Historical geology

Not to be confused with History of geology.

Historical geology or paleogeology is a discipline that uses the principles and techniques of geology to reconstruct and understand the geological history of Earth. It focuses on geologic processes that change the Earth's surface and subsurface; and the use of stratigraphy, structural geology and paleontology to tell the sequence of these events. It also focuses on the evolution of plants and animals during different time periods in the geological timescale. The discovery of radioactivity and the development of several radiometric dating techniques in the first half of the 20th century provided a means of deriving absolute versus relative ages of geologic history.

Economic geology, the search for and extraction of fuel and raw materials, is heavily dependent on an understanding of the geological history of an area. Environmental geology, including most importantly the geologic hazards of earthquakes and volcanism, must also include a detailed knowledge of geologic history.

Historical development

Nicolaus Steno, also known as Niels Stensen, was the first to observe and propose some of the basic concepts of historical geology. One of these concepts was that fossils originally came from living organisms. The other, more famous, observations are often grouped together to form the laws of stratigraphy.

James Hutton and Charles Lyell also contributed to early understanding of the Earth's history with their observations at Edinburgh in Scotland concerning angular unconformity in a rock face and it was in fact Lyell that influenced Charles Darwin greatly in his theory of evolution by speculating that the present is the key to the past. Hutton first proposed the theory of uniformitarianism, which is now a basic principle in all branches of geology. Hutton also supported the idea that the Earth was very old as opposed to the prevailing concept of the time which said the Earth had only been around a few millennia. Uniformitarianism describes an Earth created by the same natural phenomena that are at work today.

The prevailing concept of the 18th century in the West was that of a very short Earth history dominated by catastrophic events. This view was strongly supported by adherents of Abrahamic religions based on a largely literal interpretation of their religious scriptural passages. The concept of uniformitarianism met with considerable resistance and the catastrophism vs. gradualism debate of the 19th century resulted. A variety of discoveries in the 20th century provided ample evidence that Earth history is a product of both gradual incremental processes and sudden cataclysmic events. Violent events such as meteorite impacts and large volcanic explosions do shape the Earth's surface along with gradual processes such as weathering, erosion and deposition much as they have throughout Earth history. The present is the key to the past - includes catastrophic as well as gradual processes.

Brief geological history

Main article: Geologic time scale

Eon	Era	Period	Epochs	Start
Phanerozoic	Cenozoic	Quaternary	Holocene	0.0117
			Pleistocene	2.558
		Neogene	Pliocene	5.333*
			Miocene	23.030*
		Paleogene	Oligocene	33.9*
			Eocene	56.0*
			Paleocene	66.0*
	Mesozoic	Cretaceous	Late Cretaceous	100.5*
			Early Cretaceous	c. 145.0
		Jurassic	Late Jurassic	163.5 ± 1.0
			Middle Jurassic	174.1 ± 1.0*
			Early Jurassic	201.3 ± 0.2*
		Triassic	Late Triassic	c. 235*
			Middle Triassic	247.2
			Early Triassic	252.2 ± 0.5*
	Paleozoic	Permian		298.9 ± 0.2*
		Carboniferous	Pennsylvanian	323.2 ± 0.4*
			Mississippian	358.9 ± 0.4*
		Devonian		419.2 ± 3.2*
		Silurian		443.4 ± 1.5*
		Ordovician		485.4 ± 1.9*
		Cambrian		541.0 ± 1.0*
Proterozoic	Neoproterozoic	Ediacaran	Precambrian	c. 635*
		Cryogenian		850
		Tonian		1000
	Mesoproterozoic	Stenian		1200
		Ectasian		1400
		Calymmian		1600
	Paleoproterozoic	Statherian		1800
		Orosirian		2050
		Rhyacian		2300
		Siderian		2500
Archean	Neoarchean			2800
	Mesoarchean			3200
	Paleoarchean			3600
	Eoarchean			4000
Hadean				4567