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An introduction to Earth Science
•The nature of Scientific Ideas
•A brief history of Geology
•The state of Catastrophismin the 20th century
•Geology 001: the basics
The nature of scientific ideas
Science is a system of knowledge that is based on general
“truths” or facts as they are known at the time.
Scientists use observations (to gather the facts) and deduction
to offer explanations of natural phenomena.
Earth Science (Geology) focuses on natural phenomena that are
related to the Earth.
• Hypothesis: a logically derived explanation that is based on a
body of knowledge that is made up of “facts”; everything known
about what is being explained.
• Speculation: simply opinion or guess.
How do we develop and express such explanations of natural
phenomena?
•It remains tentative (possibly wrong) until further evidence
and rigorous testing proves it better than other hypotheses; are
new “facts” that become known consistent with the explanation?
• Theory: a very well verified and communicated explanation that
links together a number of separate hypotheses.
Extensively documented theories that explain many aspects of the
natural world are called paradigms.
Theories provide a basis for predicting outcomes and these
outcomes can be tested by further observation.
Ockham’s Razor (Principle of Parsimony):
Given more than one hypothesis for a given phenomenonthe
simplest hypothesis is always preferred!(the simplest has fewer
assumptions and unknowns)
“The history of knowledge has been characterized by periodic
formulation of hypotheses that generalized the most factual
informationavailable at a given time…
Science is a process of continuous refinement and testing of
such generalizations…
Hypotheses inevitably have been colored by the temperaments,
experiences and prejudices of their advocates…” Robert Dott
• Law: Theories become Laws (e.g., the Law of Gravity) when they
are shown to be absolutely correct for the conditions to which they
apply.
In a more brutally honest quote:
“There are no facts, only interpretations.”
Nietzsche
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“The greatest obstacle to progress in science is the illusion of
knowledge….the illusion that we know what’s going on but we
don’t.”
Professor Mike Disney, Cosmologist, Cardiff University
What are the implications to scientific progress?Geology 001:
the basics
Geology: a science that deals with the history of the earth and
its life especially as recorded in rocks.
The science that deals with:
•The materials that make up the Earth (rocks and minerals).
•Processes acting within the Earth and at its surface.
•The history of the Earth.
Physical Geology: involved with the processes that act to form
the Earth and the products of those processes.
Rockse.g., Minerals Plate tectonics
Earthquakes Volcanoes
Historical Geology: involved with the interpretation of the
history of the Earth.
Based on the recognition of the signature of changing
environments over time, as preserved in the rock record.
Environmental interpretation of rocks
+Age of rocks
= Earth History
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Large scale cross-bedding in 240million year old rocks in the
Central US.
= windblown sand dunes; the Central US was a desert at
thattime.
By examining the characteristics of various environments on
Earth today we can interpret the environments in which ancient
sediments were deposited.
The present is the key to the past.
Environmental interpretation:
Age of rocks:
Based on relative age (relative to associated rocks)
or
absolute age (radiometric dating).
Earth History:
The history of changing environments on Earth.
The Earth can be considered as being made up of a series of
concentric spheres, each made up of materials that differ in terms
of composition and mechanical properties.
Structure of the Earth
Geothermal gradient: the rate at which the temperature increases
with depth beneath the Earth’s crust.
Averages 20° to 30° C per kilometre in the upper part of the
crust (deepest mine: 3.5 km; 70°C).
Upper Mantle
Lower Mantle
Outer Core
The temperature is more uniform through the upper and lower
mantle, possibly due to convection.
There is a marked increase in the gradient at the top of the
outer core.
The inner core temperature is likely approximately 7000° C.
Inner Core: Solid Nickel and Iron alloy (very high pressure and
temperature)Outer Core: Nickel and Iron alloy that behaves like a
soft plastic (i.e., it flows).
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Lower Mantle: Rigid material or possibly a soft plastic that
flows under stress.Upper Mantle: Soft, plastic that flows due to
convection driven by the internal heat of the Earth.
Lithosphere:The topmost layer of the upper mantle.
The lithosphere has the composition of the upper mantle but is
rigid like the crust.
Lower temperatures and pressures allow the Lithosphere to be
rigid.
Ranges in thickness from a couple of km to over 80 km.
