Geological Time

Geologic Time Scale Geologists have subdivided geologic time into units based on fossil evidence. This division of time is called the Geological Time Scale.

The Time Scale has four major divisions. The largest divisions are referred to as eons. There have been two eons, the Precambrian, and the Phanerozoic Eons extend for hundreds of millions to billions of years.

The Precambrian represents the first 85% of Earth history. The Precambrian is essentially devoid of a fossil record for several reasons: Plants and animals were small, with no hard parts needed for preservation as fossils. Most rocks formed during that period are non-sedimentary. Igneous and metamorphic rocks have been distorted and transformed by heat and pressure, destroying fossil evidence.

The Precambrian has unique subdivisions There are several systems for describing the Precambrian eon. The fossil record is very sparse during most of Earth history Life caused profound changes in the atmosphere and oceans, which are captured in rocks and minerals formed during this time.

History and Evolution of Earth’s Atmosphere Earth was born 4.55 million years ago in the nebula that created the sun. For 500 million years, the interior of the earth was solid, composed of mostly iron and silicates with small amounts of other elements, some radioactive. As millions of years passed, Earth heated up and gradually melted the solid materials. Iron melted first – because of its high density, it sank toward the center, forcing up other material found there.

Early Earth Surface Earth’s surface was in turmoil; many volcanoes, lava flows. As Earth cooled, a stable crust developed. Oceans formed soon after Earth’s crust cooled. The primordial atmosphere was produced by outgassing volcanoes. Volcanic gases consist of H2O, CO2, N2, CH4 and other trace gases. Our current atmosphere evolved over billions of years (secondary atmosphere) O2 levels were negligible for most of Earth history – the rise of plants took a long time to produce our modern atmosphere

The Phanaerozoic Eon is subdivided into eras Eras extend for several hundred million years Marked by profound, abrupt changes in the fossil record. Major mass extinctions at the end of the Paleozoic and Mesozoic Paleozoic 8.5% of Earth history Mesozoic 3.5% of Earth history Cenozoic 1.4% of Earth history

The Cambrian Explosion

The Paleozoic Era

Lesser extinction events are marked by periods The Paleozoic, the Mesozoic, and the Cenzoic eras are divided into periods – the most common way of referring to chunks of geological time.

Periods range from 40 to 80 million years in length The ESRT have short descriptions of common extinct organisms that lived during each period. The larger extinction events are shown in brackets in the Life on Earth column.

Periods are subdivided into epochs Currently we are living in the Holocene epoch. A new epoch has been proposed: the anthropocene, as a result of dramatic increases in the rate of extinction of plants and animals. The conditions of the Eocene were much warmer than today – climate change projections are compared to this time.

Geological time is based on the fossil record Fossils provide evidence that a great many kinds of animals and plants have lived on Earth in the past under a variety of conditions. Most organisms (>99%) have become extinct; only a tiny fraction of these are found as fossils. Two conditions seem to be necessary for fossil formation: Organisms must be buried soon after death Animals must have hard shells, bones, or teeth

Paleontology: Studying Fossils Paleontologists use the principle of uniformitarianism to study conditions under which organisms live and develop, and compare them to extinct fossil species. From this comparison, past environments can be inferred. The geographic distribution of both present-day and prehistoric plants and animals is closely controlled by climate – temperature, food availability, and water.

Time Distribution of Fossils Major phyla of plants and animals are shown on the ESRT.

Geologic History of New York State Event Period, mya Insects first appeared   Extinction of trilobytes Catskill Delta forms First coral reefs formed Earliest fish appeared Dinosaur extinction First Flowering plants Grasses appear Earliest mammals (placental animals) Adirondacks uplifted The mid-Atlantic rift begins to form NY in subduction zone First animals with shells First land plants/animals Supercontinent (Pangea) Fill in the following table:

New York is home to many fine specimens of Phacops. Using your ESRT, find the following: The period of time in which it lived A landscape region in New York where it may be found.

The Catskill Delta was deposited during the Devonian Period (416-359 mya) An orogeny is a period of mountain building. The collision of two tectonic plates produced the Acadian Orogeny.

Former Coral Reef in Western NY Note that the precursors to North America were close to the equator at the time. Former Coral Reef in Western NY