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

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

theory of plate tectonics

The earth’s magnetic field behaves like a dipole magnet

The basics ofdipole magnets

Magnetic field lines encircle the earth in 3D.Field lines “come to closure” at the geographic north pole, whichis why compasses point here. What does that say about theorientation of the earth’s dipole in the core? Yes, the south end ofthe dipole is pointing north!

The 3D orientation of the earth’s magnetic field also meansthat field lines are oriented at different angles relative to theEarth’s surface; parallel at the equator, perpendicular at thepoles, approx. 45 degrees at 45 degrees N or S latitude.

So a compass can rotate in two directions, towards closure at theSouth end of the dipole (our geographic North pole) ANDvertically, parallel to the field orientation relative to the surface of the earth = angle of inclination

The relationshipbetween latitudeand angle ofinclination:

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

TheCurie Point is the name for the temp.where theminerals“lock in” to thisorientation ~450-500degrees

How sedimentary rocks record the earth’s magnetic field. Wheniron-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 burrowingorganisms, for example, so the paleomagnetic signal out of theserocks is often less reliable.

Because of the relationship between the angle of inclination andthe latitude on the Earth’s surface where an Fe-rich rockformed, we can use this information to determine the “paleo-latitude” for an iron-rich rock.

British geophysicists measured theangles of inclination of Fe-rich rocks of a wide range ofages. 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)

Of course, if the continents where you sampled the rocks are going toremain stationary, then the pole must be moving, or wandering

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?

Rocks of the same age from different continents record two different magneticpoles! 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!

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 continentaldrift and sea floor spreading

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

• Two plates can be at the same latitude, but be very far apart, or close together. How do we determine which?

(a) 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

(b) 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!

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

How similar would you expect the marine faunas of China and Laurentia to be?

What happens when the earth’s magnetic field reverses:

Strength decreases slowly, rapid change in polarity, strengthrebuilds