Transcript Continental Drift & Seafloor Spreading

Chapter 10.1
Continental Drift and
Seafloor Spreading
The Earth: a Giant Puzzle?
As early as 400 years ago, explorers sailing the oceans
brought back information about new continents and their
coastlines. Mapmakers used this information to make the
first reliable world maps. At this time, it was discovered
that there was a similarity between the continental
coastlines on either side of the Atlantic. It appeared
that the coastlines of western Africa and eastern South
America would fit together like pieces in a giant puzzle …
Continental Drift
 Continental Drift – a hypothesis proposed by Alfred
Wegener in 1912, which stated that all of the
continents we see on Earth today were once part of a
single landmass.
Alfred Wegener
Click below for a cool animation of continental drift!
http://www.ucmp.berkeley.edu/geology/anim1.html
Evidence for Continental Drift
Evidence for Continental Drift
1. The coastlines of several of the continents appear
to “fit together” like pieces of a giant jigsaw
puzzle (ex. the west coast of Africa and the east
coast of South America).
2. Similar fossils have been found on the coastlines of
those continental pieces that seem to fit together
(ex. Mesosaurus, a small, extinct reptile lived 270
million years ago in both west Africa and east
South America and could not have swum across the
ocean to get there.).
3. Rocks of similar age and type have been found on
the coastlines of those continental pieces that
seem to fit together (ex. Rocks in western Africa
and eastern Brazil are similar.).
Evidence for Continental Drift
Evidence for Continental Drift
4. Mountain ranges of similar age and type appear
to extend across the continental pieces that
seem to fit together (ex. The Appalachian
Mountains in the eastern US appear to be
contiguous with the mountains ranges of
Greenland, Scotland, and northern Europe.).
5. Geologic evidence of past climates has been
found on the coastlines of those continental
pieces that seem to fit together (ex. Evidence
of past glaciation exists along southern Africa
and South America which today are both
warm./Tropical plant fossil deposits, indicating
tropical or subtropical swamps, are found along
in Antarctica which today is much cooler.
Continental Drift “just” a hypothesis
Despite all this evidence, Wegener was unable to
explain the mechanism for Continental Drift. As
we have learned since his death, the evidence
that he needed lay on the ocean floor and the
mechanism that he needed to explain his
hypothesis is Seafloor Spreading…
The Mid Atlantic Ridge
 Mid-Atlantic Ridge – an undersea mountain range in the
center of the Atlantic with a steep, narrow valley
running down its center, which is believed to be part of
a tear in the Earth’s crust through which magma upwells
(The Mid-Atlantic Ridge is a part of a 65,000 km long
ridge system that extends around the globe.).
The Mid Atlantic Ridge
Mid-Atlantic Ridge – Two surprising trends that
scientist discovered : 1. Sediment covering the
sea floor is thinner near a ridge than away form
the ridge (suggesting that the nearer to the
ridge, the less time that sediment has had to
deposit.
2. Dating the tiny organisms trapped in ocean
floor sediments revealed that the rocks closer to
the ridge are younger than the rocks farther
away from the ridge.
Found that rocks on land (the continents) are
older(3.8 billion years old) compared to rocks on
the ocean floor (less than 175 million years old)
Bob Ballard and the Black Smokers
Undersea researcher Bob Ballard and his team discovered “black
smokers” at Mid Ocean Ridges, which play a role in enriching seawater
with minerals and contain fantastic new lifeforms with a radically
different biology … They are NOT dependant on the Sun for energy!!!
Bob Ballard
http://www.biosbcc.net/ocean/marinesci/04benthon/dsindex.htm
http://www.pbs.org/saf/1305/index.html
Iceland ... Growing and Tearing
Iceland is the only sizable
country on the Mid Atlantic
Ridge …
For the people of Iceland, the Ridge
is both a blessing and a curse …
The ridge is increasing the size of
the country , creating beautiful
volcanic landscapes, and producing
warmth and geothermal power,
however, it can also threaten the
homes of the people with
earthquakes, volcanism, and fire!!!
http://www.delargy.com/tn/Iceland.htm
Iceland ... Growing and Tearing
The youngest rocks on Iceland
would be nearest the MidAtlantic Ridge; The oldest
would be the farthest away
from it. This is because new
magma is upwelling from the
center of the ridge and
pushing the older ocean basin
crust away from the ridge.
Seafloor Spreading
 Seafloor Spreading – the hypothesis proposed by Harry
Hess in the 1950s that the ocean floor has cracks,
called ridges, in it through which new magma is
constantly upwelling. As this new magma constantly
renews the sea floor at these ridges, the older ocean
basin crust moves away from these ridges. Just as the
seafloor is spreading and moving, it was hypothesized
that continents might be “carried along” as well.
Harry Hess
Click below for a cool animation of seafloor spreading!
http://www.pbs.org/saf/1305/features/platetechintro.htm
Seafloor Spreading “just” a hypothesis
Like Wegener with his hypothesis of Continental
Drift, Hess was also unable to provide a
mechanism for his hypothesis of Seafloor
Spreading. The evidence that would later
support and provide a mechanism for Seafloor
Spreading was paleomagnetism.
Evidence for Seafloor Spreading …
Paleomagnetism
The evidence for Seafloor Spreading comes from
Paleomagnetism.
As magma cools, the magnetic orientation of
rocks with iron-bearing minerals aligns with
Earth’s current magnetic field. We know that
that magnetic field weakens and reverses
periodically. When the magnetic north pole
matches with the geographic north pole we have
normal polarity. When the magnetic north pole
matches with the geographic south pole we have
reverse polarity!
Magnetic field of the sea floor
The striped patterns of paleomagnetism on both sides of
the Mid-Atlantic Ridge (and other ridges) are mirror
images of each other, which is consistent with an even
seafloor spreading at the ridge(s). With the seafloor
spreading, it is hypothesized that the continents might be
“carried along” as well.
The End
All images came from Google and are for educational purposes only!!!