Transcript Scientific Ideas, Geology 101, the History of

An introduction to Earth Science
•The nature of Scientific Ideas
•What is Geology?
•A brief history of Geology
•The state of Catastrophism
in the 20th century
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.
Deductive reasoning: to draw conclusions regarding a
phenomenum based on all that is known about it.
Earth Science (Geology) focuses on natural
phenomena that are related to the Earth.
How do we develop and express such explanations
of natural phenomena?
• Speculation: simply opinion or guess.
• 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.
•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?
Ockham’s Razor (Principle of Parsimony):
Given more than one hypothesis for a given phenomenon
the simplest hypothesis is always preferred!
(the simplest has fewer assumptions and unknowns)
• Theory: a very well verified and communicated
explanation that links together a number of
separate hypotheses.
Theories provide a basis for predicting outcomes
and these outcomes can be tested by further
observation.
Extensively documented theories that explain many
aspects of the natural world are called paradigms.
• 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.
“The history of knowledge has been characterized
by periodic formulation of hypotheses that
generalized the most factual information
available 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
In a more brutally honest quote:
“There are no facts, only interpretations.”
Nietzsche
What are the implications to scientific progress?
“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 is geology?
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.
e.g., Minerals
Earthquakes
Rocks
Volcanoes
Plate tectonics
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
Large scale cross-bedding in 240
million year old rocks in the
Central US.
= windblown sand dunes; the
Central US was a desert at that
time.
Environmental interpretation:
The present is the key to the past.
By examining the characteristics of various environments on Earth
today we can interpret the environments in which ancient sediments
were deposited.
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.
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.
April 9, 1903 the Earth
changed for the people of
the town of Frank, Alberta.
3 km2 buried
in 100 seconds.
80 million tonnes;
30 million m3.
A Brief History of Geology
Modern Geology is Born (late 18th C)
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).
Neither correct but sparked a healthy debate!
A Brief History of Geology
Abraham Werner (1749-1817): the founder of
modern geology and the champion of Neptunism.
Werner was the first to postulate a history of the
Earth, recognizing that the Earth underwent change
over time.
James Hutton (1726-1797): introduced the
Principle of Uniformitarianism.
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.
A Brief History of Geology
Hutton 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 Geology
Cuvier 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.
First Canadian to be knighted.
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.
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!
Catastrophism in the 20th Century
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
Catastrophism begins to find its place!
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 historical
earthquake in Canada:
28 people died due to the tsunami that
was caused by the submarine landslide.
Catastrophism in the 20th Century
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
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 Century
Catastrophes and mass extinctions
Prior 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.
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.
Catastrophism in the 20th Century
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 Century
With 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.
Iridium is rare on Earth but occurs in greater abundance in
asteroids.
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
Geologists 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.
Catastrophism in the 20th Century
Studies of local variation in the intensity of the Earth’s
gravity recognized a circular structure…a crater buried
beneath sedimentary rock.
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.
Catastrophism in the 20th Century
The Chicxulub Crater is 300 km
across and 50 km deep; 20,000 km3
of rock was vaporized, melted and
ejected into the Earth’s atmosphere.
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 Boltysh
crater 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 Chicxulub
crater.
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!