cont drift:seafloor spreading

Our Ever-Changing Earth!
Part 1: Continental Drift and Seafloor
For centuries, people have wondered about the
strange things happening on the surface of our
How did the continents and oceans form?
Why were fossils of seashells and other sea creatures found on
the tops of mountains?
How do mountains form?
What causes earthquakes and volcanoes?
Why do they occur in some places but not others?
What is it like at the bottom of the ocean?
What is it like under the surface of the land?
Moving Continents?
Since as early as 1598,
as better maps were
drawn, people
wondered why some
continents appeared
to “fit” together so
nicely, especially
South America and
Finally, in 1915, a German meteorologist named
Alfred Lothar Wegener was the first to try to explain
the possibility of “moving” continents.
Alfred Lothar Wegener (1880-1930)
Studied meteorology,
astronomy, climatology and
Very intelligent and
interesting man.
Explored Greenland several
Actually died (froze to death
on a rescue mission) while
on Greenland!
In 1915, Wegener published his book describing
his Theory of Continental Drift
And he filled the pages
with evidence to support
his theory.
He used information from
fossils, rocks, ancient
climates, and more, to
back up his ideas.
Alfred Wegener’s Theory of Continental Drift.
The Evidence!
One of the most interesting
fossils he studied was
Glossopteris, an extinct tree
that had large seeds – too big
to be carried by wind and too
weak to survive a long ocean
How was that a possible clue?
And he found that fossils of
many other plants and
animals matched up too.
He also noticed similarities
in rock formations from
South America and Africa.
And other people helped
him by noticing that ancient
mountain chains lined up.
But they could not explain
the “gaps” in the ocean.
Alfred thought he could!
He also noticed similarities
in glacial deposits from
hundreds of millions of
years ago.
He found that many other
deposits, including coal
deposits, matched up
perfectly as well
Why might coal be so
fascinating to Wegener?
Wegener suggested that all the continents were
once connected into one supercontinent he
called Pangaea.
Pangaea is a word that means
“all land”.
He believed it existed about 300
million years ago.
(We now
believe it was more like 225
million YA)
He believed it then separated
into two continents – one called
Gondwanaland and one called
Laurasia, separated by the
Tethys Sea.
Gondwanaland and Laurasia
later separated into the
continents we know today.
Unfortunately for Alfred Wegener, his theory of
continental drift was rejected by most people.
Many considered his ideas
crazy, and Wegener a
His theory had evidence,
but it did not explain HOW
such massive things
(continents) could move.
For years, long after his
death, his theory was
mostly ignored.
Click on image for video
But a very interesting thing
happened in the 1940s and
1950s that got people
thinking again about Alfred
Wegeners theory.
It is called sonar.
During WW II (1940s) sonar
was used to look for
It also gave people their
first look at the bottom of
the ocean.
What they found surprised
Mid-ocean ridges.
Miles of them! They are the
longest mountain chains in
the world – and they are
They are like the seams of a
baseball, running all around
the Earth.
They have several parallel
ridges with a steep valley
down the middle (twice as
deep as the Grand Canyon).
They also found deep sea
These are the deepest
valleys in the world – and
they are also under the sea.
They have extremely steep
Some of them are 3 to 5
miles deep – the deepest
spots in the ocean.
In 1960, American geologist Harry Hess made a
startling hypothesis.
Hess suggested that the
ocean floor was separating
at these ridges.
He proposed that molten
magma rises there, causes
the spreading, cools,
hardens, and becomes new
sea floor (oceanic crust).
Harry Hess
Throughout the 1960’s, more and more evidence
supported the idea of seafloor spreading.
The deep-sea submersible
ALVIN (from Woods Hole
Oceanographic Institute)
observed evidence of
“pillow lava” at the ridges.
Drilling Samples:
The age of the crust varied
as you moved farther from
a ridge, but the crust was
never older than
continental crust.
More evidence...
Depth of Sediments:
Sediments were thinner
nearer the ridge and thicker
farther away. Why?
Magnetic Striping:
The newly forming ocean
crust was actually
“recording” the magnetic
reversals of the Earth every
600,000 years.
But one question remained:
If new crust is forming at the midocean ridges, why isn’t the Earth
getting larger and larger?
The Answer: Subduction Zones.
Subduction is where ocean
crust is being forced under
other crust.
It is believed to be melted
back into magma.
Subduction zones also
explain those deep sea
Click on image for video (4:02)
Finally, when it was all added together…
The Theory of Plate Tectonics!