Crust: the outermost layer of the earth, a hard outer shell.
Oceanic crust: relatively thin, varying from 5 to 8 km (but
thinner at Oceanic ridges). Made up of relatively dense, heavy
rocks.
Continental Crust: thicker and more variable in thickness than
oceanic crust.
Thickness ranges from 20 km to about 75 km (beneath mountain
ranges).
Made up of “lighter” rocks than oceanic crust.
Mineral: a solid, crystalline substance that occurs naturally
and has a specific chemical composition that determines its
physical properties.
Naturally occurring: synthetic gemstones and most mineral
supplements are not real minerals.
Solid, crystalline substance: excludes gases and liquids (e.g.,
oil and gas); the internal structure of atoms is ordered.
Solid Earth materials that are not crystalline are said to be
glassy or amorphous (lacking structure).
Specific chemical composition: every mineral is characterized by
a specific chemical composition that determines the properties of
the mineral.
E.g., Halite, rock salt, always has the same chemical
composition (NaCl).
Approximately 3,500 minerals are known; only about 20 make up
over 95% of the crust of the Earth.
Mineral: a solid, crystalline substance that occurs naturally
and has a specific chemical composition that determines its
physical properties.
Igneous Rock: any natural rock that forms by cooling of a magma
(molten rock state) into a crystalline solid.
E.g. Granite
Igneous rocks are made up of interlocking mineral crystals.
Rock: a mass of naturally occurring solid material that is made
up of minerals. The minerals may occur as interlocking crystals or
cemented “grains”.
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Interlocking crystals in an igneous rockIgneous rocks commonly
occur as large bodies such as the pluton on the left formed as a
mass of magma pushed into the Earth’s crust from deeper within the
Earth. This pluton has been exposed due to erosion.
Smaller igneous bodies include dykes (oblique) and sills
(horizontal), linear bodies of igneous rock that form when magma is
injected into cracks in pre-existing rocks.
The formation of an igneous pluton with associated sill and
dyke. This pluton has become exposed at
the Earth’s surface after years of erosion.
Just an aside…….
Geologists often study rocks in thin section under a microscope
to determine the minerals that are present.
Thin sections are 30 micron (30/1000 mm) thick slices of rock
through which light can be transmitted.
Click here to see how a thin section is made.
http://faculty.gg.uwyo.edu/heller/Sed%20Strat%20Class/SedStratL1/thin_section_mov.htm
Sedimentary Rock: any natural rock that forms from the products
of broken down pre-existing rocks that have become “cemented”
together.
E.g., conglomerate(lithified gravel)
Made up of discrete particles or interlocking crystals if the
rock material crystallizes by precipitation from water.
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Igneous Rock
Clastic Sedimentary Rock
Individual grains are separated by regions of cement.
Sedimentary rocks normally occur in laterally continuous layers
or strata. The Niagara Escarpment is made up of layered sedimentary
strata (Decew Falls Power Station outcrop is shown below).
Where sedimentary rocks are deformed by tectonic processes their
layering may vary from horizontal to vertical.
Deformation of any rock can vary from simple tilting to complex
folding as shown below.
Metamorphic Rock: any natural rock formed by the modification of
a pre-existing rock by heat and/or pressure associated with burial
deep within the Earth and/or tectonic forces.
Gneiss:a banded metamorphic
rock.
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Igneous Rocks
Sedimentary Rocks Metamorphic Rocks
The geologic cycle
Involves:•Cooling of magma (to form igneous rocks).
•Heat and pressure inside the earth (metamorphism or melting to
form a new magma).
•Uplift of buried rocks by tectonic processes (e.g., mountain
building).
A concept that relates the three rock types through processes
that act in their formation.
•Weathering: the breakdown of a rock exposed at the Earth’s
surface.
•Transport of weathering products (e.g., by rivers).
•Deposition of transported material (as loose sediment) to where
it can no longer be transported.
The geologic cycle….
•Burial and compaction: covered by subsequent deposition and
pushed into close contact due to the weight of overlying
sediment.
•Cementation: the binding together of sedimentary particles by
minerals that act as a cement.
The geologic cycle….
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Fossil: any evidence of past life.
