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 ofBerlin 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, inAustria. 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 theAtlantic. 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.

http://www.uwgb.edu/dutchs/202ovhds/platetec.htm

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.

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

http://virga.sfsu.edu/courses/gm309/images/world.map2.gif

PLATE BOUNDARIES INFLUENCE

GLOBAL TOPOGRAPHY

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 FIREBlue 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/tectonics.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. Text and diagrams from: http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part1.html

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 fromhttp://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 fromhttp://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/northpole_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).

Text and images : http://www.geolab.nrcan.gc.ca/geomag/long_mvt_nmp2_e.shtml

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.

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

Text and image from http://www.geolab.nrcan.gc.ca/geomag/reversals_e.shtml

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.

It all starts with a ridge that runs down the middle of the Atlantic and smack through Iceland.

SO HOW DOES THIS FIT INTO PLATE TECTONICS?

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 mid-

ocean ridges preserved different magnetic anomalies. Text adapted from http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part9.html. Diagrams from http://www.uwgb.edu/dutchs/202ovhds/platetec.htm

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 http://volcano.und.nodak.edu/vwdocs/vwlessons/plate_tectonics/part9.html.

DISCOVERY OF SEA FLOOR SPREADING

Source of images & Excellent website on Earth’s paleomagnetismhttp://sorcerer.ucsd.edu/es160/lecture8/web6/node1.html also seehttp://tlacaelel.igeofcu.unam.mx/~mary/geomagnetic_field_reversals.htm

Geomagnetic Reversal Timescale

Global data for last 5 Million years based on sediment and volcanic rock analyses.

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 plate-tectonics 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. 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 canend abruptly and "transform" into major faults that slip horizontally. A well-known example of such a transform-fault boundaryis the San Andreas Fault zone. Unlike ridges and trenches, transform faults offset the crust horizontally, without creating or

destroying crust.

Text adapted and images from http://pubs.usgs.gov/publications/text/Wilson.html.

LOIHI SEAMOUNT: The Next Hawaiian Island

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

for lava erupted underwater.

Images from http://www.soest.hawaii.edu/GG/HCV/loihi.html

HOTSPOTSHotspots 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

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.

SEAFLOOR SPREADING ON THE PACIFIC COAST

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?

This answer lies in the magnetite in volcanic rock. Grains of magnetite -- behaving like littlemagnets -- 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 magneticorientation or polarity (normal or reversed) at the time of cooling.http://pubs.usgs.gov/publications/text/developing.html

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

http://www.ngdc.noaa.gov/mgg/global/crustage.HTML

CRUSTAL AGE OF WORLD’S OCEANS

Examples of converging crust

http://www.google.ca/search?q=cache:l2op6CXHs2IJ:www.enchantedlearning.com/subjects/dinosaurs/glossary/Contdrift.shtml+origin+india+continental+drift&hl=en&ie=UTF-8