An introduction to Earth Science The nature of

An introduction to Earth Science • The nature of Scientific Ideas • Geology 001: the basics • A brief history of Geology • The state of Catastrophism in the 20 th 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. 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

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. 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.

Structure of the 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.

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). The temperature is more uniform through the upper and lower mantle, possibly due to convection. Upper Mantle Lower Mantle There is a marked increase in the gradient at the top of the outer core. The inner core temperature is likely approximately 7000° C. Outer Core

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).

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. 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). Naturally occurring: synthetic gemstones and most mineral supplements are not real minerals.

Mineral: a solid, crystalline substance that occurs naturally and has a specific chemical composition that determines its physical properties. 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 (Na. Cl). Approximately 3, 500 minerals are known; only about 20 make up over 95% of the crust of the Earth.

Rock: a mass of naturally occurring solid material that is made up of minerals. The minerals may occur as interlocking crystals or cemented “grains”. Igneous Rock: any natural rock that forms by cooling of a magma (molten rock state) into a crystalline solid. Igneous rocks are made up of interlocking mineral crystals. E. g. Granite

Interlocking crystals in an igneous rock

Igneous 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 preexisting 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%20 Strat%20 Class/Sed. Strat. L 1/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. Made up of discrete particles or interlocking crystals if the rock material crystallizes by precipitation from water. E. g. , conglomerate (lithified gravel)

Clastic Sedimentary Rock Individual grains are separated by regions of cement. Igneous Rock

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.

Igneous Rocks Sedimentary Rocks Metamorphic Rocks

The geologic cycle A concept that relates the three rock types through processes that act in their formation. 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).

The geologic cycle…. • 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.

Fossils Fossil: any evidence of past life. Fossils may be preserved in a variety of ways: Body Fossils: The entire organism preserved.

Fossils… Trace Fossil: preserving evidence of an organism’s behavior. E. g. walking traces, grazing traces, shelters. 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.

April 9, 1903 the Earth changed for the people of the town of Frank, Alberta. 3 km 2 buried in 100 seconds. 80 million tonnes; 30 million m 3.

A Brief History of Geology Modern Geology is Born (late 18 th 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 19 th 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 20 th 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 1920 s but came to be widely accepted in the early 1960 s. “Continental drift” did not fit the Uniformitarian mold!

Catastrophism in the 20 th 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 20 th 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 1950 s 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 20 th 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 20 th 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 20 th Century 100 km 3 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 km 2.

Catastrophism in the 20 th 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 20 th 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 20 th 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 20 th 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 20 th 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 20 th Century The Chicxulub Crater is 300 km across and 50 km deep; 20, 000 km 3 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!