EARTH SCI.

Harry Hess

(1960s) A gentleman by the name of Harry Hess proposed that not only were the continents moving, but the sea floor was also moving.

Lithosphere

(Geological Science) the rigid outer layer of the earth, having an average thickness of about 75 km and comprising the earth's crust and the solid part of the mantle above the asthenosphere

Evidences of continental drift

*Wegener noticed that the continents seemed to fit together, not at the continuously changing shoreline, but at the edge to their continental shelves. He derived this hypothesis from the observation that the continents in the southern hemisphere exhibit an identical pattern of rock and fossils known as the "Gondwana sequence". The most logical explanation was that the continents themselves were once parts of a much larger "super-continent" which was named Pangaea! *A second idea supporting movement of the continents was the glacial till deposits in the southern hemisphere. With the continents in their present positions, the till deposits indicate erratic glacier motion. When the continents are fitted together, they show a much more streamlined motion of the glacier from southern Africa and Northern Australia outward. Wegener collected ancient climate information to confirm his glacial data. For the glaciers to have been present with the continents in their current positions, much of the world should have been under ice. But this wasn't the case. Sedimentary rock examination showed a change in climates, and the only possible explanations were continental drift or movement of the poles.

Earth's magnetic field

Earth's magnetic field (also known as the geomagnetic field) is the magnetic field that extends from the Earth's inner core to where it meets the solar wind, a stream of energetic particles emanating from the Sun. Its magnitude at the Earth's surface ranges from 25 to 65 µT (0.25 to 0.65 G). It is approximately the field of a magnetic dipole tilted at an angle of 11 degrees with respect to the rotational axis—as if there were a bar magnet placed at that angle at the center of the Earth. However, unlike the field of a bar magnet, Earth's field changes over time because it is generated by the motion of molten iron alloys in the Earth's outer core (the geodynamo).

Convection

Hess's explanation of the motion of the sea floor was mantle convection. It has been understood that the Earth's mantle behaves plastically. At the ridge crest in the sea floor, hot magma rises because it is less dense than the surrounding magma. When it reaches the surface it cools and hardens becoming the sea floor. As more magma rises, the present sea floor is pushed aside. At plate boundaries, the cold and more dense rock sinks and melts, becoming magma once again. This mantle convection is the driving force for the motion of the sea floor and the continents themselves.

Normal Magnetic Polarity

Normal polarity is defined as the magnetic minerals in the rock having the same polarity as the Earth's present magnetic field (i.e. the north end of the rock's "compass needle" is pointing in the same direction as Earth's current magnetic north).

Paleomagnetism

A later piece of evidence that was given in support of Wegener's ideas. One source of evidence was that of paleomagnetism. When hot magma rises to the earth's surface and cools, the minerals themselves (especially magnetite) become magnetized in alignment with the Earth's magnetic field. Rocks that were formed at different places on the Earth's surface have different magnetizations. Using this information, when rock layers were uncovered with a magnetization that did not agree with it's position on the Earth, questions were raised. It was proposed that the Earth's magnetic poles wandered. Paleomagnetic data was collected from North America and Europe. Using the alignment of magnetite minerals in the rock layers, the movement of the magnetic poles could be traced through the different geologic periods.

The theory of continental drift

A term, no longer used by geologists, that refers to the fact that continents are not stationary, but move across the Earth's surface. This was proposed by Alfred Wegner. Continental drift is one feature of the modern theory of plate tectonics. (See Pangaea.)

The Theory Of Plate Tectonics

A theory in geology: the lithosphere of the earth is divided into a small number of plates which float on and travel independently over the mantle and much of the earth's seismic activity occurs at the boundaries of these plates

Asthenosphere

A thin semifluid layer of the earth (100-200 km thick), below the outer rigid lithosphere, forming part of the mantle and thought to be able to flow vertically and horizontally, enabling sections of lithosphere to subside, rise, and undergo lateral movement. See also isostasy

Alfred Wegner

German geophysicist, meteorologist, and explorer who proposed the theory of continental drift. He posited the existence of the supercontinent Pangaea in his book The Origin of Continents and Oceans (1915).

Reverse Magnetic Polarity

In contrast, reversed polarity is defined as the magnetic minerals in the rock having opposite polarity to the Earth's present magnetic field (i.e. the north end of the rock's "compass needle" is pointing south).

Magnetic Stripes

Magnetic stripes refer to stripes of alternating normal and reversed polarities frozen into oceanic crust, caused by earth's changing magnetic poles.

Sea floor spreading

Motion of the sea floor in a conveyor belt fashion explains the phenomena of the youngest rocks being found only at the mid-ocean ridges, and the rocks get progressively older as you move away from the ridge. If the sea floor is spreading, then clearly the continents could diverge from one another. The rock of the sea floor also exhibited magnetic properties. Not only did the magnetism of these rocks change over time, they completely reversed which supported the motion of the magnetic poles.

Oceanic Crust

Oceanic crust is the part of Earth's lithosphere that surfaces in the ocean basins. Oceanic crust is primarily composed of mafic rocks, or sima, which is rich in iron and magnesium. It is thinner than continental crust, or sial, generally less than 10 kilometers thick, however it is denser, having a mean density of about 2.9 grams per cubic centimeter[1] as opposed to Continental which has a density of about 2.7 grams per cubic centimeter.

Pangea

Pangaea or Pangea was a supercontinent that existed during the late Paleozoic and early Mesozoic eras. It formed approximately 300 million years ago and then began to break apart after about 100 million years.

Continental Crust

Te continental crust is the layer of igneous, sedimentary, and metamorphic rocks which forms the continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial because there is more felsic, or granitic, bulk composition, which lies in contrast to the oceanic crust, called sima because of the mafic or basaltic rock. (Based on the change in velocity of seismic waves, it is believed that at a certain depth sial becomes close in its physical properties to sima. This line is called the Conrad discontinuity.) Consisting mostly of granitic rock, continental crust has a density of about 2.7 g/cm3 and is less dense than the material of the Earth's mantle (density of about 3.3 g/cm3), which consists of mafic rock. Continental crust is also less dense than oceanic crust (density of about 2.9 g/cm3), though it is considerably thicker; mostly 25 to 70 km versus the average oceanic thickness of around 7-10 km. About 40% of the Earth's surface is now overlaid by continental crust. [1] Continental crust makes up about 70% of the volume of Earth's crust.[2]