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Lecture

Module A.pdf


Department
Earth and Ocean Sciences
Course Code
EOSC 326
Professor
Michael Wheeler

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1. GEOLOGICAL TIME
a. Age of the Earth
In the western tradition, one of the earliest and most influential figures in the
interpretation of geological time was James Ussher (1581 - 1665), Archbishop
of Armagh. An important historical figure in his own right, Ussher also
published a chronology of Earth's history using all dates mentioned in the
Bible to establish a timeline. Using this technique, he established the first
day of creation to be October 22, 4004 BC. This date would make the Earth a
little over 6,000 years old!
Figure 1.1 James Ussher (1581 - 1665), Archbishop of Armagh. Image from
Wikipedia.
As science continued to develop during the 1700s, people started to become
dissatisfied with Ussher's estimated age for the planet. One such scientist
and notable natural historian was George Louis De Buffon (1707 - 1788).
Believing the Earth to have been initially as a hot molten mass, Buffon
heated iron spheres (which he thought was a reasonable model for the
structure of the planet) and calculated the time they took to cool. Using this
method Buffon believed the Earth to be around 75,000 years old.
Figure 1.2 George Louis De Buffon (1707 - 1788). Image from Wikipedia.
Irish Geologist John Joly published a paper in 1899 in which he estimated the
Earth's oceans (which he believed to be the same age as the planet) to be
about 90 million years old. He calculated this by estimating how long it would
take for the oceans to reach their current salinity (from an original fresh water
state) as salt is added via the erosion of minerals in rocks. (Today we
understand that the Earth's oceans have not been getting increasingly salty
with time. As such, present day salinity levels cannot be used as a gauge to
estimate the passage of geological time.)
Later in his career Joly was to work with Ernest Rutherford using radioactive
decay in minerals to estimate the age of rocks. This technique provides us

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with the current age of the Earth at 4.6 Ga (Giga-anum: billions of years).
Figure 1.3 Ocean water. Photo by S. Sutherland.
b. Deep Time
4.6 Ga is a vast amount of time, especially when the oldest recorded human
being was only 122 years old when he died. It is this vast amount of time that
the geologist and paleontologist must consider when trying to understand the
evolution of the Earth and its biological systems. This is often referred to as
the concept of deep time.
Analogy is often used to help people grasp the vast tracts of time that have
passed since our planet formed. If you compress all of Earth's history into
one year, these are some of the significant events in our planets history:
January 1: Earth accretes out of Solar disk
February 1: Formation of oldest rock (preserved until today)
November 15: Creatures with shells first appear
December 15: Dinosaurs evolve
December 26: Dinosaurs become extinct
December 31, 23:59:18: end of last Glaciation
December 31, 23:59:46: Birth of Christ
Figure 1.4 An artist's rendition of Geological Deep Time. Image from
Wikipedia.
2. GEOLOGICAL CONCEPTS AND
TERMINOLOGY
Like any science, geology and paleontology has its own terminology and
fundamental concepts. Before we can explore the evolution of Earth through
deep time, you need to become familiar with these concepts and the

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language we use to describe them.
Rocks and Minerals
As cells and tissues are some of the fundamental units to a biologist, so
rocks and minerals are the fundamental units to a geologist.
A mineral can be defined as "a naturally occurring crystalline solid with a
characteristic chemical composition, a highly ordered atomic structure, and
specific physical properties."
Figure 1.5 Quartz (left) and feldspar (right) are the most common minerals in
the Earth's crust. Image from Wikipedia.
A rock on the other hand is an aggregate of minerals. Rocks fall into a basic
three fold classification:
a. Igneous;
b. Metamorphic; and
c. Sedimentary
a. Igneous Rocks
Igneous [audio] rocks (derived from the Latin "ignis" meaning fire) are rocks
that have formed by the cooling of magma or lava. Igneous rocks are
generally composed of interlocking crystals of varying sizes.
Figure 1.6 An image of granite [audio], an igneous rock composed of the
minerals quartz, feldspar, and mica, as seen from under a microscope.
Image from Wikipedia.
If a magma cools within the Earth's crust it is referred to as an intrusive
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