Making Connections:Science in
the Mountains
Plate Tectonics, Earth’s Magnetism
and Radioactivity
P.Benham
Yoho-Burgess Shale Foundation
© 2003
Part I: Continental Drift
Alfred Lothar Wegener, 1880-1930 the
Formulator of the theory of continental drift.
Born on November 1, 1880, Wegener earned a
Ph.D in astronomy from the University of
Berlin in 1904. However, he had always been
interested in geophysics, and also became
fascinated withthe developing fields of
meteorology and climatology. During his life,
Wegener made several key contributions to
meteorology: He pioneered the use of balloons
to track air circulation. In 1906 and 1912
Wegener joined expeditions to Greenland to
study polar air circulation. In 1914 he was
drafted into the German army, but was released
from combat duty after being wounded, and
served out the war in the Army weather
forecasting service. In 1924 he accepted a
specially created professorship in meteorology
and geophysics at the University of Graz, in
Austria. Wegener made what was to be his last
expedition to Greenland in 1930. While
returning from a rescue expedition that brought
food to a party of his colleagues camped in the
middle of the Greenland icecap, he died, a day
or two after his fiftieth birthday.
Text and image
http://www.ucmp.berkeley.edu/history/wegener.html.
Alfred Lothar Wegener, 1880-1930 the Formulator of the theory of continental drift.
In the autumn of 1911, Wegener was browsing in the university library when he came across a
scientific paper that listed fossils of identical plants and animals found on opposite sides of the
Atlantic. Intrigued by this information, Wegener began to look for, and find, more cases of similar
organisms separated by great oceans. Orthodox science at the time explained such cases by
postulating that land bridges, now sunken, had once connected far-flung continents. But Wegener
noticed the close fit between the coastlines of Africa and South America. Might the similarities
among organisms be due, not to land bridges, but to the continents having been joined together at one
time? As he later wrote: "A conviction of the fundamental soundness of the idea took root in my
mind."
Such an insight, to be accepted, would require large amounts of supporting evidence. Wegener found
that large-scale geological features on separated continents often matched very closely when the
continents were brought together. For example, the Appalachian mountains of eastern North America
matched with the Scottish Highlands, and the distinctive rock strata of the Karroo system of South
Africa were identical to those of the Santa Catarina system in Brazil. Wegener also found that the
fossils found in a certain place often indicated a climate utterly different from the climate of today:
for example, fossils of tropical plants, such as ferns and cycads, are found today on the Arctic island
of Spitsbergen. All of these facts supported Wegener's theory of "continental drift." In 1915 the first
edition of The Origin of Continents and Oceans, a book outlining Wegener's theory, was published;
expanded editions were published in 1920, 1922, and 1929. About 300 million years ago, claimed
Wegener, the continents had formed a single mass, called Pangaea (from the Greek for "all the
Earth"). Pangaea had rifted, or split, and its pieces had been moving away from each other ever
since. Wegener was not the first to suggest that the continents had once been connected, but he was
the first to present extensive evidence from several fields.
Text from http://www.ucmp.berkeley.edu/history/wegener.html.
FIT OF THE CONTINENTS A CLUE TO CONTINENTAL DRIFT
Wegener came up
with a theory for
why the world’s
continents fit
together like a
puzzle if you
removed the
intervening oceans.
http://www.uwgb.edu/dutchs/202ovhds/platetec.htm
Fossils of the same species were found on several different continents. Wegener proposed that the species
dispersed when the continents were connected and later carried to their present positions as the continents
drifted. For example, Glossopteris, a fern, was found on the continents of South America, Africa, India, and
Australia. If the continents are reassembled into Pangaea, the distribution of Glossopteris can be accounted
for over a much smaller contiguous geographic area. The distribution of other species can also be accounted
for by initially spreading across Pangaea, followed by the breakup of the supercontinent, and movement of
the continents to their present positions.
From: http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part3.html
104 Million year old Cretaceous
dinosaur remains found in
southeastern Australia are
intriguing because at that time
Australia lay within the Antarctic
Circle. The global climate was
much warmer then. Indeed
permanent icecap didn’t form on
the south pole until around 30
Million years ago.
CONTINENTAL DRIFT- THE GLACIATION CLUE
Glaciation in South America, Africa, India, and Australia is best explained if these
continents were once connected. Glaciers covered all or part of each of these continents
during the same time period in the geologic past.
If the continents were in their present position, a major glaciation event that covered
nearly all of the continents and extended north of the equator would be required.
