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.
III. 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.