Download Pangaea and Seafloor Spreading Notes

Objective1. Students will explain the theory of Continental Drift and Pangaea.
2. Students will explain seafloor spreading and describe the evidence used to
support it.
IV.
Pangaea and Seafloor Spreading
A. Pangaea is the idea put forth by Wegener that, once an incredibly long time
ago, all the continents were connected as one large landmass.
1. Wegener formed the idea of Continental Drift based on his observations
that many fossils of the same type were found on continents thousands
of miles apart, that rock types and rock formations were the same on
continents thousands of miles apart, that the continents shapes
seemed to have edges that could readily fit into each other, and that
the climates many continents were now experiencing would not
support the fossil types found there. Continental drift states that all
the continents moved from their place of origin-Pangaea, to where they
are presently located.
2. Wegener’s idea was not supported by the scientists of his day because
he could not explain how, when or why such large pieces of Earth
could move. Not until Harry Hess discovered seafloor spreading, in the
early 1960’s did Wegener get the credit he deserved.
B. Seafloor spreading is the proposed theory that hot, less-dense material in the
mantle is forced upward to the surface at the mid-ocean ridge. The material
then turns and flows sideways, pushing the seafloor with it in both directions.
As the seafloor spreads apart, magma located a few kilometers below the ridge
moves upward, flows from the crack, solidifies, and forms new seafloor. The
seafloor that is pushed away from the ridge cools, contracts, and becomes
denser than the asthenosphere below it. The new seafloor begins to sink,
forming trenches.
1. Age evidence to support seafloor spreading came in 1968, from the
Glomar Challenger – a drilling research ship. Researchers found rocks
on the seafloor no older than 180 million years, whereas, continental
rocks are almost 4 billion years old. The youngest rocks were found at
the mid-ocean ridges, then progressively got older, the farther traveled,
from the ridges on both sides
2. Magnetic clues found in the iron-bearing basalt rock from the ocean
floor support seafloor spreading. Iron minerals in the basalt align
themselves according to the magnetic field orientation when they form.
So, as the magnetic field reverses, the iron minerals become deposited
in reverse as well.