Geologists have found no evidence of glacial action in the northern hemisphere during
this time period. In fact, during this time period, the climate in North America was
warm.
Text and images from :
http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part5.html
PLATE BOUNDARIES INFLUENCE
GLOBAL TOPOGRAPHY
http://virga.sfsu.edu/courses/gm309/images/world.map2.gif
Mid Oceanic ridges marking growth of new crust due to spreading.
Simple model of main tectonic plates and their boundary relationships
http://neic.usgs.gov/neis/general/seismicity/world.html
RING OF FIRE
Blue lines mark subduction zones where oceanic crusts are sliding under the
continental crust. Orange marks area of high volcanic activity.
This is a reconstruction
of how the continents
may have drifted through
time.
For high quality animations of continental
drift through time visit
http://www.ucmp.berkeley.edu/geology/tec
tonics.html and http://www.scotese.com/
(rotatable views of Earth at different times
in history
Continental drift has ripped India from the
continent of Antarctica over 135 million years
ago. Fossil record shows unique fauna and flora
(including dinosaurs) evolved during this time.
India slammed into Asia about 10 million years
ago. The Himalayas are a result of this collision
THE BASICS OF PLATE TECTONICS
1) Crust consists of rigid moving plates
2) These can move apart, creating new crust, strike slip past each other (like San Andreas fault), or converge
on each other and consume crust (subduction)
3) Continents are passive, with deformation at the edges unless rifting begins (I.e. Great Rift Valley in
Africa). They contain the oldest rocks.
4) Ocean Basins are active and have thinner crust. No ocean crust older than Jurassic due to recycling by
continental drift.
5) Convection is the driving force.
The diagram, from a 1969 paper by Isaacs, Oliver and Sykes, shows the
different types of plate interaction
http://www.uwgb.edu/dutchs/202ovhds/platetec.htm
Part II: Magnetism and the Earth
LAYERS OF THE EARTH
The Earth is divided into three chemical layers: the core, the mantle and the crust. The core is composed
of mostly iron and nickel and remains very hot, even after 4.5 billion years of cooling. The core is
divided into two layers: a solid inner core and a liquid outer core. The middle layer of the Earth, the
mantle, is made of minerals rich in the elements iron, magnesium, silicon, and oxygen. The crust is rich
in the elements oxygen and silicon with lesser amounts of aluminum, iron, magnesium, calcium,
potassium, and sodium. There are two types of crust. Basalt is the most common rock on Earth. Oceanic
crust is made of relatively dense rock called basalt. Continental crust is made of lower density rocks,
such as andesite and granite.
Picture of dipolar magnet. Iron filings
mimic lines of magnetic force.
The earth behaves just like a dipolar magnet
because it has a solid iron core
THE MAGNETOSPHERE
Our planet is connected with our sun with more than light. ON THE NEXT SLIDE , it
appears the sun and earth are connected by the stream of charged particles that come from
the sun. The Sun produces a hot gas that travels through space at a million miles per hour,
carrying particles and magnetism outward past the planets. In essense, the Earth is immersed
in the Sun's atmosphere. Changes on the Sun affect the solar wind flow; for example, solar
flares, which are explosions associated with sunspots, cause strong gusts of solar wind.
The space around our atmosphere is alive and dynamic because the Earth's magnetic field
reacts to changes in the solar wind. The interaction between the solar wind and the plasma of
the magnetosphere acts like an electric generator, creating electric fields deep inside the
magnetosphere. These fields in turn give rise to a general circulation of the plasma within
the magnetosphere and accelerate some electrons and ions to higher energies.
During periods of gusty solar wind, powerful magnetic storms in space near the Earth cause
vivid auroras, radio and television static, power blackouts, navigation problems for ships and
airplanes with magnetic compasses, and damage to satelites and spacecraft. Events on the
Sun and in the magnetosphere can also trigger changes in the electrical and chemical
properties of the atmosphere, the ozone layer, and high-altitude temperatures and wind
patterns.
Text from
http://science.msfc.nasa.gov/ssl/pad/sppb/edu/magnetosphere/mag3.html
THE MAGNETOSPHERE
Image from http://science.msfc.nasa.gov/ssl/pad/sppb/edu/magnetosphere/mag3.html
WHY CARE ABOUT THE EARTH’S MAGNETOSPHERE?
We've learned that Earth's atmosphere is protected from the solar wind by our magnetosphere.
Even so, some solar wind energy does enter our magnetosphere and atmosphere and can cause a
small amount of our atmosphere to be launched into space. We need to understand this loss of our
atmosphere in order to understand our planet's environmental stability over a long time period.
