Chapter 2 Pangaea pieces of earth fit together to make a supercontinent then fragmented and drifted apart continental drift Wegener believed that Pangaea existed until end of Mesozoic Era fit of continents continents all fit together with few gaps or overlap distribution of similar aged fossils species evolved independently on different conti nents but they could have migrated across land on Pangaea Wegener plotted fossils and found that they had existed on several continents so they had to have been adjacent to one another matching geologic units folded mountain belts of similar age etc when continents were joined matching rock groups could have been adjacent to each other forming continuous blocks or belts Ex Appalachian Mountains match mountain belts in Greenland and Great Britain which would have been adjacent to each other Figure 2 4 indicates that if a newly formed rock could record the magnetic field lines wherever it formed on the earth s surface at the time the rock formed you d expect to see a strong latitude influence on how steep the fossilized earth s magnetic field was in that rock See Figure 2 5 for examples from cooling lavas Looking back at Figure 2 4d if a lava flow cools at the earth s equator today and you later study the orientation of the magnetic field locked in to the rock the field lines should be horizontal in that lava flow because the orientation of the earth s field at the equator today is horizontal parallel to the earth s surface and pointing North If a lava flow erupts at the equator during a time of Reversed polarity you d see a horizontal remanent mag netic field but one that points South since that is how the field lines run during a Reversed polar ity episode If you let the lava flow erupt onto parking areas in Boston during a time of Normal Polarity like today s at latitude about 40o North of the equator you d see remanent magnetic field orientation that was pointing North as today and with the north direction pointing down into the earth at about 50 degrees to the surface see Fig 2 4d Paleomagnetism is the study of ancient magnetic fields recorded in igneous sedimentary or metamorphic rocks If you can a date the rock precisely because there is good fossil age control or good radioactive dating be havior and b a clean paleomagnetic signal that you can interpret easily you can use that rock to help build polar wander curves studies that show how plates have moved over time In general paleomagnetic studies can constrain the latitude of origin of old rocks much better than they can identify the longitude because the earth s field is very symmetrical all the way around the north or south poles See Figure 2 6 If you find the concept of polar wander curves confus ing you re right about where you should be it is not simple to visualize what the evidence is trying to tell you The main thing to realize is that it is not the earth s magnetic poles that wan der although they do a little but in a highly mobile plate tectonic environment lithosphere plates can move many hundreds of km as they move away from divergent plate boundaries and move towards convergent plate boundaries If you imagine rocks forming steadily on a moving plate over geologic time if you use paleomagnetism and the age of each rock to guesstimate where the earth s North magnetic pole must have been at Time X given the dip of magnetic field lines in a rock that formed at time X and then you plot the apparent location of where the earth s North magnetic pole must have been at Times Y Z A B C you have a polar wan der curve If the poles are remaining more or less stationary and you still have an extensive track across the globe it must be the plate that is moving not the pole Mid ocean ridges submarine mountain ranges lie 2 km below sea level crest is called ridge axis Deep ocean trenches deep ocean depths 8 12 km Volcanic arcs curving chains of active volcanoes Seamount chains isolated submarine mountains which were once volcanoes but no longer erupt Fracture zones narrow bands of vertical cracks and broken rock Abyssal plains broad flat regions of the ocean that lie at a depth of 5 km below sea level Very thin sediment cover on the ocean ridges themselves but progressively thicker sedi ment layers the farther you look away from ocean ridges Basalt dominated rock type for oceanic crust as opposed to more felsic rock types and a wider range of rock types found on continental crust Highest heat flow values are localized along the mid ocean ridges and the farther you look to either side the colder the rock is lower heat flow with greater distance from the spreading center Most earthquakes in the ocean basins are localized either along mid ocean ridges or else along fracture zones where two plates slide past each other in opposite directions Lithospheric plates Pieces of Earth s rigid plates Plate boundaries breaks between the plate active margins Rock along plate boundaries undergoes intense deformation as plate grinds against other plates so continents move as well Earthquake belts define the position of plate boundaries Some plates consist of entirely oceanic lithosphere and some consist of oceanic and continen tal lithosphere Divergent boundary two plates move away from the axis of a mid ocean ridge and new oceanic lithosphere forms Convergent boundary two plates more toward each other downgoing plate sinks beneath over riding plate Transform boundary two plates slide past each other on a vertical fault surface The opposing plate motions generate continuous shear stress across the fault which deforms rock until the stored energy breaks the weakest rock in the area and an earthquake occurs Continental shelf surface of thick accumulations of sediment that covers thinned crust basalt magma originates in divergent plate boundaries because hot asthenosphere rises beneath the ridge and begins to melt so magma forms Some magma solidifies to make gabbro some rises and fills vertical cracks and makes dikes of basalt long term production of basalt beneath an ocean ridge magma from mantle rises to surface at the ridge forms oceanic crust then moves laterally away from ridge Tension breaks the crust resulting in the formation of faults divergent boundary earthquakes and younger ocean floor Subduction when two plates move towards each other one sinks below the other only oceanic continental cannot sink As the lithosphere sinks asthenosphere flows out of its way plates get wedged into floor called an