EPSC201 - Lecture 8 Notes
Continental Drift (Wegner, 1920s)
is explained by plate tectonics
Key line of evidence for plate tec-
tonics was the alternating magnet-
ic pattern observed on the Atlantic
seafloor. It was anomalously posi-
tive or negative, but in equal mag-
This was unexpected because
there is only one type of rock on
the seafloor. Basalt – lava that
flows out onto the ocean
Why are these patterns observed?
Halfway between Canada and Eu-
rope, there is the Mid-Atlantic
Ridge. It is a topographical high
point in the sea floor.
The pattern of anomalies is the
same moving outwards from the
ride. It is symmetric about the mid
ocean ridge. The rocks get older
away from the ridge, and at the
ridge, the rocks are young. There
is lava flowing out of the ridge
forming basalt. There are volcanoes along the ridge.
Iceland is part of the Mid-Atlantic ridge that
has come above the surface. Iceland is loaded
with active volcanoes. They actually shut down
air travel over the Atlantic after volcanic erup-
tions. The country uses geothermal energy.
Key Point – the earth is shifting its magnetic
field, and the rocks are recording it. It is symmetrical about the ridge because new rock is being created at the ridge, pushing the old crust
outwards. The new rock is formed by lava flowing out, forming basalt. This formation of new oceanic crust
at the mid Atlantic ridge pushes the continents apart. This is
called the sea floor spreading theory. At every new
ocean ridge, the old crust is being pushed
apart, moving the continents apart.
The reversal of the magnetic field is a
rapid transition. In the transition state,
solar radiation is able to enter the at-
mosphere. The transition state is on
the magnitude of a few hundred to a
couple thousand years. Exact amount
of time is not known.
It’s difficult to take samples from the
bottom of the ocean. Finding surface
rocks is much easier. Find an area
with lots of lava flow from different time periods. An old
cliff would be a good example Sampling
different layers will tell you about the
Earth’s magnetic field history.
We are able to date these changes
They can determine the age of a rock, and its distance
from the ridge, which gives you the rate at which new crust is being created. The age is
determined by the magnetic striping. You can measure the