Solar wind energy in our magnetosphere can also cause what are known as space plasma storms.
Thesestorms can cause communication and science satellites to fail. They can also cause damage
to electric power systems on the surface of the Earth.
A large space storm in 1989 made currents on the ground that caused a failure in the Hydro-Quebec
electric power system. This prevented 6 million people in Canada and the US from having
electricity for over 9 hours. The same storm caused the atmosphere to inflate and dragged the
LDEF satellite to a lower orbit earlier than expected.
Text from
http://science.msfc.nasa.gov/ssl/pad/sppb/edu/magnetosphere/mag6.html
WANDERING POLES
The Earth’s magnetic pole wanders on a daily basis (right). The daily rotation of
the magnetic pole is influenced by the spinning of the Earth. The core is thought to
consist of solid iron and rotate at a different speed from ther surface of the Earth.
This results in daily, short and long term movement.
For more information on this topic visit
http://www.geolab.nrcan.gc.ca/geomag/no
rthpole_e.shtml
WANDERING POLES
The Earth’s magnetic pole also wanders on a longterm basis (bottom left). By
2050 the pole could drift into Russian territory (bottom right). The rapid drift
that has occured since 1904 may proceed a major internal reorganization of the
Earth’s magnetic field that will lead to a flip of magnetic poles
Source: http://www.geolab.nrcan.gc.ca/geomag/northpole_e.shtml
WANDERING POLES
It appears that the North Magnetic Pole moved southeast a distance of approximately 860 km
between 1760 to 1860 (bottom right). Prior to that is was located in a relatively confined area near
75° N, 110° W. It was then “stationary” until 1904 before beginning to drift northwards. The change
in velocity of the North Magnetic Pole since the early 1970s has been remarkable – 9 km/yr to 41
km/yr (bottom left).
Changes in the magnetic field
characterized by an abrupt change in
the secular variation have been
named (geo) magnetic jerks" or
"geomagnetic impulses". Six jerks of
global extent have occurred during
the past century: in 1901, 1913, 1925,
1969, 1978 and 1992. The last three
jerks can be seen clearly as abrupt
changes in the slope of the annual
change in H at Resolute Bay. The
1969 jerk corresponds to the start of
the increase in the speed of the NMP
and the two subsequent jerks,
especially that near 1992, appear to
correlate with additional increases in
the speed.
Text and images : http://www.geolab.nrcan.gc.ca/geomag/long_mvt_nmp2_e.shtml
PLATE TECTONICS AND “FOSSIL” MAGNETISM
(PALEOMAGNETISM)
•Magnetic minerals retain their magnetic properties unles heated above their CURIE
TEMPERATURE (e.g. 580 degrees C for magnetite and 780 degree C for pure iron)
•The temperature gradient for the Eath’s crust is approximately 30 degrees C / km; the Curie
temperatrure for iron is reached about 25 km below the Earth’s surface/ We should not
expect permanent magnetism below this depth.
•At hot magma crystallizes, magnetic minerals cool to below the Curie temperature and
become aligned with the Earth’s magnetic field
•The Earth’s magnetic field has “flipped” or REVERSED POLARITY over geological time.
•If the magnetic mineral is reheated above this tmperature then the alignment is RESET in
the new field.
•These concepts are critical in explaining how magnetic striping would occur on the ocean
floor
Magnetic Reversals
The Earth's magnetic field is aligned roughly along the spin axis and has an
approximate dipole shape, similar to that of a bar magnet, with north and
south magnetic poles. This is the normal state of affairs, but occasionally the
magnetic field switches polarity, the north and south magnetic poles reverse,
and the field settles down in the opposite state. The process goes by several
names – "magnetic field reversal" and "polarity transition" are the most
common.
Reversals have been documented as far back as 330 million years. During
that time more than 400 reversals have taken place, one roughly every
700,000 years on average. However, the time between reversals is not
constant, varying from less than 100,000 years, to tens of millions of years. In
recent geological times reversals have been occurring on average once every
200,000 years, but the last reversal occurred 780,000 years ago. At that time
the magnetic field underwent a transition from a "reversed" state to its present
“normal state".
A full magnetic reveral may take from 1000 to 8000 years to occur. Geologically instantaneous but not in
the context of human perception.
Text and image from
http://www.geolab.nrcan.gc.ca/geomag/reversals_e.shtml
SO HOW DOES THIS FIT INTO PLATE TECTONICS?
