James Hutton

1. James Hutton Plutonism and the depths of time

2. Hutton and an angular unconformity

3. 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.”

4. 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).

5. 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.

6. 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?

7. 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!)

8. 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.)

9. 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.

10. 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.

11. 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:

12. Angular Unconformity at Siccar Point

13. 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.

14. 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.

15. 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?

16. 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.