tectonics
 Current theory of earth formation
(1960’s)
Lithosphere is divided in plates
that move along the magma of
the asthenosphere
Scientists don’t know exactly
what causes the plates to move
Possible Forces that
Cause Plate Movement
 Ridge Push – gravity causes the oceanic
lithosphere to sink into the asthenosphere
Ridge Push
Possible Forces that
Cause Plate Movement
 Slab pull - because the oceanic lithosphere
is denser than the asthenosphere the edge
sinks and pulls the rest of the tectonic plate
with it
Slab Pull
Possible Forces that
Cause Plate Movement
 Convection – hot rock deep in the Earth
rises while cooler rock sinks, which causes
the oceanic lithosphere to move sideways
away from the oceanic ridge
Convection