It all starts with a ridge that runs down the middle of the
Atlantic and smack through Iceland.
DISCOVERY OF SEA FLOOR SPREADING
After World War II, detailed surveys of the ocean floor revealed complex topography that required
geologists to rethink completely their concepts about the structure of the ocean basins. Previously, the
ocean floors were assumed to be stable flat plains.In reality the topography was incredibly diverse,
consisting of mountain ranges and deep trenches that dwarfed anything seen on land. By the mid-1960s
the extent of these submarine features had been mapped, and six major topographic features were
defined: mid-ocean ridges, fracture zones/transform faults, seamounts, trenches, abyssal plains and
continental margins. The single largest topographic feature on the planet is the mid-ocean ridge (MOR)
system. This huge mountain range extends for 65,000 km and cuts across all major ocean basins. At its
widest, the MOR is nearly 1500 km across and reaches heights of 3 km above the ocean floor
Text adapted from http://www.courses.psu.edu/geosc/geosc001_tfl3/exercises/Tectonics.htm . Diagrams
from http://www.uwgb.edu/dutchs/202ovhds/platetec.htm
OCEAN FLOOR TOPOGRAPHY
DISCOVERY OF SEA FLOOR SPREADING
In the late 1950's, scientists mapped the present-day magnetic field generated by rocks on the floor of
the Pacific Ocean. The volcanic rocks which make up the sea floor have magnetization because, as they
cool, magnetic minerals within the rock align to the Earth's magnetic field. They found positive and
negative magnetic anomalies. Positive magnetic anomalies are places where the magnetic field is
stronger than expected. Positive magnetic anomalies are induced when the rock cools and solidifies
with the Earth's north magnetic pole in the northern geographic hemisphere (and vice versa for negative
anomalies). The anomalies produce a zebra-striped pattern of parallel positive and negative bands. The
pattern was centered along, and symmetrical to, the mid-ocean ridge. In 1963 by Fred Vine &
Drummond Matthews proposed that lava erupted at different times along the rift at the crest of the midocean ridges preserved different magnetic anomalies.
Text adapted from http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part9.html.
DISCOVERY OF SEA FLOOR SPREADING
Vine and Matthews proposed that lava erupted on the sea
floor on both sides of the rift, solidified, and moved away
before more lava was erupted. If the Earth's magnetic
field had reversed (changed from one geographic pole to
the other) between the two eruptions, the lava flows
would preserve a set of parallel bands with different
magnetic properties. The ability of Vine and Matthews'
hypothesis to explain the observed pattern of ocean floor
magnetic anomalies provided strong support for sea floor
spreading.
Text and diagrams adapted from
Geomagnetic Reversal Timescale
Global data for last 5 Million years based on
sediment and volcanic rock analyses.
Source of images & Excellent website on Earth’s paleomagnetism
http://sorcerer.ucsd.edu/es160/lecture8/web6/node1.html also see
http://tlacaelel.igeofcu.unam.mx/~mary/geomagnetic_field_reversals.htm
Youngest ocean crust coloured red and oldest is blue. Note spreading
zones and offsetting (transform) faults.
Canadian geophysicist J. Tuzo Wilson
(1908-1993)
In 1963, he developed a concept crucial to the
plate-tectonics theory. He suggested that the
Hawaiian and other volcanic island chains may
have formed due to the movement of a plate over a
stationary "hotspot" in the mantle. This hypothesis
eliminated an apparent contradiction to the platetectonics theory -- the occurrence of active
volcanoes located many thousands of kilometers
from the nearest plate boundary. His idea was
considered so radical that his "hotspot" manuscript
was rejected by all the major international scientific
journals. This manuscript ultimately was published
in 1963 in a relatively obscure publication, the
Canadian Journal of Physics, and became a
milestone in plate tectonics.
Text adapted and images from
http://pubs.usgs.gov/publications/tex
t/Wilson.html.
Two years later he proposed that there must be a
third type of plate boundary to connect the
oceanic ridges and trenches, which he noted can
end abruptly and "transform" into major faults
that slip horizontally. A well-known example of
such a transform-fault boundary
is the San Andreas Fault zone. Unlike ridges and
trenches, transform faults offset the crust
horizontally, without creating or
destroying crust.
