GEOLOGIC FOUNDATIONS OF THE EARTH
Objectives:
⋆ Discuss the geological history of the Earth.
⋆ Compare and contrast each stage of geologic.
⋆ Emphasize the significance of the geological events and developments in the past with the present times
1. Geological History of the Earth:
How everything began?
⋆ The Pregeologic Period
Geological history of
earth is studied by
dividing it in eons, then
further dividing eons in
eras, era in periods,
period in epochs. Life
appeared in
Archaeozoic eon.
Multicellular life
appeared during
Proterozoic eon.
Currently it is
Phanerozoic eon, that
started about 540
million years ago.
Geologic history of Earth
Evolution of the continents, oceans, atmosphere, and biosphere. The layers of rock at Earth’s surface contain evidence
of the evolutionary processes undergone by these components of the terrestrial environment during the times at which
each layer was formed. By studying this rock record from the very beginning, it is thus possible to trace their
development and the resultant changes through time.
Geologic History of Earth
the history of Earth spans approximately 4.6 billion years.
The oldest known rocks—the faux amphibolites of the Nuvvuagittuq greenstone belt in Quebec, Canada—
however, have an isotopic age of 4.28 billion years.
Factors which have to be considered: the age of formation at 4.6 billion years ago, the processes in operation until
4.3 billion years ago, the bombardment of Earth by meteorites, and the earliest zircon crystals.
It is widely accepted by both geologists and astronomers that Earth is roughly 4.6 billion years old.Earth is roughly
4.6 billion years old; age has been obtained from the isotopic analysis of many meteorites as well as
of soil and rock samples from the Moon by such dating methods as rubidium–strontium and uranium–lead.
Models developed from the comparison of lead isotopes in meteorites and the decay of hafnium-182 to tungsten-
182 in Earth’s mantle, however, suggest that approximately 100 million years elapsed between the beginning of
the solar system and the conclusion of the accretion process that formed Earth. These models place Earth’s age at
approximately 4.5 billion years old.
An exciting discovery was made in 1983 by William Compston and his research group at the Australian National
University with the aid of an ion microprobe. Compston and his associates found that a water-laid clastic sedimentary
quartzite from Mount Narryer in western Australia contained detrital zircon grains that were 4.18 billion years old.
In 1986 they further discovered that one zircon in a conglomerate only 60 km (about 37 miles) away was 4.276
billion years old; 16 other grains were determined to be the same age or slightly younger.
In 2014 American geochemist John Valley and colleagues discharged lead atoms at one of the zircon crystals
discovered in western Australia’s Jack Hills and discovered that the crystals are more than 4.4 billion years old.
o Time Scales
The geologic history of Earth covers more than 4.5 billion years of time. Different types of phenomena and events in
widely separated parts of the world have been correlated using an internationally acceptable, standardized time
scale.
o
o Earth’s Evolution
Earth and the rest of the solar system formed about 4.6 billion years ago from a huge, spinning cloud of gas
and dust.
Over a period of about 10 million years, the dense center of the cloud grew very hot. This massive center became
the sun. The rest of the particles and objects continued to revolve around the sun, colliding with each other in clumps.
Eventually, these clumps compressed into planets, asteroids, and moons. This process generated a lot of heat.
o Two geologic time scale:
Relative, or Chronostratigraphic = evolved since the mid-1800s and concerns the relative order of strata. Important
events in its development were the realization by English engineer and geologist William Smith that in a horizontal
sequence of sedimentary strata what is now an upper stratum was originally deposited on a lower one
and the discovery by Scottish geologist James Hutton that an unconformity (discontinuity) indicates a significant gap
in time. Furthermore, the presence of fossils throughout Phanerozoic sediments has enabled paleontologists to construct
a relative order of strata. As was explained earlier, at specific stratigraphic boundaries certain types of fossils either
appear or disappear or both in some cases. Such biostratigraphic boundaries separate larger or smaller units of
time that are defined as eons, eras, periods, epochs, and ages.
Absolute or Chronometric = more recent origin. It was made possible by the development of mass spectrometers
during the 1920s and their use in geochronological laboratories for radiometric dating; based on specific units of
duration and on the numerical ages that are assigned to the aforementioned chronostratigraphic boundaries;
