Continental Drift to Plate Tectonics

EAS-100-51: Earth Science

CONTINENTAL DRIFT TO PLATE TECTONICS

A. Earliest theories of moving continents

1. During the 17th century, some European map-makers speculated that some continents seemed to "fit" together like a jigsaw puzzle (such as South America and Africa), and must have once been joined together.

B. Wegener's theory of continental drift (1910)

1. Alfred Wegener, a German meteorologist, proposed a theory of continental drift , stating that the positions of the Earth's continents changed relative to each other over geologic time. Wegener claimed that continents on either side of the Atlantic were once joined together, and that all other continents were once joined together in the geologic past as one giant super-continent, which he called Pangaea .

Geological evidence supporting Wegener's theory during the early 20th century

i. Similar (or closely-related) fossils were found on either side of the Atlantic Ocean, implying that the continents were once joined together.

ii. Fossil tropical land plants were found in rocks which are now in the polar regions.

iii. Similar rock types were found in mountain ranges on either side of the Atlantic Ocean (Appalachians and Swiss Alps)

Wegener spent much of his life trying to convince the academic world that the continents drifted over geologic time, but he was unable to describe a convincing mechanism or force that was powerful enough to move land masses. Most geologists of that time believed that continents were immobile. However, geologists in South Africa supported many of Wegener's ideas because local geological conditions matched many of his predictions. He died in 1930 while on a field expedition in Greenland.

C. Theory of seafloor spreading (Arthur Holmes, 1928)

British geologist Arthur Holmes proposed that convection currents in the Earth's mantle carry molten rock up towards the ocean floor. The lava extrudes through cracks in the seafloor and is deposited on either side of this zone, forming mid-oceanic ridges. As time goes on, the newly- extruded lava pushes the older rock further away from the mid-ocean ridge, and the seafloor grows wider, or spreads. However, like Wegener, Holmes was unable to prove that his proposed mechanism could work, due to technological limitations of the time.

C. Paleomagnetism (1930s)

It had been known for centuries that he Earth actually has properties of a huge magnet, with a North and a South pole close (but not coinciding with) the geographic poles. During the 1930s, rocks and stone tools containing magnetite from several archeological digs were analyzed with new techniques that showed the earth's magnetic field of the past was reversed relative to the present. When these objects were heated in a fire (or by some other process such as volcanism), the magnetite grains align themselves to the Earth's magnetic field; when cooled, the objects became "records" of the geomagnetic field for that particular time. Further investigation revealed that the Earth's magnetism changed in polarity and intensity repeatedly over geologic time.

From modern evidence provided by the study of seismic waves and other geophysical evidence, the earth's magnetic field is now believed to be generated by slow convection currents in the liquid iron outer core (which spins at a different speed and direction relative to the solid iron inner core).

III. Theory of plate tectonics (1960s)

A. Oceanographic and geophysical evidence establishes plate tectonics

During the 1960's, the results of ocean floor geophysical surveys revealed symmetrical patterns of magnetic reversals in the mid-ocean ridge rock, indicating that the youngest ocean floor rock was at the ridge, and older rock was further away. This conclusion was further corroborated by drill cores of deep ocean sediments, which showed the same age pattern relative to the mid-ocean ridge. Furthermore, no ocean floor rock was found to be older than 200 million years, although the age of the Earth was known to be 4.6 billion years old. This evidence led to the wide acceptance of plate tectonics as a theory. Some examples of the many unifying features of plate tectonics include: an explanation of why volcanism and seismic tremors are associated with plate boundaries, why mountain ranges and geologic uplift occur, why the development of past life was affected by plate movement, etc.

B. The layered Earth

Geophysical evidence depicts the Earth's interior as being constructed of layers, somewhat like an onion. The thin outermost layer, the crust, consists of two type of rock, broadly generalized as continental and oceanic crust. The two rock types differ in mineral composition, and therefore, in density. Continental crust, which approaches the composition of granite, is less dense than oceanic crust, which approaches the composition of basalt. The crust is broken up into about a dozen blocks, or plates , averaging about 70 km in thickness. The word "tectonics" comes from the study of regional changes in the Earth's structure; plate tectonics incorporates the motions of these plates over geologic time.

C. Spreading Centers and Subduction Zones

The older models of seafloor spreading and continental drift may be integrated with plate tectonics: continental crust, being less dense, "rides" on top of oceanic crust. As the seafloor grows larger at mid-ocean ridges, ocean floor is being destroyed elsewhere; these areas are called subduction zones - a good example is the perimeter of the Pacific Ocean. The margin of the continental crust subducts , or forces the oceanic crust and sediments downwards into the Earth's interior, where they are eventually remelted.