Exploring Geologic Time

The 4.6 billion years of Earth’s history can be

divided into 2 major time segments called eons.

Precambrian 4600 Ma to 570 MaPhanerozoic 570 Ma to present

Ma means million of years ago

Precambrian Eon

85% of Earth’s history intense volcanic activity; oceans form;

meteorite bombardment atmosphere develops from primitive through

out-gases from volcanoes to one with free oxygen (and ozone layer)

life begins with unicellular organisms (blue-green algae and bacteria) and evolves to multi-cell organisms like worms

few fossils exist because most did not have hard parts

The Phanerozoic Eon is divided into time segments

called eras.

Paleozoic 570 Ma to 245 MaMesozoic 245 Ma to 66.4 MaCenozoic 66.4 Ma to present

The Paleozoic Era most significant breakup period for first

supercontinent with development of smaller continents and finally formation of Pangaea

sea level rises and falls covering and uncovering the new continents

six periods are marked by significant differences in lifeforms

appearance of fish, insects, amphibians, and some reptiles

The Mezozoic Eraknown as age of reptiles; dinosaurs are

prominent lifeformslast part of this period (Cretaceous)

witnesses large-scale extinction of marine and flying reptiles as well as dinosaurs; extinction may be due to massive asteroid collision with Earth

Pangaea breaks up

The Cenozoic Eraage of mammalsclimates cool generating widespread

glaciationhumans evolvehigh level of tectonic activity as plates

separate and evolve

Each era can be subdivided into periods. Periods can be

subdivided into epochs. Each time segment is

characterized by changes in landforms (tectonics

changes), climate, and in life forms.

Here is an example of a geologic timeline with

important events noted.

http://3dparks.wr.usgs.gov/coloradoplateau/timescale.htm

Another site that provides information about

characteristics and events in various geologic periods:

http://geomaps.wr.usgs.gov/parks/gtime/gtime2.html

Information about rocks of a particular geologic time can

be displayed on a map called a geologic map.

The most striking features of geologic maps are its colors. Each color represents a different geologic unit. A geologic unit is a volume of a certain kind of rock of a given age range. So a sandstone of one age might be colored bright orange, while a sandstone of a different agemight be colored pale brown. Many geologic units are givennames that relate to where their characteristics are best displayed, or where they were first studied.

http://ncgmp.usgs.gov/ncgmpgeomaps

Here is a site to access geologic maps of Maryland:

http://www.mgs.md.gov/esic/geo/