James Hutton

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James Hutton

• Plutonism and the depths of time

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Hutton and an angular unconformity
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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.

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Huttons 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).

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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 enlightenments social circles.

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Huttons 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?

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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!)

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The driving force of the cycles

• The newly formed rock is lifted up to form new
  land by the heat energy in the earths 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.)

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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.

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Crosscuttings

• Recall Stenos 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.

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The Earths Age

• The theme of the earths great age becomes a key
  trope in geology from Hutton on.
• Why? Consider an angular unconformity

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Angular Unconformity at Siccar Point
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What does this represent, for Hutton?

• Think of the cycles that Hutton proposes for the
  earths 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.

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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.

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Time in an angular unconformity

• 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?

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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 theres no
  cause to suppose that these 4 are all the cycles
  there have been.
• The earth is immensely, staggeringly old.