Fossils may be preserved in a variety of ways:
Body Fossils:The entire organism preserved.
Fossils
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Trace Fossil: preserving evidence of an organism’s behavior.
E.g.walking traces, grazing traces, shelters.
Fossils…
Dinosaur tracks Trilobite tracks
Fossils…
Coprolite: fossil excrement!
A Brief History of Geology
Herodotos (500 BC): recognized that the Nile River deposited
sediment during flood (i.e., the land surface was modified by
natural processes).
Aristotle (384-322 BC): recognized that fossils in rocks
resembled many living organisms; concluded that fossils were
once-living animals. Implies that rocks formed since there was life
on Earth.
Theophrastus (374-287 BC): wrote the first book on the minerals
that make up rocks.
A Brief History of Geology
St. Thomas Aquinas (1225-1274): explained all geological
phenomena in terms of the biblical account.
Landscape on Earth today was formed during creation or modified
during the biblical Flood.
Fossils in rocks in the mountains were washed there by the
Flood.
Leonardo da Vinci (1452-1519): deduced that fossils represented
past life and that they could not have been transported by floods
to mountainous areas where they were found.
A Brief History of Geology
James Ussher (1581-1665):
Provided the first estimate of the age of the Earth…based on the
genealogies of the Bible.
Concluded that the Earth formed on October 23, 4004 BC.
…..at 9:00 in the morning.
The Earth was created and remains pretty much as it was on Day 1
except for changes due to catastrophic events.
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April 9, 1903 the Earth changed for the people of the town of
Frank, Alberta.
3 km2 buriedin 100 seconds.
80 million tonnes;30 million m3.
A Brief History of Geology
Neptunism versus Plutonism
Neptunists: believed that all rocks (including igneous rocks)
were formed by deposition from an early global ocean. The Oceans
receded and the Earth surface has been much the same ever
since.
Plutonists: believed that rocks formed by igneous processes
(i.e., all rocks formed by crystallization from a magma).
Modern Geology is Born (late 18th C)
Neither correct but sparked a healthy debate!
A Brief History of Geology
Werner was the first to postulate a history of the Earth,
recognizing that the Earth underwent change over time.
Abraham Werner (1749-1817): the founder of modern geology and
the champion of Neptunism.
That the Earth changes very slowly over time in response to
natural processes that we can see acting today as they have always
acted on Earth. E.g., river valleys are cut slowly by the streams
that occupy them.
James Hutton (1726-1797): introduced thePrinciple of
Uniformitarianism.
A Brief History of GeologyHutton believed that everything about
the Earth must be interpreted in terms of the processes that act in
a slow, ongoing manner over time.
Georges Cuvier (1769-1832)
The father of modern Catastrophism and first to recognize that
extinctions have taken place over Earth history.
Suggested that the Earth has always been more-or-less the same
but it periodically undergoes “Revolutions”(catastrophes); periods
of rapid and extensive change.
A Brief History of GeologyCuvier did not have an explanation for
the “revolutions”, he just recognized evidence that major events
had taken place.
Charles Lyle (1779-1875):
Prolific author and follower of Hutton’s Principle.
These ideas appeared in the first edition of Principles of
Geology (1830) and in all of the 12 editions that followed.
Lyle’s writings shaped geology by the end of the 19th
century.
The Principle of Uniformitarianism was entrenched making it
difficult for contrary ideas to be accepted by geologists.
Canadian Profile
Sir William Logan(1798-1875)
Ranked the top Canadian scientist in history.
Conducted extensive field work mapping the geology of
Canada.First to recognize (and promote) the mineral wealth of
Canada.Founded the Geological Survey of Canada.
First Canadian to be knighted.
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Mount Logan
Source: Natural Resources Canada/KGS-2262© Natural Resources
Canada. Photo: Courtesy of Natural Resources Canada.nlc-10607
Catastrophism in the 20th Century
Alfred Wegner (1880-1930):
Gathered a mass of evidence suggesting that the continents had
moved over time and were once together as a supercontinent situated
near the south pole.
Wegener’s theory was refuted in the late 1920s but came to be
widely accepted in the early 1960s.
“Continental drift” did not fit the Uniformitarian mold!
J. Harlan Bretz (1927): published a theory explaining the
formation of the Channeled Scablands of western Washington
State.
The Scablands are dominated by deep valleys, abandoned river
channels and giant boulders; all inconsistent with the processes
acting there today.
Catastrophism in the 20th Century
Bretz Postulated that landscape was formed catastrophically due
to a massive flood of water (although he didn’t know the
source).
Bretz’s ideas challenged the Principle of Uniformitarianism so
they were quickly discarded by his peers.
In the 1950s a source of the flood was found: a large glacial
lake that was dammed by ice. The ice dam broke releasing a
discharge greater than all of the world’s rivers combined.
Catastrophism in the 20th Century
The discovery of turbidity currents.
In 1929 an earthquake caused a massive slump on the continental
slope off the coast of Newfoundland.
The earthquake caused the largest loss of life of any
historicalearthquake in Canada:
28 people died due to the tsunami that was caused by the
submarine landslide.
Catastrophism in the 20th CenturyCatastrophism begins to find
its place!
An outcome of the earthquake that remained unexplained for 25
years was the sequential breakage of undersea telephone lines
immediately after the earthquake.
In 1954 it was realized that the breaks were due to a quickly
flowing mass of sediment away from the centre of the
earthquake.
Catastrophism in the 20th Century
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100 km3 of sediment was involved in the current.
The turbidity current flowed for hundreds of kilometres,
snapping underwater telephone and telegraph cables.
The current flowed at speeds up to 95 km/hr and covered an area
of 100,000 km2.
Catastrophism in the 20th CenturyCatastrophes and mass
extinctionsPrior to 1980 asteroid impacts (major catastrophic
events) were not considered seriously as possible causes of mass
extinctions, the rapid loss of a large number of species from
Earth.
Catastrophism in the 20th Century
Luis Alvarez (a physicist) and his son Walter Alvarez (a
geologist) examined the geochemistry of rocks in Italy that were
deposited just as the dinosaurs and much other life became extinct
on Earth.
A marked feature of the chemistry of the rocks that are exactly
the age of the extinction had a very high concentration of
Iridium…a rare element on Earth but not so rare in asteroids.
Catastrophism in the 20th CenturyWith further investigation this
same elevation in Iridium (called the Iridium Anomaly) was found in
rocks the same age at dozens of locations around the world.
The Alvarezs suggested that the Iridium anomaly was due to a
large asteroid impact that sent Iridium loaded dust into the
atomsphere. As the dust settled it elevated Iridium concentrations
in sediments being deposited world-wide.
Such a collision was postulated to be the “catastrophe” that
caused the mass extinction.
Catastrophism in the 20th Century
Iridium is rare on Earth but occurs in greater abundance in
asteroids.
Catastrophism in the 20th CenturyGeologists found these ideas to
be interesting but asked: “Such a massive impact must have made a
mark on Earth…..so where’s the crater?”
Within a couple of years a huge crater was discovered in rocks
exactly the right age − 65 million years (my) − off coast of
Yucatan Mexico.
Studies of local variation in the intensity of the Earth’s
gravity recognized a circular structure…a crater buried beneath
sedimentary rock.
Catastrophism in the 20th Century
Core samples showed that rocks were present that showed signs of
a major impact.
Thought to have formed with the impact of a 10 km diameter
asteroid.
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The Chicxulub Crater is 300 km across and 50 km deep; 20,000
km3of rock was vaporized, melted and ejected into the Earth’s
atmosphere.
Catastrophism in the 20th Century
Proof that this major, catastrophic event had occurred and had a
global impact on life.
But, could there be more to this…..
The 24 km wide Boltyshcrater in the Ukraine had been dated at 88
my old.
Improved dating techniques show it to be 65 my old, within
250,000 years of the Chicxulubcrater.
It’s likely that collisions so close in time were a part of a
swarm of impacts that took place over a very short period of
time.
The Shiva Crater in the Arabian Sea has been dated at 65 million
years.
600 by 450 km in size.
Likely formed by the impact of a 40 km diameter object.
Yet another massive impact among a possible “cluster” of large
impacts.
Modern thinking is that the demise of the dinosaurs was due to
the effects of a large number of significant impacts over the span
of several hundred thousands of years.
Catastrophism has found a new place in geological thinking!
