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An introduction to Earth Science
•The nature of Scientific Ideas
•A brief history of Geology•The state of Catastrophism
in the 20th century
•What is Geology?
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.
Deductive reasoning: to draw conclusions regarding a phenomenum based on all that is known about it.
• 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)
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“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
“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?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.
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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.
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.
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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 buriedin 100 seconds.
80 million tonnes;30 million m3.
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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.
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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.
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
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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 Century100 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
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Catastrophes 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 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.
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.
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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.
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.
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.
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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!
