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What Wegener Knew
and what he didn’t know
Earth Science for KS4
Earth Science Education Unit
www.earthscienceeducation.com
© The Earth Science Education Unit
Copyright is waived for original material contained herein if it is required for use within the laboratory or classroom. Copyright material included from other publishers rests with them.
What Wegener knew - and what he didn’t know
How Science Works: Alfred Wegener’s ‘Continental Drift’
theory
- based on his 1929 book
Find out -what Wegener knew:
continental coastline shape
continental geology
fossil evidence
biological evidence
geophysical evidence
palaeoclimate evidence
longitude evidence
© The Earth Science Education UnitCopyright is waived for use of this presentation within the laboratory or classroom
According to UK copyright law (detailed on http://www.mda.org.uk/cbasics.htm), copyright on literary works expires 70 years after the death of the author, even if the work is republished elsewhere. All the diagrams used here are from Wegener’ republished work and, since he died in 1930, are out of copyright. They are taken from the translation by John Biram published by Dover (New York) in 1966. However, if you believe your copyright
is being infringed, please contact us. We welcome any information that will help us to update our records.
What Wegener knew - and what he didn’t know
what Wegener didn’t know:• sloping zones of earthquakes
beneath trenches• earthquakes and volcanic
activity at ocean ridges• ocean floor magnetic stripes• age of the ocean floor• lithosphere and
asthenosphere• modern movement of
continents
All this - and more - through this interactive PowerPointNote: Wegener is pronounced as ‘vain’ (without the ‘n’) followed by ‘gun’ and ‘er’
about Wegener:• contrasts between
Wegener’s ‘continental drift and today’s ‘plate tectonics’
• why Wegener wasn’t believed
• Wegener’s adventurous life
• In 1910, Wegener noted, as others had before him, that the coastlines on either side of the Atlantic had similar shapes
• Hypothesis? - what explanations might account for this?
Similar shapes of coastlines - the ‘jigsaw fit’
• the continents were once together and have drifted apart
or
• there was once a continent in between, that has sunk
or
• it is coincidence
or
• the continents and coastlines were made that way
The geology - the ‘picture on the jigsaw’
• Wegener found a scientific paper published in 1927 by du Toit (pronounced ‘dew toy’) showing that the geology of the South American and African coast areas matched closely
• Hypothesis? - what might account for this match?
• the continents were once together and have drifted apart
or
• there was once a continent with similar geology in between, that has sunk
or
• it is coincidence or they were made that way
The fossils - more ‘picture on the jigsaw’
• Wegener knew about the published evidence that Glossopteris and related plants were found on different southern hemisphere continents - as shown in green on this ‘reconstruction’ of these continents (evidence from other fossils is also shown)
• Hypothesis? - what could have caused this distribution?
• the continents were once together and have drifted apart
or• there were once ‘sunken continents’ or
‘land bridges’ between the continents
or• vegetation rafts, with their ‘cargo’ of
animals floated on ocean currents between the continents
The biology - more ‘picture on the jigsaw’• Wegener found publications showing the
distributions of modern-day species, like this one for a garden snail species by Ökland (1927). He found similar published data across the north Atlantic Ocean for an earthworm species, a fresh-water fish species (perch), a fresh-water mussel species and the common heather species
• the continents were once together and have drifted apart
or
• Hypothesis? - what could have caused these distributions?
• there were once ‘sunken continents’ or ‘land bridges’ between the continents - like these shown for the South Atlantic
• Wegener knew about the work by geophysicists showing that the continents seem to be ‘floating’ on material beneath the Earth’s surface (the theory of isostasy). So when a continental area is weighed down by an ice sheet and the ice melts, the continent bounces back, as in Scandinavia today
• Hypothesis? - what does this show about ‘sunken continents’ and ‘land bridges’?
Geophysical evidence - can there be sunken continents?