LOIHI SEAMOUNT: The Next Hawaiian Island
Images from http://www.soest.hawaii.edu/GG/HCV/loihi.html
Topographic map of the volcanic cone (above) that
comprises the Loihi Seamount. Note the three craters at the
peak. Pillow shaped blobs of rock (right) are typical shape
HOTSPOTS
Hotspots are a persistent magma convection point in the Earth’s Mantle. As the
crust glides over it, driven by continental drift, it periodically bursts through in
volcanic eruptions. A classic example is the trail of Hawaiian islands. As the
Pacific Plate drifts eastwards and is consumed by the North American plate a
series of islands are forming in the middle of the ocean. The Island of Hawaii is
the youngest, but not the last of the chain. Loihi will be the next to emerge.
Note m.y. stands for Million years. Pictures from
http://www.uwgb.edu/dutchs/202ovhds/platetec.htm
Diagram illustrating movement of oceanic
crust over hot spot an chain of islands that
form.
San Andreas is an
example of a
transform fault
SEAFLOOR SPREADING ON THE PACIFIC COAST
There was once a continuous ridge system
off the West Coast.
The West Coast was once occupied by a
continuous subduction zone.
Most of the ridge and all the crust on the
other side of it has been subducted
beneath North America. The small ridge
and subduction zone in the Pacific
Northwest are the last remnants of these
former plate boundaries.
There was once a junction of three ridges
in the North Pacific. The anomalies off
California tell of east-west spreading, but
those off Alaska imply north-south
spreading. There must have been an
additional ridge to accomodate the
differences in plate motion.
Text and diagram from http://www.uwgb.edu/dutchs/platetec/kula.htm
SEAFLOOR SPREADING ON
THE PACIFIC COAST
Geologists deduce that over an 80 m.y. period a
triple spreading junction formed in the Pacific and
was consumed under North America
Text and diagram from
http://www.uwgb.edu/dutchs/platetec/kula.htm
A theoretical model of the formation of magnetic
striping. New oceanic crust forming continuously at
the crest of the mid-ocean ridge cools and becomes
increasingly older as it moves away from the ridge
crest with seafloor spreading : a. the spreading ridge
about 5 million years ago; b. about 2 to 3 million
years ago; and c. present-day.
Early in the 20th century, paleomagnetists (those who
study the Earth's ancient magnetic field) -- such as
Bernard Brunhes in France (in 1906) and Motonari
Matuyama in Japan (in the 1920s) -- recognized that
rocks generally belong to two groups according to
their magnetic properties. One group has so-called
normal polarity, characterized by the magnetic
minerals in the rock having the same polarity as that
of the Earth's present magnetic field. This would
result in the north end of the rock's "compass needle"
pointing toward magnetic north. The other group,
however, has reversed polarity, indicated by a
polarity alignment opposite to that of the Earth's
present magnetic field. In this case, the north end of
the rock's compass needle would point south. How
could this be?
http://pubs.usgs.gov/publications/text/developing.html
This answer lies in the magnetite in
volcanic rock. Grains of magnetite -behaving like little
magnets -- can align themselves with the
orientation of the Earth's magnetic field.
When magma (molten rock containing
minerals and gases) cools to form solid
volcanic rock, the alignment of the
magnetite grains is "locked in," recording
the Earth's magnetic
orientation or polarity (normal or
reversed) at the time of cooling.
Hot springs and volcanic activity on spreading ridges provide
chemical nutrition to a wide variety of specially adapted life forms.
Tube worms (top left) for example survive in boiling temperatures
and generate food by processing toxic hydrogen sulphide gases
through bacteria in their guts. The giant clam Calyptogena behaves
in a similar way
Diagram of subduction: oceanic crust
being consumed under continental crust
as two plates converge. Oceanic crust
melts and rises to surface where it is
erupted. This is what is happening on
the west coast of North America. Mount
St Helens, Mount Baker and Mount
Garibaldi are all periodically spitting out
molten oceanic crust
Subduction is occurring underneath
Japan too. The frequent earthquakes
are aresult of friction as the two
plates slide past each other.
Scientists record the earthquakes
and map out where the subduction
is occurring
Magnetic record of sea
floor spreading at Juan
de Fuca Ridge of the
coast of BC
ADDITIONAL DIAGRAMS
CROSS-SECTION OF PLANET EARTH
Another view of the Earth acting like a
dipolar magnet
CRUSTAL AGE OF WORLD’S OCEANS
http://www.ngdc.noaa.gov/mgg/global/crustage.HTML
Examples of converging crust
http://www.google.ca/search?q=cache:l2op6CXHs2IJ:www.enchantedlearning.com/su
bjects/dinosaurs/glossary/Contdrift.shtml+origin+india+continental+drift&hl=en&ie=
UTF-8