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Where do those plate reconstructions through time come from?. What data is used to make these reconstructions? Paleomagnetism, paleoclimates, rock distributions, biogeographic distribution of organisms. Polar Wandering.

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where do those plate reconstructions through time come from

Where do those plate reconstructions through time come from?

What data is used to make these reconstructions?

Paleomagnetism, paleoclimates, rock distributions, biogeographic distribution of organisms

polar wandering

Polar Wandering

Along with sea floor spreading, “APW curves” are the two data sets that lead to the development of the theory of plate tectonics

slide4

Magnetic field lines encircle the earth in 3D.

Field lines “come to closure” at the geographic north pole, which

is why compasses point here. What does that say about the

orientation of the earth’s dipole in the core? Yes, the south end of

the dipole is pointing north!

slide5

The 3D orientation of the earth’s magnetic field also means

that field lines are oriented at different angles relative to the

Earth’s surface; parallel at the equator, perpendicular at the

poles, approx. 45 degrees at 45 degrees N or S latitude.

slide6

So a compass can rotate in two directions, towards closure at the

South end of the dipole (our geographic North pole) AND

vertically, parallel to the field orientation relative to the

surface of the earth = angle of inclination

slide7

The relationship

between latitude

and angle of

inclination:

slide8

How rocks can record the earth’s magnetic field when they form.

When magma cools the iron present in mafic minerals orient

parallel to the field lines at that time and place

The

Curie Point is

the name

for the temp.

where the

minerals

“lock in”

to this

orientation

~450-500

degrees

slide9

How sedimentary rocks record the earth’s magnetic field. When

iron-rich grains settle out of the water, if the water is “quiet” they

will orient themselves parallel to the earth’s magnetic field. Of

course, the grains may be disturbed after deposition (by burrowing

organisms, for example, so the paleomagnetic signal out of these

rocks is often less reliable.

slide10

Because of the relationship between the angle of inclination and

the latitude on the Earth’s surface where an Fe-rich rock

formed, we can use this information to determine the “paleo-

latitude” for an iron-rich rock.

British geophysicists measured the

angles of inclination of Fe-

rich rocks of a wide range of

ages. What did they find?

Each rock recorded a different angle

of inclination!

How did they interpret this data:

from one time period to another,

the Earth’s magnetic field moved =

Polar Wandering

an polar wandering pw curve showed how the magnetic pole moved or wandered over time figure a below
An polar wandering (PW) curve showed how the magnetic pole moved, or wandered, over time (figure a below)

Of course, if the continents where you sampled the rocks are going to

remain stationary, then the pole must be moving, or wandering

alternatively the pole could be stationary and the continent moving figure b below
Alternatively, the pole could be stationary, and the continent moving (figure b below)

Which interpretation is correct? What happens if you sample rocks from a different continent?

slide13

Rocks of the same age from different continents record two different magnetic

poles! In green is shown where rocks from Eurasia say the magnetic pole

should be, and in red are the data points from North America. Obviously,

there is only 1 magnetic pole, so if you move the continents together, the

lines tracing the location of the magnetic pole through time for these two

continents coincide. Thus, it must be the continents that moved, not the poles!

slide14

Thus, proving that the poles have not moved over the Earth’s surface over time, but the continents moved, re-focused attention on the idea of “continental drift” only now there was a completely new data set from the ocean floor as well: sea floor spreading

Sea floor spreading: new ocean crust is created at ridges and old crust is destroyed at trenches. The sea floor is continually “recycling” material from the mantle.

The theory of plate tectonics synthesizes the two theories of continental

drift and sea floor spreading

how do we use paleomagnetism to locate a plate on the earth s surface
How do we use paleomagnetism to locate a plate on the Earth’s surface?
  • You must know the age of the rock
  • The rock must be Fe-rich, so you can determine the angle of inclination of the Earth’s magnetic field at the time it formed
  • You must know which plate you are on (think for example, of California in the Blakely website. If you determined the paleolatitude of rocks in California, it wouldn’t tell you where N. America was, but where Wrangellia was!)
paleomagnetism helps with latitude only not longitude
Paleomagnetism helps with latitude only, not longitude
  • Two plates can be at the same latitude, but be very far apart, or close together. How do we determine which?
  • How similar are there geologic histories. For example, if in several million years they share an orogeny, then they must have been relatively close together
  • Do they share similar marine animals? The ocean between them must have been relatively narrow; if they share terrestrial animals, then they were very close together!
slide17

How close to Laurentia, or North America, was Baltica?

The fact that they shared the same marine animals suggests that

the Iapetus Ocean was narrow at this time

slide19

What happens when the earth’s magnetic field reverses:

Strength decreases slowly, rapid change in polarity, strength

rebuilds