• continents and ‘land bridges’ cannot sink into ocean basins
or• there must be great masses of unusually heavy material on top of
‘sunken continents’ and ’land bridges’ holding them down
The palaeoclimate - more ‘picture on the jigsaw’
• Wegener and his father-in-law, Köppen, studied modern and ancient (palaeo-) climates. They noted that large ice sheets are only found in polar regions today. But published papers showed that 300 million year old rocks on the southern continents and India have evidence showing they were covered by ice sheets then (scratched rocks and ancient moraine and till deposits)
• Hypothesis? - what could have caused this pattern?
• the continents were once near one of the poles, but have drifted away
or
• most of the Earth was covered by an ice sheet at the time
or• Earth rotated differently, so the poles were in different places
The palaeoclimate - more ‘picture on the jigsaw’
• Wegener and Köppen found from published scientific papers that around 270 million years ago coal was forming on all the southern continents and India - indicating tropical conditions. At the same time salt, gypsum and desert sands were being deposited in North America and Europe - indicating dry desert conditions - as shown on Wegener’s ‘reconstruction’
• Hypothesis? - what could have caused this pattern?
• the continents have moved from their present positions
or
• Earth rotated differently, so the poles, Equator and the belts of climate around the Earth were in different places
• Earth was hotter, so there were tropical and desert conditions nearer the poles
or
Longitude evidence- a test of movement over years
• Wegener knew that the longitude of Greenland had first been measured in 1820, then in 1870. He was an assistant on the Greenland expedition of 1906/8 when longitude was measured again. These measurements showed that Greenland was moving west at about 20 metres per year
• Hypothesis? - what could account for this measured movement?
• Greenland is moving at around 20 m per year (and so are the other continents)
or• the measurements were inaccurate
or• the calculations based on the measurements were incorrect
Wegener’s ‘Continental drift’ hypothesis
Wegener described his ‘continental drift’ hypothesis as:• ‘South America must have lain alongside Africa and formed a ..
block that was split into two…; the two parts must then have become increasingly separated over … millions of years, like pieces of a cracked ice floe in water.’ (p 17)
• ‘… India … was joined to Asia by a long stretch of land, … this … became increasingly folded by the continuing approach of present day India to Asia; it is now the largest folded range on earth, ie. the Himalaya …’ (p 20)
• ‘In the westward drift of both Americas, their leading edges were compressed and folded by the … ancient Pacific floor …’ (p 20)
• ‘… the continents undergo a displacement towards the equator under the action of a force causing flight from the poles.’ (p172)
So, to Wegener, the continents ‘drifted’ by a force moving them from the poles and piled up mountain ranges ahead of them as they moved
Wegener’s ‘Continental drift’ hypothesis
• This is the ‘drift’ of continents shown in Wegener’s book
• Click here to find out how we think plates move today(Flash animation will open in web browser)
Contrasts between Wegener’s ‘continental drift’ hypothesis and today’s ‘plate tectonic’ theory
• Plate tectonics shows that the plates do not drift like ice floes on water, they are carried by currents in the mantle
• We now think that there was no land between India and Asia, but that the Himalayan mountains were formed of sea sediments crumpled up by the collision
• We now know that the ocean floors are not ancient, they are younger than the continents. The American mountain ranges were formed by plate tectonic destructive margin processes
• The ‘flight from the poles’ force is now known to be much too small to cause the movement of continents
Wegener described his ‘continental drift’ hypothesis as:
• ‘South America must have lain alongside Africa and formed a .. block that was split into two…; the two parts must then have become increasingly separated over … millions of years, like pieces of a cracked ice floe in water.’ (p 17)
• ‘… India … was joined to Asia by a long stretch of land, … this … became increasingly folded by the continuing approach of present day India to Asia; it is now the largest folded range on earth, ie. the Himalaya …’ (p 20)
• ‘In the westward drift of both Americas, their leading edges were compressed and folded by the … ancient Pacific floor …’ (p 20)
• ‘… the continents undergo a displacement towards the equator under the action of a force causing flight from the poles.’ (p172)
So, to Wegener, the continents ‘drifted’ by a force moving them from the poles and piled up mountain ranges ahead of them as they moved
Why wasn’t Wegener believed?
