Theory Development

Grade 8 Science Unit: 07 Lesson: 01 Theory Development

Abraham Ortelius Lived 1527–1598 Dutch map maker and Geographer

1596 - Ortelius suggested that North and South America, Eurasia, and Africa were once joined and have since drifted apart "by earthquakes and floods". His "evidence" was the jigsaw fit of the continents. This fit is especially close when the underwater continental shelves of the continents are matched.

Nicolaus Steno Lived 1636–1686 Danish Anatomist and Geologist

Nicolaus Steno observed the changes in a sequence of rock layers in the mountains of Italy. Steno's observations became known as the Law of Superposition – in a sequence of sedimentary rock layers, each layer of rock is older than the layer above it and younger than the rock layer below it.

James Hutton Lived 1726–1797 Scottish Chemist and Geologist

1785 - Uniformitarian Principle – the geological forces at work in the present day are the same as those that operated in the past. Relative Age Dating of Strata – Hutton used the Steno’s law of superposition to develop this theory. It compares rock layers to rocks above and below the rock. Generally, sedimentary rocks below are older, and rocks above are younger. The Law of Crosscutting – stated that if a fault or other body of rock cuts through another body of rock, then it must be younger in age than the rock through which it cuts and displaces.

Alfred Lothar Wegener Lived 1880–1930 German Meteorologist and Geophysist

Pangaea Theory – claimed that the continents had formed a single landmass, which split apart. Wegener was not the first to suggest that the continents had been one, but was the first to present evidence from several fields.

1912 - Continental Drift Theory – Wegener noticed that there were similar glacial deposits in South America and Africa. He also found that fossils often indicated a climate very different from the climate of today, such as fossils of tropical plants found on an Arctic island. Fossil remains of a specific reptile were found on continents that once touched. All of these facts supported theory of continental drift. Wegener's hypothesis lacked a geological mechanism to explain how the continents could drift across the Earth’s surface. ©2011, TESCCC 10/20/11 page 2 of 4 Grade 8 Science Unit: 07 Lesson: 01

Arthur Holmes Lived 1890–1965 British Geologist

1929 - Convection Currents Theory – about the time Wegener's ideas began to be dismissed for lack of a mechanism of movement, Holmes elaborated on the idea that the mantle undergoes thermal convection. As magma is heated, it tends to rise, then cools and sinks again. This repeated heating and cooling results in a current which may be enough to cause continents to move. Arthur Holmes compared this thermal convection to a conveyor belt. The pressure from the upwelling of magma could break apart a continent and then force the broken continent in opposite directions carried by the convection currents. This idea received very little attention at the time.

Harry Hess Lived 1906–1969 American Geologist

1962 - Sea Floor Spreading Theory – idea that the seafloor itself moves and carries continents with it, as it expands from a central point. The theory is well-accepted now. It is caused by convection currents in the molten, very weak upper mantle, or asthenosphere.

Hess described a geologic mechanism to account for Wegener's moving continents. He said it was possible that molten magma from beneath the earth's crust could ooze up between the plates in the rift in the ocean floor. As the hot magma cooled in the ocean water, it would expand and push the plates beside it. North and South America would move to the west and Eurasia and Africa would move to the east. The Atlantic Ocean would get wider, but the coastlines of the landmasses would not change dramatically.

Dan McKenzie Lives 1942– English Geophysicist

1968 - Theory of Plate Tectonics – is a combination of two earlier ideas: continental drift and sea-floor spreading. This theory was formed as new information was learned about the nature of the ocean floor, Earth's ancient magnetism patterns, the location of volcanoes and earthquakes, the flow of heat from Earth's interior, and the worldwide distribution of plant and animal fossils.

Earth's outermost layer, the lithosphere, is broken into 7 large, rigid pieces called plates: the African, North American, South American, Eurasian, Australian, Antarctic, and Pacific plates. Several minor plates also exist. The plates are all moving in different directions and at different speeds, from 2 cm to 10 cm per year in relationship to each other.

The main features of plate tectonics are: The Earth's surface is covered by a series of crustal plates. The ocean floors are continually moving, spreading from the center, sinking at the edges, and being regenerated. Convection currents beneath the plates move the crustal plates in different directions. The source of heat driving the convection currents is from radioactivity deep in the Earth’s mantle.