Earth History GEOL 2110

Lecture 4Development of

Geological Concepts, Part II

Toward a Unified Theory of the EarthThe Cosmogonists

Developed all-encompassing and largely speculative theories of the origin of the cosmos that were no longer tied to scripture, but based on observations of nature and emerging ideas about physics;

The era of speculative cosmogonies ran from AD 1600-1700. In this period a number of comprehensive cosmogonies were proposed. These were long on armchair speculation and short on substantive supporting evidence. These cosmogonies were part of the new emphasis of science in seeking rational explanations of the features of the world. Rene Descartes (1596-1650)

“I think, therefore I am”

Toward a Unified Theory of the EarthThe Cosmogonists

G.L. Buffon – 1749 Histoire Naturelle – 34 volume treatise on the origin of earth and the cosmos. Speculated that the planets were created by the impact of a large comet with the sun (influenced by Halley’s comet drive-by in 1682 and its predicted return in 1758.

• Estimated the Earth to be 75,000 years old based on its cooling from an originally molten state

• Envisioned 7 epochs of earth formation of core, hydrosphere, atmosphere and crustal layers

• Rejected the literal interpretation of scripture as a source of geological insightGeorge Louis Leclerc,

Comte de Buffon

Toward a Unified Theory of the EarthThe Neptunists

A.G. Werner (1787) - Developed a chronostratigraphy of the Earth’s crust that was based on progressive deposition of rocks from a gradually subsiding ocean. The theory was nearly universally accepted in the late 1700’s. Defined five crustal units:

Abraham Gottlob Werner1749-1817

Primitive Series – crystalline rocks considered to be the first precipitates from the ocean before the emergence of land.Transition Series - more indurated sedimentary sequences that were the first orderly deposits from the ocean. Secondary Series - the remaining, obviously stratified fossiliferous rocks and certain associated "trap" rocks. These were thought to represent the emergence of mountains from beneath the ocean and were formed from the resulting products of erosion deposited on their flanks.Alluvial Series - poorly consolidated sands, gravels and clays formed by the withdrawal of the oceans from the continents.Volcanic Series - younger lavas flows demonstrably associated with volcanic vents. Werner believed that these rocks reflected the local effects of burning coal beds.

Toward a Unified Theory of the EarthThe Neptunists

Interesting Interpretations: - Primitive granites, highly metamorphosed rocks, basalts flows and

diabase dikes are crystalline precipitates from the universal ocean- Mountains reflect the original chaotic landscape of the earth; they

are static, fixed in space and time- Volcanoes are minor, geologically unimportant elements of the crust created by the subterranean combustion of coals seams.

Toward a Unified Theory of the EarthThe Vulcanists

Early Challenges to Werner’s theory: - Where did the water go???- The Auvergne volcanoes rest on granite (primitive rocks)!- Italian and French scientist showed that “trap rock” interlayered with

sedimentary rocks were identical to recent basalt flows and thus were formed from molten lava and were not precipitates from

seawater.

Auvergne Volcanoes , S. France

Toward a Unified Theory of the EarthThe Plutonists

James Hutton (1726-1797) – The Father of Modern GeologyA member of the Edinburgh Oyster Club that included economist Adam Smith, mathematician John Playfair, philosopher John Hume, and chemist Joseph Black1788. Theory of the Earth; or an investigation of the laws observable in the composition, dissolution, and restoration of land upon the Globe.

Principal Concepts• The current landscape is a balance between

rejuvenation (uplift) and destruction (erosion) of the earth’s surface

• The earth is eternally dynamic and ever-changing (“No Vestige of a Beginning, No Prospect of an End”)

• The internal heat of the earth is responsible for uplift of mountains and the igneous origin of granite and basalt; reinforced by melting experiments of Hall (1792)

Toward a Unified Theory of the EarthThe Plutonists

Angular Unconformity at River Jed, Scotland

Angular Unconformity at Siccar Point, Scotland

“The mind seemed to grow giddy looking so far into the abyss of time” John Playfair (1802)

Illustrations of the Huttonian Theory of the Earth

Toward a Unified Theory of the EarthThe Catastrophists

Cuvier's (1812) section for the Paris Basin.

From the their biostratigraphic mapping in the Paris Basin, Cuvier and Brongniart interpreted the many mass extinctions, dramatic changes in rock type, and obvious unconformities found in the Paris Basin sedimentary sequence as evidence that the earth experience repeated catastrophic revolutions between periods of relative quiescence.

Cuvier believed that all life was present at the beginning and that selective preservation and extinctions gave rise to present day life forms.

Toward a Unified Theory of the EarthThe Uniformitarians

Charles Lyell (1797-1875) – Railed against catastrophism and instead argued that the Earth is in a steady-state equilibrium with uniformity in the types and rates of processes over all geological time - Gradualism

12 Editions 1830-1875

Toward a Unified Theory of the EarthThe Uniformitarianists

Frontpiece Illustration from Principles of Geology (1857)

Toward a Unified Theory of the EarthThe Evolutionists

Charles Darwin (1809-1882) - The directionalism inherent in biological evolution went against the basic notion of static equilibrium that Darwin’s friend and colleague Lyell advocated.

On the Origin of Species1859

Introduced the scientific theory that populations evolve over the course of generations through a process of natural selection.

Toward a Unified Theory of the EarthThe Physicists

Sir William Thompson (The Lord Kelvin) (1824-1907) - Renown physicist of the late 19th century best known for determining the value of absolute zero temperature and for estimating the age of the earth based on principles of thermodynamics applied to an initially molten globe – his estimates ranged from 400 million to 20 million.

Kelvin thought Lyell’s notion of perpetual equilibrium was totally implausible based on the principles of physics applied to a cooling earth.

Toward a Unified Theory of the EarthThe Actualists

Today, earth scientist recognize that:• the basic principles of nature are uniform through

geological time• rates and scales of geological processes and events can

change over time and be nonlinear• some changes are cyclical (glaciation), some are

unidirectional (growth of continents) and irreversible (oxygenation of the atmosphere, evolution)

• the simplest explanation is the best, unless data suggest otherwise

• In the context of actualism, present-day processes can be useful analogs to the geologic past

Why is Actualism Important?Geology strive to establish not only the present condition of the Earth, but also the past and the future. The principles

of Actualism provide us with guiding principles about how to read the

chapters of the Earth’s past.

Not only is the present the key to the past, but the reverse is true as well.

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EVOLUTION