Exploring Geologic Time

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Exploring Geologic Time
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The 4.6 billion years of Earths history can be
divided into 2 major time segments called eons.

• Precambrian 4600 Ma to 570 Ma
• Phanerozoic 570 Ma to present

Ma means million of years ago
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Precambrian Eon

• 85 of Earths 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

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The Phanerozoic Eon is divided into time segments
called eras.

• Paleozoic 570 Ma to 245 Ma
• Mesozoic 245 Ma to 66.4 Ma
• Cenozoic 66.4 Ma to present

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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

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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

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The Cenozoic Era

• age of mammals
• climates cool generating widespread glaciation
• humans evolve
• high level of tectonic activity as plates
  separate and evolve

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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.
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Here is an example of a geologic timeline with
important events noted.http//3dparks.wr.usgs.go
v/coloradoplateau/timescale.htm
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Another site that provides information about
characteristics and events in various geologic
periodshttp//geomaps.wr.usgs.gov/parks/gtime/g
time2.html
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Information about rocks of a particular geologic
time can be displayed on a map called a geologic
map.
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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
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Here is a site to access geologic maps of
Marylandhttp//www.mgs.md.gov/esic/geo/