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Lecture

ESS102H1 Lecture Notes - Fitz Roy, Fault Scarp, Geophysical Global Cooling


Department
Earth Sciences
Course Code
ESS102H1
Professor
C.Banks

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October 14
19th Century views on mountains
1. “Craters of elevation”
- von Buch (1774-1853)
ostudent of Werner
oobservations in the Canary Islands
domal uplift due to subsurface intrusion of magma
cracking releases magma to surface
collapse of dome to form a crater
remaining elevated rim = mountain
extended mountain chains would require a linear intrusion (along a
crack)
hypothesis would require all mountains to be associated with
volcanism
so a new question: what makes volcanoes?
2. Contracting Earth
- (Jean-Baptiste-Armand-Louis-Leonce) Elie de Beaumont (1798-1874)
oprofessor of geology at Ecole des Mine, Paris and vice president of Conseil-
General des Mines
opublished geological map of Freance (1840, with Dufrenoy)
- 1852: “Notice sur les systems des montagnes” = On Mountain Systems (prevalent
thinking in the 19th century)
- Earth has been cooling down
- shrinkage of inner Earth has caused stress buildup in outer rigid crust
- mountains compared to wrinkles on a drying apple
- According to Beaumont:
omountains are formed by stacking of thrust sheets
oEarth’s circumference has shrunk by 200-600km
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ouplift of mountains is rapid and violent due to release on compressional
stress, i.e breakage of the crust under the stress (a Catastrophist approach to
geology)
ofaunal “catastrophies” and extinctions attributed to these violent uplifts
3. Earthquakes and mountains
- Charles Darwin
ovoyage of the Beagle
- aside: correct explanation of formation of coral reeds circling a lagoon
o(??opposing view??): have built up from submerged volcanoes, volcanoes
had once broken above sea level
- observations:
oearthquakes generated 1-2m fault scarp and elevated shorelines
20 Feb 1883: “There can be no doubt that the land round the Bay of
Concepcion was upraised two or three feet... About thirty miles distant
[...] Captain Fitz Roy found beds of putrid mussel-shells still adhering
to the rocks, ten feet above high-water mark: the inhabitants had
formerly dived at lower-water spring-tides for these shells. At
Valparaiso [...], similar shells are found at the height of 1300 feet: it is
hardly possible to doubt that this great elevation has been effected by
successive small uprisings, such as that which accompanied or
caused the earthquake this year.”
oDarwin’s conclusion:
mountains can be produced by repeated earthquakes over a long
period of time – a uniformitarian approach despite the catastrophic
nature of individual earthquakes.
he was right: the uplift of many mountain ranges manifests itself as
earthquakes. He did not speculate, he just argued the evidence.
He converted the problem of mountain building to the problem of the
origin of earthquakes – a reductionist step towards the solution
4. Mountains and isostsasy
- isostasy:
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