Transcript James Hutton and geological time

James Hutton
Plutonism and the depths of time
Hutton and an angular
unconformity
The Scottish Enlightenment
• Hutton was one of many active scientists and
business people associated with the ‘Scottish
enlightenment,’ which lasted from the early 18th
Century to its end.
• James Watt (steam engine), Francis Hutcheson
(moral philosophy), David Hume (philosophy),
Adam Smith (economy and moral philosophy),
Joseph Black (chemist), John Playfair (physicist)
and others made major contributions to science,
philosophy and political thought.
• Voltaire remarked at the time, “We look to
Scotland for all our ideas of civilisation.”
Hutton’s Career
• A student first of humanities and then
medicine (receiving his M.D. at 23), with a
liking for chemistry.
• A business venture extracting ‘sal
amoniac’ (ammonium chloride– NH4Cl)
from chimney soot made him wealthy
before he was 30.
• Decided to become a farmer (he had
inherited a farm).
More career
• Began to study farming more or less as an
apprentice farmer.
• Traveled widely to study how agriculture is done
in different regions.
• Took an interest in the geology of the different
regions.
• Lived on his farm 14 years (1754-1768), and
retired to Edinburgh.
• There he became an enthusiastic member of the
Scottish enlightenment’s social circles.
Hutton’s providentialism
• The earth is a system intended to
preserve/provide the necessities of life.
• Hydrological cycle: Seas receive rivers, and are
the source of ‘vapours’ that return water to the
land.
• Decay is part of the process. In particular,
mountains erode, providing material to refresh
the soils of lowlands.
• But over time, the mountains will be worn down;
how can the system of the world be maintained?
Renewing the earth
• Land today is built on rocks that were laid down
under the oceans.
• So how did it come to be above the sea today?
• Begin with the rock: how was it converted from
soft sediment to stone?
• Rather than invoke water (as the Neptunists
had), Hutton invoked heat (and pressure).
(Some sedimentary rocks are cemented with
insoluble minerals!)
The driving force of the cycles
• The newly formed rock is lifted up to form new
land by the heat energy in the earth’s centre.
• The power required to do this work is evident in
the contortions and fractures we find in strata.
• Veins of minerals and igneous rocks crossing
sedimentary rock and reaching high up into the
mountains demonstrate the power these forces
have (consider Mount Etna, for example).
• This has been going on for a very long time
(ancient lavas, and igneous formations deep
below the surface now exposed in valleys etc.)
Another bit of providentialism
• Hutton held that volcanoes actually served
as ‘safety valves,’ releasing pressure from
below that would otherwise disrupt the
surface much more violently.
Crosscuttings
• Recall Steno’s principle of original
continuity for sedimentary strata.
• This implies that, when we see intruded
veins and sheets (dykes) of igneous rock,
these are younger than the strata they cut
across.
The Earth’s Age
• The theme of the earth’s great age
becomes a key trope in geology from
Hutton on.
• Why? Consider an angular unconformity:
Angular Unconformity at Siccar
Point
What does this represent, for
Hutton?
• Think of the cycles that Hutton proposes for the
earth’s history:
• In each cycle, we begin with a world with things
more or less as they are now.
• Gradually the mountains are worn down and
carried off to the sea by erosion, and the
landscape reduced to a low lying plain.
• Meanwhile, sedimentary rock is being formed
under the ocean by the heat and pressure of
deep burial.
Continuing the cycle:
• The heat and pressure build up below the
sea, raising the sea bed to form new
lands.
• The old land founders, sinking below the
waves.
• The slow processes of erosion and
sedimentation begin again.
Time in an angular unconformity
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We see two beds of sediment here.
One is on top of the other.
An erosional horizon lies between them.
The lower bed is tilted to an extreme
angle.
• How would Hutton account for this?
Cycles upon cycles
• First, the bottom sedimentary layers were laid down in
the ocean during a cycle long ago.
• Second, they were lifted up, tilted and eroded; this
constitutes a second cycle.
• Third, they sank again, and new sedimentary layers
were formed on top of them– a third cycle.
• Finally they have been lifted up once more and are now
part of a fourth cycle.
• But each cycle is immensely long, and there’s no cause
to suppose that these 4 are all the cycles there have
been.
• The earth is immensely, staggeringly old.