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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.
©2011, TESCCC
10/20/11
page 1 of 4
Grade 8
Science
Unit: 07 Lesson: 01
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.
©2011, TESCCC
10/20/11
page 3 of 4
Grade 8
Science
Unit: 07 Lesson: 01
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.
©2011, TESCCC
10/20/11
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