Exploring Geologic Time

The 4.6 billion years of Earth’s history can be divided into 2 major time segments called eons.  Precambrian4600 Ma to 570 Ma  Phanerozoic570 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.  Paleozoic570 Ma to 245 Ma  Mesozoic245 Ma to 66.4 Ma  Cenozoic66.4 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 Era  known as age of reptiles; dinosaurs are prominent lifeforms  last 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 Era  age of mammals  climates cool generating widespread glaciation  humans evolve  high 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. loradoplateau/timescale.htm loradoplateau/timescale.htm

Another site that provides information about characteristics and events in various geologic periods: rks/gtime/gtime2.html rks/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 age might be colored pale brown. Many geologic units are given names that relate to where their characteristics are best displayed, or where they were first studied.

Here is a site to access geologic maps of Maryland: