CHAPTER 1 - INTRODUCTION TO PHYSICAL GEOLOGY AND THE ENVIRONMENT
Overview
This chapter begins with an introduction to the history of geology, examines why the discipline developed and looks at some of the important figures in the development of geology in Canada. Two key concepts in geology are then briefly examined as these are fundamental to the discipline – plate tectonics and the geological time scale. The discussion then turns to examine the range of work modern geoscientists may find themselves doing and how this differs from the ‘traditional’ image of the geologist working in the bush. The exciting challenges that lie ahead in the field of environmental geoscience are emphasized. Regardless of the type of project being worked on, geoscientists apply the scientific method.
A natural starting point for a text covering Physical Geology and the Environment is a discussion of the Earth’s origin and formation within our solar system. The characteristics of the early Earth and the formation of the atmosphere are also examined. Formation of the Earth involved the process of differentiation which allowed materials of different densities to separate and create the internal structure of our planet consisting of core, mantle and crust. The crust and uppermost rigid mantle are broken into a number of lithospheric plates which move across the surface of our planet. This movement of plates probably began early in the Earth’s history and continues to the present day. As a consequence the Earth is a dynamic planet and has undergone many important events in its 4.6 billion year history. Some of the important events in the Earth’s geological history are identified. The chapter ends with a look at the rock cycle and how the formation of rocks involves both geological and surface processes and is closely linked with Plate Tectonics.
Learning Objectives
1. Geology is the scientific study of the earth. Physical geology is that division of geology concerned with earth materials, changes in the surface and interior of the earth, and the dynamic forces that cause those changes.
2. Geology developed as a scientific discipline in the late eighteenth century as a result of new demands for resources. Sir William Logan became the first director of the Geological Survey of Canada in 1842 and was the first to systematically describe Canada’s geology.
3. Two fundamental concepts of geology are plate tectonics and the geologic time scale.
4. Plate tectonics is a theory that views the earth's lithosphere as broken into plates that are in motion. The movement of continents was first proposed by Alfred Wegener in 1912 and a Canadian geologist, J.Tuzo Wilson, contributed significantly to the development of the theory. Geologists now recognize that no place on Earth is fixed or immobile and that plate tectonic theory explains most of the geological processes and events observed today.
5. Geology involves deep time, vastly greater than human lifetimes or even human contemplation. The earth is about 4.5 billion years old. Most geological processes are slow and take place over many million years. Fast, to a geologist, is an event or process completed in a million years or less. Plate motions are relatively fast. Complex life forms have existed on the earth for the past 545 million years. Humans have only been on earth for about 3 million years.
6. Geoscientists now work in a number of fields including mining and exploration geology, environmental geoscience and engineering geology. These geoscientists employ the scientific method in their investigations.
7. The earth is one of nine planets in the solar system. The solar system formed by collision and accretion of particles of dust and gas within the solar nebula. The Earth is one of the dense, terrestrial planets in the solar system. Meteorites are thought to represent materials present in the original solar nebula and are isotopically dated at 4.6 billion years
8. Accretion of debris, gravitational compression and decay of radioactive isotopes caused extreme heating and partial melting of the early Earth. This allowed dense materials, such as iron and nickel, to sink to the planet’s centre and the less dense materials (silica and oxygen) to float to the surface. This process is called differentiation. The internal structure of the Earth
9. The Earth’s early atmosphere was derived from water and gases released during volcanic eruptions and contained little oxygen. Evolution of photosynthetic life forms allowed development of an oxygen-rich atmosphere.
10. The Earth's interior comprises three concentric zones: crust (thin, oceanic crust and thicker, continental crust), mantle, and core (inner-solid, outer-liquid). Lithosphere is the rigid crust and upper mantle that is broken into plates. Tectonic forces cause vertical and horizontal movement of these plates, events which are recorded in the geological record by suture zones and mountain ranges.
11. Rocks formed within the earth are brought to the surface by tectonic and erosive forces. New sediments are formed as these rocks are broken down by surficial processes. Over time, sediments become lithified and create sedimentary rocks; these rocks may be buried and changed by the effects of heat and pressure to form metamorphic rocks. Metamorphic rocks may partially melt to form magma which produces igneous rocks upon cooling. This recycling of geologic materials is called the rock cycle and is closely connected to plate tectonic processes.
Boxes
1.1 IN GREATER DEPTH – WILLIAM SMITH
William Smith was one of the founders of the science of geology. He worked as a surveyor in England during the late 1700’s and created the first comprehensive geological map of England in 1815.
1.2 IN GREATER DEPTH – SIR WILLIAM LOGAN
Sir William Logan became the first director of the Geological Survey of Canada in 1842 and is credited with compiling the first systematic layout of the geology of Canada. He traveled extensively across Canada making an inventory of Canada’s mineral resources.
1.3 IN GREATER DEPTH – ALFRED WEGENER
Alfred Wegener was a German climatologist who first proposed the concept of continental drift in 1912. He suggested that all the present continents formed a supercontinent named Pangea around 250 million years ago which later broke up. His ideas were largely discarded by the geologic community until the 1960’s when deep ocean drilling identified possible mechanisms for continental drift.
1.4 IN GREATER DEPTH – J. TUZO WILSON
J. Tuzo Wilson was a Canadian geologist who studied transform faults and hot spots and recognized their significance as indicators of active plate movements. His work contributed substantially to the development of the Plate tectonic theory.
1.5 IN GREATER DEPTH – A DAY IN THE LIFE OF AN EXPLORATION AND MINING GEOSCIENTIST
Dave works for a mining company collecting geological data in order to find gold in the Canadian Shield. He collects and analyses samples of glacial sediment that overlies bedrock and has to be able to reconstruct former ice flow directions to find the source of any gold particles he finds. He works with geophysicists and supervises drilling operations, often conducted in the cold winter months when the ground is frozen. One of his tasks is to describe and record drill core and collect samples to be sent to the lab for mineralogical analysis. Back in head office he is responsible for compiling all of the analytical data and communicating with economic geologists who can determine the feasibility of developing a particular prospect.
1.6 IN GREATER DEPTH – A DAY IN THE LIFE OF AN ENVIRONMENTAL GEOSCIENTIST
Cheryl is an environmental geoscientist working with an environmental consulting company who are cleaning up a contaminated industrial site. She must understand the three-dimensional layering of different sediment types below the site and identify which layers contain and transmit contaminants. To do this she must work with drillers and geophysicists. She also has to decide what action is necessary to control and deal with the contamination and for this she must understand subsurface water flow. Her work requires an understanding of provincial water quality standards and she is often directly involved with local residents and politicians.
1.7 IN GREATER DEPTH – A DAY IN THE LIFE OF AN ENGINEERING GEOLOGIST
Jason is an engineering geologist working on the construction of a major subway tunnel through glacial sediments. He works closely with a glacial geologist who understands the composition of the sediment as the boring machine used to excavate the tunnel has problems when large boulders are encountered. Jason is responsible for determining the engineering properties of the sediment ahead of the boring machine and may make small changes in the elevation of the tunnel to ensure the most effective pathway is obtained. Engineering geologists also work on projects related to bridge construction, earthquake resistance and slope stability.
1.8 ASTROGEOLOGY – METEORITES Meteorites are meteors that have survived the earth's atmosphere and strike its surface. Three types occur: stony (most common), stony-iron and iron (rarest). About 90% of stony meteorites contain chondrules, round grains mostly of olivine and pyroxene, and resemble terrestrial peridotite. These chondrites are distinguished from the achondrites that lack chondrules. Carbonaceous chondrites are stony meteorites that have up to 5% carbon, hydrocarbons, and amino acids, but they are generally regarded as inorganic. Iron meteorites are alloys of iron and nickel, while stony-iron meteorites have equal amounts of silicate minerals and iron-nickel alloy. Meteorites are interpreted as originating from a fragmented planetary body. Most provide radiometric dates of 4.5 billion years, the presumed age of the earth.
1.9 IN GREATER DEPTH – CANADIAN ROCKY MOUNTAINS
The Canadian Rocky Mountains record the accretion of several microcontinents onto the western seaboard of North America. Their evolution began around 180 million years ago when the North Atlantic Ocean began to open and North America began to drift westward. Intense compression caused by the repeated accretion of microcontinents caused thrusting, folding and uplift of the area that now forms the Canadian Rockies. There were at least two episodes of compression (orogenies) during the late Jurassic and early Tertiary. The spectacular Rocky Mountains that we know today, with deep valleys and steep sided peaks, formed only recently as glacial and fluvial erosion processes dissected the deformed and uplifted rock layers.
1.10 IN GREATER DEPTH – BURGESS SHALE
The Burgess Shale is exposed in Yoho National Park in the Canadian Rocky Mountains and contains some of the world’s most important fossils. The soft-bodied fossils lived around 540 million years ago (Cambrian age) and consist of a remarkable assemblage of arthropods, sponges, mollusks, worms and some of the earliest chordates. This diverse and spectacular assemblage of well preserved fossils has allowed scientists to learn a great deal about the evolution of complex life forms during the ‘Cambrian explosion’.
Short Discussion/Essay
1. How did geology develop as a scientific discipline?
2. From your own perspective, what are the major challenges facing geoscientists today?
3. Describe the evolution of the solar system
4. Explain how the process of differentiation affected the internal structure of the Earth.
5. Explain how the various rock types are related to one another through plate tectonic processes.
Longer Discussion/Essay
1. Describe the role of the geoscientists in modern society.
2. Describe the scientific method
3. How did the early Earth differ to the modern Earth?
4. Why is the lithosphere constantly changing through geologic time?
5. What are some of the important events in the development of the North American continent?
Selected Readings
Most of the material in this chapter is covered in detail in later chapters; appropriate references are given in the summaries of those chapters. The references listed below are appropriate to this chapter specifically.
Garland, G. D., 1995. John Tuzo Wilson: Biographical Memoirs of Fellows of the Royal Society, v. 41, 535-552
Gore, Pamela Jeanne Wheeless, 1997. "Using the World-Wide Web in The Geology Classroom." Journal of Geological Education 45 (3): 246-251.
Gould, Stephen Jay, 1989. Wonderful Life: The Burgess Shale and the Nature of History. W.W. Norton, New York. 347pp.
Kobluk, D.R. 1993. "Enhancing contact with students in high enrollment Geology courses with electronic bulletin boards," Journal of Geological Education 41 (1): 32-34.
LeGrand, H.F., 1994. Drifting Continents and Shifting Theories. Cambridge University Press. 169pp. An interesting view of geology before and after Wegener.
McPhee, J. 1981. Basin and Range. New York: Farrar, Straus and Giroux. McPhee presents a vivid (and accurate) picture of how geologist works and portrays some of the most interesting geologic characteristics of the United States.
Rhodes, F.H.T. and R.O., Stone, eds., 1981. Language of the earth. New York: Pergamon Press (paperback). An anthology of writings bearing on geology. Some of the chapter headings are: "Geology and Poetry"; "Humor in Geology"; "Geology and the Arts"; "Geopolitics." Authors include Mark Twain, Herbert Hoover, Ernest Hemingway and Charles Darwin.
Schrum, S. A., 1991. To Interpret the Earth: Ten Ways To Be Wrong. New York: Cambridge University Press.
Sullivan, M. A., and Y. Dilek, 1997. "Enhancing Scientific Literacy Through the Use of Information Technology in Introductory Geoscience Classes", Journal of Geological Education 45 (4): 308-313.
Winchester, Simon, 2001. The Map that Changed the World: William Smith and the Birth of Modern Geology. Harper Collins, New York. 329pp