The ruins of an older world are visible in the present structure of our planet, and the strata which now compose our continents have been once beneath the sea, and were formed out of the waste of pre-existing continents. The same forces are still destroying, by chemical decomposition or mechanical violence, even the hardest rocks, and transporting these materials to the sea, where they are spread out, and form strata analogous to those of more ancient date. -- The Theory of the Earth (1788) James Hutton

Review course policies, syllabus.

A Review of the Course, in 60 Seconds:

Historical Geology:
Use of geological principles to reconstruct the history of EArth, its environment, and its life. 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 Timescale: 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

The most key concepts of this course:

1. The environments on the surface of the Earth and its inhabitants have changed greatly over the course of geologic time
2. Rocks and their contents are the record of this change

Some of the core truths of Earth history (based on Robert Hazen's The Story of the Earth):

1. Earth is made of recycled and recycling atoms.
2. Earth is immensely old when compared with human time frames.
3. Earth is three-dimensional, and most of the action is hidden from view.
4. Rocks are the record keepers of Earth history.
5. Earth systems--rocks, oceans, atmospheres, and life--are complexly interconnected.
6. Earth history encompasses long periods of stasis punctuated by sometimes sudden, irreversible events.
7. Life has changed and continues to change Earth's surface.

Discovery of Earth History
Traditional (i.e., pre-scientific) View of Earth: the origin of the Earth and its structures (mountains, oceans, seas, rivers, etc.) are totally distinct from modern disasters (earthquakes, floods, landslides, volcanoes, etc.). The former was thought to be a singular event at the Dawn of Time, and completed; the latter were ongoing processes due to divine, diabolic, astrologic, etc., influences. (Indeed, "disaster" means "bad star"). Earthquakes, volcanoes, floods, and the like were thought to have no more relation to the formation of the Earth and its structures than diseases have to do with the birth of a child.

Various philosophers proposed "Theories of the Earth": several models beginning in the 17th Century questioned a single Creation week as the entirety of the formation of the Earth and its structure. Did the Earth itself have a history?

Two main competing philosophies among the "Theories of the Earth":

• Catastrophism: Natural laws of the past were not the same as the current natural laws
  • Did not require that the transitions between phases of Earth's history were necessarily "catastrophes" in the common use of the word.
  • However, many did interpret these transitions as disasters, with Noah's Deluge being the last major phase change.
• Uniformitarianism: Natural laws today are the same as in the past
  • "The Present is the Key to the Past"

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: Continue to form in the present world

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: 19 January 2012