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Historical Geology:
Study of the evolution of Earth, its environments, and its life through time
Combines many subdisciplines of geology

• Sedimentology
• Stratigraphy
• Structural Geology
• Plate Tectonics & Paleogeography
• Paleontology

Examines geologic history as a pattern (the particular changes in sequence throughout time) and as process (rules or principles that govern these changes).

Some of the topics covered:

• Paleoenviroment analysis: interpreting ancient environmental conditions from the rock record (e.g.: river & lake systems, deltas, lagoons, deep marine, etc.)
• Geologic Time: how to determine numerical and relative dates in time
• The Geologic Column: its construction and its operation
• Orogenesis: the rise (and fall) of mountain ranges
• The principles of organic evolution
• The formation and distribution of continental rocks through time
• Origin, diversification, and extinction of the major groups of organisms in Earth's history

Discovery of Earth History
Different views of Earth History in early scientific times:

• Catastrophism – different natural laws operated in the past then at present
  • (Does NOT imply that these different laws are marked by “catastrophes” in the vernacular sense)
• Uniformitarianism – same natural laws operated in the past as at present

Many early geologists recognized several main types of rocks:

• Crystalline rocks (essentially plutonic igneous and metamorphics)
• Lithified sediments (hard sedimentary rocks)
• Unlithified sediments (soft sedimentary rocks)
• Volcanic rocks

In 1759 Giovanni Arduino put these into an historical context, modified by Abraham Gottlob Werner in 1787. This model came to be called neptunism (after Neptune, the Roman God of the Sea). The model is catastrophist (in that it assumes different natural laws in past than at present).

The Neptunist model of rock formation:

• Primary Rocks - Crystalline rocks, formed by unknown chemical process (some type of precipitation) out of the primeval ocean
• Transitional Rocks – Non-fossiliferous sedimentary (and metasedimentary) rocks: mostly chemical but partly mechanical origin
• Secondary Rocks – Fossiliferous sedimentary rocks, mostly mechanical but partly chemical in origin
• Tertiary Rocks – Alluvial (poorly consolidated sedimentary rocks), the only sedimentary rocks in the Neptunist scheme formed by modern-style processes
• Volcanic Rocks

In one scheme, Primary (and Transitional?) rocks were pre-Noah's Flood, Secondary rocks were the record of the Flood, and Tertiary and Volcanic rocks were post-flood.

In any case, under neptunism, rock type and rock age were related: all rocks of a certain type (say secondary rocks) were formed during a certain period of time; similarly, all rocks of a certain time would be the same kind of rock.

Also, all mountains built of Primary rock were formed in Primary time; the Secondary mountains (built of Secondary rock) crystallized on the sides of the Primary mountains; and the Tertiary hills and volcanoes were all deposited later. Thus, under Neptunism, the conditions to form big mountains were under a different set of natural laws than those of the current world.

Others developed a uniformitarianist alternative to neptunism. Most influential was James Hutton, and his work of 1785. Because it recognized crystalline rocks as being formed underground rather than at sea, called plutonism (after Pluto, Roman God of the Underworld) instead of neptunism.

• Demonstrated that volcanic rocks & plutonic rocks represent the same types of rock (i.e., igneous rocks), only cooled on different scales.
• Because of this, crystalline rocks were NOT crystallized form a primeval ocean, but instead are cooled molten rock.
• Mountains represent upheavals of material due to internal forces, not deposits from the sea
• Volcanism shows that these forces are still active: the same forces that created the past operate in the present
• A continuous cycle of uplift, erosion, deposition, burial, and re-uplift.

Thus, under plutonism, rock type and rock age are NOT related: there can be extremely ancient sedimentary rocks (Secondary in the neptunist scheme) which are much older than some crystalline rocks (Primary in the neptunist scheme). Futhermore, mountain building is still going on today.

Work of Hutton was developed upon and further popularized by Charles Lyell & his Principles of Geology (1830). Lyell was famous for an extreme form of uniformitarianism, which assumed the same rates of operation in the past, rather than merely the same natural laws. (Lyell also predicted that organisms now extinct would reappear in the future!).

Most geologists operate under the uniformitarian philosophy of actualism: that the same natural laws now recognized operated in the past, but at different rates according to particular conditions.

Although the plutonists showed that rock age and rock type are not related, their recognition that the forces that produce rocks in the past are the same as those today allowed geologists to use modern observations to establish ancient environments. By discovering this, they were able to unravel the sequence of events in the past: historical geology.

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Last modified: 9 January 2006