Earth Science Notes Geological Time

Objectives I can… Explain how the geological time scale is structured. Describe what each division is based on. Explain the relationship between divisions of geological time. Recall some major details of geological time.

Clarification There is a lot of information so I put the most important things in red (make sure you know it). I do not expect you to know the number of years with each division of time. Only the names of the Eons and Eras are you required to know. The objective of learning about GT is that you understand the structure of it, that you know where it comes from; what its based on.

The Geological Time Scale The GTS is based on the biotic composition of rock strata. This means the life forms that are fossilized in layers of rock characterize each division if the GTS.

The Geological Time Scale Using a variety of techniques and dating methods (relative and absolute dating), geologists have been able to ascertain (or infer) the age of the Earth, as well as major eras, periods, and epochs within Earth's history. These dates are used to study, among other things, the tempo or rates of environmental and biologic change occurring on Earth.

The Geological Time Scale Beginning in the late 1700's, scientists recognized that fossils appeared in an orderly fashion in rock layers. The types of fossils found depended on how old the rock was. Older layers had “simpler” life forms.

The Geological Time Scale Geologist observed fossils in deeper layers as being quite different than in rock layers near the surface. Some fossils were of extinct organisms. Some fossils were only seen in the rock strata for a short period of time. Fossils in later (generally higher) rock strata seemed to be more “advanced” than fossils in earlier rock strata.

The Geological Time Scale The majority of fossils did not match modern groups of organisms; this led to the classification of three major eras in the most recent division of geological time (Phanerozoic Eon). The point here is that the divisions of GT are linked to layers in the rock strata.

The point here is that the divisions of GT are linked to layers in the rock strata.

Geological Time These are the divisions of Geological Time Eons – longest subdivision Based on abundance of certain fossils found in strata Eras – subdivisions of eons Based on major changes in types of fossils found Periods – subdivisions of eras Based on types of life existing worldwide Epochs – subdivision of periods Based on major events or changes in climate

GTS: Eons Eons - longest subdivision of GT Based on abundance of certain fossils The Eons of GT are: Precambrian (precedes the Cambrian Period) Proterozoic Eon (latest) Archaean Eon Hadean (earliest) Phanerozoic

Eons: The Precambrian Events Hadean Eon - 4.6 to 3.8 Billion years 4.6 BYA -- Formation of Earth and Moon (as indicated by dating of meteorites and rocks from the Moon) 4 BYA -- Likely origin of life This is the "hidden" portion of geologic time as there is little evidence of this time remaining in Earth's rocks. Archean Eon - 3.8 to 2.5 Billion years The eon of first life 3.8 BYA -- Oldest known rocks 3.5 BYA -- Oldest known fossils 3.2 BYA -- First known plants (algae) Proterozoic Eon - 2.5 Billion to 570 Million years The eon of the first multi-celled life 1.2 BYA -- First known animal (jellyfish) End of the Pre-Cambrian The oldest known traces of life. A 3.5 billion year-old fossilized photosynthetic bacterium similar to contemporary cyanobacteria (blue green algae) from Western Australia. New discoveries have pushed the origin of cellular organisms further into the past, leaving less time for pre-cellular phases of evolution.

GTS: Eons These three Eons are collectively called the Precambrian. This period is about five times longer than all the geologic time that follows.

GTS: Eons Phanerozoic Eon - 570 Million years to the Present The eon of complex life Made up of three major Eras Paleozoic Era Mesozoic Era Cenozoic Era

GTS: Eons  Eras Eras are subdivisions of eons Based on major changes in types of fossils Each of the names of the Eras reflects the relative stage in the development of life.

GTS: Eons  Eras The Paleozoic Era (Ancient life) The Paleozoic Era is the oldest of the three Eras and dates from 540 Million to 248 Million Years Ago. During the Paleozoic Era multi-celled living things acquired hard body parts, bones, vertebral columns, mandibles, and teeth. Common in the Paleozoic Era were trilobites, crinoids, brachiopods, fish, insects, amphibians, and early reptiles.

GTS: Eons  Eras The Mesozoic Era (Middle life) The Mesozoic Era extended from 248 Million to 65 Million Years Ago. The Mesozoic Era was important for the fossil remains of the dinosaurs and other reptiles that lived. The Mesozoic Era landscape was also occupied by insects, early mammals, plants such as conifers and ferns, fish, and finally flowering plants and early birds.

Mesozoic Life

GTS: Eons  Eras The Cenozoic Era (New life) The Cenozoic Era began 65 Million Years Ago with the extinction of the dinosaurs and continues into the Present. The extinction of the dinosaurs at the end of the Mesozoic Era opened up vast new habitats and environments for early mammals and birds to adapt to and occupy, which allowed them to become the prevailing life form. We live in the Cenozoic Era.

Cenozoic Life

GTS: Eons  Eras  Periods Periods are divisions of Eras Periods are often named after places where fossils that characterize that period were first found. Ex: Pennsylvanian

GTS: Eons  Eras  Periods Periods are Based on types of fossil remains found in rock strata world wide. These fossils that define periods of GT are called index fossils.

GTS: Eons  Eras  Periods Index fossils are used to draw correlations. Correlation is the matching of rock layers from one area to another.  Because fossil types change over time in the rock strata we can use them to date rock formations in different areas of the world.

GTS: Eons  Eras  Periods We can use index fossils to correlate rock layers that are the same age.   By doing this we can then place other layers of rocks in order of their relative ages to find the oldest and youngest rocks in a series of outcrops. Cambrian Period

Periods

GTS: Eons  Eras  Periods  Epochs Epochs – subdivision of geological periods Marked by notable events Ex: Paleocene  extinction and high levels of iridium (meteorite?) Ex: Eocene  large scale marine extinction due to possible climate change. EX: Miocene  marked by a gradual cooling from the warmer Oligocene to a cooler Pliocene. Ex: Pleistocene  repeated glacial events EX: Holocene  began at the end of the last ice age.

GTS: Eons  Eras  Periods  Epochs Rock layers corresponding to epochs in the Cenozoic era are most reliable due to the fact weathering and erosion has not affected them as much as older rocks.

The Geological Time Scale

Things to Remember About Geological Time. It is based on layers of rock. Divisions of time relate to the life forms (or other clues) found in specific layers. Relative dating and absolute dating methods are used to order and date rock layers We will study radiometric dating and Laws of Relative Dating of Rocks There are gaps in the rock record Well talk about unconformities soon.

Things to Remember About Geological Time. For the majority of time, not much happens (the Precambrian). Most events happen relatively recently 

Assessment Can I… Explain how the geological time scale is structured. Describe what each division is based on. Explain the relationship between divisions of geological time. Recall some major details of geological time.