• It wasn’t until 40 - 50 years later, in the 1960s, that scientists started to believe that the continents had moved, and developed the theory of ‘plate tectonics’
• Hypothesis? - why do you think Wegener wasn’t believed at the time?
Why wasn’t Wegener believed?
It is difficult to get into the minds of scientists at the time to answer this question - but the following have been said:
• the scientific consensus view at the time was that everything was ‘fixed’ - continents might move up and down or be crumpled by a shrinking Earth, but they couldn’t be moved laterally
• he was a meteorologist - how could he come up with ground-breaking ideas in geology?
• he was German - at a time when many nations had been at war with Germany
• he published in German - a language that was not widely read by scientists - none of his ideas, first formulated in 1912, were translated into English until 1924
• he was wrong about the rate of drift of Greenland• the influential British physicist, Sir Harold Jeffreys, said that the
continents didn’t have enough strength for ‘drift’• the forces that Wegener proposed as the cause of ‘drift (the ‘flight
from the poles’) were nothing like strong enough
What Wegener didn’t know
• What evidence do we now have for plate tectonic theory that Wegener didn’t know about?
What Wegener didn’t know
Wegener didn’t know that:• there was a zone of earthquakes that sloped
down from oceanic trenches, discovered by Benioff in 1954. This is where plates are taken down (subducted) at destructive plate margins
• the ocean ridges have shallow focus earthquakes, volcanic eruptions, high heat flow and a rift valley. Hess used this data to propose his ‘sea floor spreading’ hypothesis in 1962
• the ocean floor has magnetic stripes, shown by Vine and Matthews in 1963 to be evidence for sea floor spreading
What Wegener didn’t know
Wegener didn’t know that:• the rocks of the ocean floors are young
near oceanic ridges and become steadily older outwards, supporting the ‘sea floor spreading’ hypothesis
• there is a zone between about 100 and 250 km deep in the Earth where earthquake waves slow down, indicating that, although solid, it is weak and can flow - the asthenosphere. The strong lithosphere ‘floats’ on top
• positions of the continents can now be fixed very accurately, showing that the continents are moving over time - at cm per year
The age of the sea floor (red = youngest, blue = oldest)
© Dale S. Sawyer http://zephyr.rice.edu/plateboundary/home.html
Wegener - the background
1880 1890 1900 1910 1920 1930
Born in Berlin, Germany on 1st November 1880
Went to Köllsches secondary school
Studied at Universities of Heidelberg, Innsbruck and Berlin
First became an astronomer, then a meteorologist
Made a record balloon flight of 52.5 hours with his brother in 1906
Became a polar explorer in 1906, second expedition, 1912
First described his ‘continental drift’ theory in a lecture - Jan 1912
Married Else Köppen, daughter of a meteorology professor, in 1912
Joined the German army in 1914, shot in the arm and the neck
Given leave from the army in 1915, ‘unfit for active service’
Published his ideas in a book with four editions 1915, 1920, 1922, 1929 - the last two editions translated into English
Became a professor of meteorology in 1924
Began his third polar expedition to Greenland in 1930
Died on the Greenland icecap in November 1930, aged 50
Comment
The following slides offer comments
on the ‘hypothesis’ slides
Similar shapes - comment
• Although the coastlines of the continents bordering the Atlantic do match closely, there is an even closer match at the 1000m depth contour line - marking the edge of the continental shelf. This work was done in the mid-1960s - there was insufficient depth data before then. Most of the overlaps are accounted for by sediments being deposited after the continents separated
The geology - comment
• As more geological data has become available a closer match is seen. In particular, the rock sequences in South America and Africa are very similar until around 150 million years ago (when the continents separated) - but differ after that
The fossils - comment
• Extra evidence comes from the distributions of land and freshwater reptiles shown in the diagram
• Vegetation rafts carried by ocean currents have been seen, but they could never carry the range of animals/plants that have been found on different continents
