Transcript full spreading rate

When I was a kid at school I noticed that South
America seemed to be fit neatly into Africa
Now we know they did 230 million years ago after the
Triassic period and then drifted apart
This great animation is at www.rtmsd.org/page1845
Penncrest High School - Pennsylvania
sdsu- physica.org
The Earth’s Polarity changes direction and is
locked into the solidifying magma as it emerges
It is mirrored on both sides of the mid atlantic
ridge where it emerges …it is much like a
historical tape recorder
www.kidcyber.
com.au
Triassic fossils such as Cynognathus are found both in
Africa and South America!
There is more evidence for
Evolution than any other fact
we have ever discovered
Anti-Evolution is an attack all science as the
millions of pieces of evidence come from every
scientific discipline
Chemistry
Physics
Geology
Geography
Paleontology
Oceanography
Vulcanology
etc
.3.1 Linear magnetic anomalies – a record of tectonic movement
At the time that sea-floor spreading was proposed, it was also known
from palaeomagnetic studies of volcanic rocks erupted on land that
the Earth's magnetic polarity has reversed numerous times in the
geological past. During such magnetic reversals, the positions of the
north and south magnetic poles exchange places. In the late 1950s, a
series of oceanographic expeditions was commissioned to map the
magnetic character of the ocean floor, with the expectation that the
ocean floors would display largely uniform magnetic properties.
Surprisingly, results showed that the basaltic sea floor has a striped
magnetic pattern, and that the stripes run essentially parallel to the
mid-ocean ridges (Figure 6). Moreover, the stripes on one side of a
mid-ocean ridge are symmetrically matched to others of similar width
and polarity on the opposite side.
Figure 6 A modern map of symmetrical magnetic anomalies about the
Atlantic Ridge (the Reykjanes Ridge), south of Iceland. (Adapted from
Hiertzler et al., 1966)
Long description
In 1963, two British geoscientists, Vine and Matthews (Box 1),
proposed a hypothesis that elegantly explained how these magnetic
reversal stripes formed by linking them to the new idea of sea-floor
spreading. They suggested that as new oceanic crust forms by the
solidification of basalt magma, it acquires a magnetisation in the same
http://openlearn.ope
n.ac.uk/mod/oucont
ent/view.php?id=398
588&section=2.3.1
The two tapes move
apart and the
magnetic striping is
symmetric…as it
should be
As molten rock wells up at the Mid Atlantic Ridge to form new
ocean floor it continually separates the continents much like a
travelator. The direction of the Earth’s magnetic field is locked
in when the floor solidifies, creating a magnetic tape record of
the history of the Earth’s field, which periodically switches, and
tells how long ago it was since the continents separated
http://openlearn.open.ac.uk/mod/oucontent
/view.php?id=398588&section=2.3.1
Magnetic stripes not only tell us about the age of the oceans, they can
also reveal the timing and location of initial continental break-up. The
oldest oceanic crust that borders a continent must have formed after the
continent broke apart initially, and just as sea-floor spreading began. In
effect, it records the age when that continent separated from its
neighbour. In the northern Atlantic, for example, oceanic crust older
than 140 Ma is restricted to the eastern USA and western Saharan
Africa, therefore separation of North America from this part of Africa
must have commenced at this time. The oldest oceanic crust that
borders South America and sub-equatorial Africa is only about 120 Ma
old. Accordingly, it follows that the North Atlantic Ocean started to form
before the South Atlantic Ocean.
If new sea floor is being created at spreading centres, then old sea floor
must be being destroyed somewhere else. The oldest sea floor lies
adjacent to deep ocean trenches, which are major topographic features
that partially surround the Pacific Ocean and are found in the peripheral
regions of other major ocean basins. The best known example is the
Marianas Trench where the sea floor plunges to more than 11 km
depth. Importantly, ocean trenches cut across existing magnetic
anomalies, showing that they mark the boundary between lithosphere of
differing ages. Once this association had been recognised, the fate of
old oceanic crust became clear – it is cycled back into the mantle, thus
preserving the constant surface area of the Earth.