Geologic Time

The Geologic Timescale was initially built from many observations of rock sequences from many parts of the world; no single locality contains the entire geologic time sequence, and correlation of geologic units across different localities allowed geologists to construct a composite timescale. This was heavily based on sedimentary rocks that contained fossils, which were placed in a relative time sequence based on Darwin's ideas of evolution. In broad terms, the fossil-based geologic timescale is shown below. The different Eras are marked by major life extinctions in the fossil records, including the famous demise of the dinosaurs at the Mesozoic-Cenozoic boundary, as well as an even larger extinction of marine life at the Paleozoic-Mesozoic boundary. The beginning of the Paleozoic was defined as the occurrence of the first hard-shelled fossils, including marine fauna such as the trilobite, the Wisconsin State Fossil.

Geologic Era	Characteristic Life
Cenozoic ('late life")	Mammals
Mesozoic ("middle life")	Reptiles, dinosaurs
Paleozoic ("early life")	Invertebrate Marine life (clams, etc.), amphibians, first land plants
Precambrian (time before the Cambrian, the first geologic period with hard-shelled fossils)	No hard-shelled fossils

(You should know this table)

There have been several prior attempts at placing absolute ages on the Earth and the Geologic Timescale. In 1654, Archbishop James Usher reconstructed biblical records and issued the "official" age of the Earth from the Christian Church's view, where the Earth was created at 9:00 am Oct. 26, 4004 BC. This caused difficulty with many geologists because it required such rapid rates of sedimentation, mountain range uplift, and erosion, as compared to current rates on Earth. The guiding principle followed by many geologists in the 1800's was that current rates of geologic processes were more likely than not to be similar to those in the past - this principle is called uniformitarianism, and was proposed by geologist James Hutton. A famous "scientific mainstream" age of the Earth was calculated by Lord Kelvin in 1892, who assumed that the Earth cooled slowly form an initially molten ball; this age was 20-40 m.y. ago, and contrasted strongly with the Church's view. The older age of the Earth made much more sense to mainstream geologists, who could not reconcile the very great thicknesses of sedimentary rocks (tens of kilometers) with modern rates of sedimentation if the Earth was only 6,000 years old, as advocated by the Church.

In 1896 radioactivity was discovered, and the heat generated by radioactive decay in the Earth was the major reason why Lord Kelvin's calculations were wrong (his calculation was done before the discovery of radioactivity). However, it was not until the early 1950's that geologists had the technology that allowed them to apply radioactive decay to determining the absolute ages of rocks. For example, the isotope 238U decays over a fixed rate to the isotope 206Pb; by measuring the amount of U and Pb in a mineral, coupled with knowledge of the decay rate constant, one can calculate the absolute age (in years) that the mineral formed. This method is best applied to igneous rocks, where crystals form "from scratch" by crystallization of magmas. Minerals in metamorphic or sedimentary rocks were derived from older pre-existing rocks, and generally contain remnant records of this earlier radioactive history. Geologists immediately started determining the ages of volcanic rock layers that were interbedded with the fossil-containing sedimentary rocks that were used to determine the first geologic timescale, adding absolute ages to the geologic eras and periods.

YOU MUST KNOW THE PARTS IN RED FROM TABLE BELOW FOR THE SECOND EXAM!!

Eon	Era	Period	Epoch	Age at Start
Phanerozoic	Cenozoic	Quaternary	Holocene	10,000 yr B.P.
		Quaternary	Pleistocene	2 m.y.
		Tertiary	Pliocene	5 m.y.
			Miocene	24 m.y.
			Oligocene	37 m.y.
			Eocene	58 m.y.
			Paleocene	65 m.y.
	Mesozoic	Cretaceous		144 m.y.
		Jurassic		208 m.y.
		Triassic		245 m.y.
	Paleozoic	Permian		286 m.y.
		Pennsylvanian		320 m.y.
		Mississippian		360 m.y.
		Devonian		408 m.y.
		Silurian		438 m.y.
		Ordovician		505 m.y.
		Cambrian		545 m.y.
Precambrian	Proterozoic			2,500 m.y.
	Archean			3,800 m.y.
	Hadean			4,550 m.y.

Note: The table is NOT scaled to time!!!

The first radioactive (absolute) ages surprised many scientists in that the fossil record (Paleozoic Era to present) comprised only the last 1/5th of Earth's history, whereas the Precambrian Eon, where no hard-shelled fossils have been found, sans the majority of Earth's history. The 4,550 m.y. age of the Earth was determined not on rocks found on Earth, but on meteorites that reflect the initial formation of the Solar System; rocks just slightly younger than this have been found on the Moon, confirming an age of ~ 4.5 b.y. for formation of the planets.

The oldest rocks on Earth come from Australia (actually, mineral grains contained in a younger quartzite), with ages as old as 4,400 m.y. The oldest significant exposures of very old rocks are found in Greenland, at 3,850 m.y. old, although there is one small outcrop of 4,000 m.y. old rocks in northern Canada.

Spring 2003 Semester