The biology - comment
• Extra evidence comes from the distribution of marsupial mammals (existing in South America and Australia, and present as extinct fossils in Africa)
Geophysical evidence - comment
• Gravity measurements, also known to Wegener, support the isostasy hypothesis. These show, for example, that mountain chains, like the Himalayas, made of low density continental rocks, have ‘roots’ of low density rock beneath them to support them, like an iceberg has a ‘root’ underwater to support it
• Low density rocks give a gravitational ‘low’ - as in the ‘root’ still present beneath Scandinavia, resulting from the previous ice sheet
The palaeoclimate - comment
• The palaeoclimate data does not generally provide evidence that the continents were once together, only that they have moved over time
• However, evidence of ice-movement directions sometimes indicates that the ice came ‘from the sea’, where there must once have been a continent
The palaeoclimate - comment
Evidence from the rock sequences in the UK show that climate changed as:
Ma = million years ago
• climate today• Mediterranean conditions around 100 Ma• Desert conditions (like the Sahara) around 250Ma• Equatorial conditions around 350 Ma• Desert conditions (like the Kalahari) around 400 Ma• Mediterranean conditions around 450 Ma
youngest rocks
oldest rocks
showing the movement north of the UK area over time
Longitude evidence - comment
• These calculations were wrong. Measurements using lasers show that Greenland is moving west at cm per year, not tens of metres. Measurement with this precision was impossible in Wegener’s time
• Probably the errors in the measurements and calculations quoted by Wegener gave rise to the incorrect figure of 20 metres
CopyrightCover from ‘The Origin of Continents and Oceans’ © Dover Publications Inc
Similar shapes of coastline © Peter Kennett
The Geology of the South American and African coast areas © Dover Publications Inc
Distribution of land/freshwater animals and plants in the continents of ‘Gondwanaland’ - reproduced with kind permission of U.S. Geological Survey (USGS), redrawn by ESEU
An alternative explanation of the distribution of fossil plants and animals © Continental Drift: A Study of the Earth’s Moving Surface (1972) © Harmondsworth: Penguin Books
‘Garden-snail distribution according to Ökland © Dover Publications Inc
'Post-glacial elevation contours (in meters) for Fenno-scandia (according to Hogborn) © Dover Publications Inc
Traces of the Permo-Carboniferous inland glaciation … © Dover Publications Inc
Ice, bog and desert in the Permian © Dover Publications Inc
Displacement of Greenland © Dover Publications Inc
Reconstruction of the map of the world according to drift theory for three epochs © Dover Publications Inc
Ocean-ocean convergence (subduction zone) - reproduced with kind permission of USGS, redrawn by ESEU
Mid ocean ridge © Press & Siever, redrawn by ESEU
Magnetic anomalies over the Reykjanes Ridge © Geoscience, redrawn by ESEU
Magnetic evidence for ocean floor spreading © This Dynamic Earth: the Story of Plate Tectonics, USGS, redrawn by ESEU
The age of the sea floor © Dale S. Sawyer: http://plateboundary.rice.edu/home.html
Asthenosphere and Lithosphere- reproduced with kind permission of USGS, redrawn by ESEU
The Continental jigsaw, continental shelf © Andrew McLeish in ‘Geological Science’
Distribution of younger rocks across South America and Africa - source unknown, redrawn by ESEU
Distribution of ancient rocks across South America and Africa © Andrew McLeish in ‘Geological Science’
Gravity anomaly in Scandinavia (according to Born) © Dover Publications Inc
The Continental jigsaws (former distribution of ice across the Gondwana continents) © Andrew McLeish in ‘Geological Science’
What Wegener knew - and what he didn’t know
How Science Works: Alfred Wegener’s ‘Continental Drift’
theory
- based on his 1929 book
www.earthscienceeducation.com
What Wegener Knew
and what he didn’t know
Earth Science for KS4
Earth Science Education Unit
www.earthscienceeducation.com
© The Earth Science Education Unit
Copyright is waived for original material contained herein if it is required for use within the laboratory or classroom. Copyright material included from other publishers rests with them.
