Chapter 1 The Science of Historical Geology Introduction The Earth formed about 4.6 billion years ago. Homo sapiens appeared on Earth between about 300,000 and 150,000 years ago. Humans ask questions about their surroundings. How did the Earth form? Why do earthquakes occur? What lies beneath the land and below the ocean floor? Curiosity leads to exploration. Why Study Earth History? The Earth has changed through time. Understanding past geologic events will help us predict future geologic events. Past geologic events include: • • • • • • • • Earthquakes. Volcanic eruptions. Continents flooded by inland seas. Drifting and colliding continents. Glaciers have covered large parts of continents. Meteorite and asteroid impacts. Changes in chemistry of oceans and atmosphere. Changes to life on Earth through time - sometimes slow, sometimes swift and deadly. Geology Geology is the study of the Earth. Two major branches of geology: • Physical Geology - deals with Earth materials and processes • Historical Geology - deals with origin and changes of Earth and life through time and space. What do Geologists Do? • Study the structure of mountain ranges • Attempt to predict geologic hazards like earthquakes and volcanic eruptions. • Identify minerals in meteorites to learn how Earth formed. • Study rivers, floods, glaciers, and underground water. • Look at results of past events and work backward in time to discover causes of those events. • Search for fossil fuels and mineral resources. Scientific Method in Geology Science operates through the use of the Scientific Method. The scientific method is a method for finding answers to questions and solutions to problems. Scientists work like detectives to gather data, to try to figure out what happened. The data may be obtained through observations and/or experiments, which can be repeated and verified by others. Summary of Scientific Method 1. A question is formulated. 2. Observations (collect data) 3. Develop multiple working hypotheses (ideas to explain the observations) 4. Test the hypotheses by experimenting and either accept, reject, or modify the hypothesis. The simplest explanation is best. 5. When a hypothesis has considerable experimental or observational support, it is accepted and others are rejected, and it may become a theory. 6. A theory ultimately may become a scientific law. What is a Theory? A hypothesis that survives repeated challenges, and is supported by a large body of evidence, may be elevated to the status of a theory. A theory is not just an wild idea or a guess. Theories have survived close examination, and can be accepted with confidence. A theory has a very high probability of being correct. Examples of theories include the theory of relativity, plate tectonics theory, evolutionary theory, and atomic theory. Major Themes in Earth History 1. Deep time 2. Plate tectonics 3. Evolution of life Deep Time • Recognition of immensity of geologic time is geology's most important contribution to human knowledge. • The science that deals with determining the ages of rocks is called geochronology. Methods of Dating Rocks • Absolute age - The actual age. Quantifying the age of the rock or mineral in years. • Relative age - Determining which rocks are older and which are younger. Absolute Age The discovery of radioactivity in 1896 gave us the tools to find the absolute age of a rock. Radiometric dating involves analysis of the breakdown of unstable radioactive elements in rocks. Radioactive elements decay by releasing subatomic particles from their nuclei. Through this process, the unstable radioactive element is converted to a stable "daughter" element. Example: Uranium-235 decays to form lead-207. Radioactive Decay Many radioactive elements can be used as geologic clocks. Each radioactive element decays at its own nearly constant rate. The rate of decay can be measured. Once this rate is known, geologists can determine the length of time over which decay has been occurring by measuring the amount of radioactive parent element and the amount of stable daughter elements. Half Life Each radioactive element has its own unique half-life. A half-life is the time it takes for half of the parent radioactive element to decay to a daughter product. Example: Uranium-235 has a half-life of about 704 million years. Uranium-235 decays to form lead-207. Uranium235 has a half-life of about 704 million years. 1. After 704 million years, only half (50%) of the uranium atoms in the mineral remain. (The rest have decayed to lead-207.) 2. After another 704 million years, only half of that amount (or 25%) of the uranium atoms remain. 3. So, a rock with 25% uranium-235 and 75% lead-207 must be 1,408 million years old (or 1.408 billion years old). Using radiometric dating, some rocks found in Canada's Northwest Territories have been dated at 4.04 billion years old. Relative age • Determining which rocks are older and which are younger. “Rock unit A is older than rock unit B". • The geologic time scale was developed through relative dating. • Relative age determinations provide a framework or geologic time scale in which to place events of the geologic past. • Using radiometric dating, actual dates in years have been determined for the geologic time scale. Major Themes in Earth History 1. Deep time 2. Plate tectonics 3. Evolution of life Plate Tectonics The theory of plate tectonics has revolutionized the understanding of geology. Plate tectonics explains many large scale patterns in the Earth's geological record. It is a "great unifying theory" in geology. Plate Tectonics The Earth's surface or lithosphere is divided into plates (about 7 large plates and 20 smaller ones). Plate Tectonics The lithosphere is about 100 km thick and consists of the rigid, brittle crust and uppermost mantle. Rigid lithospheric plates rest (or "float") on the asthenosphere, the easily deformed, or partially molten part of mantle below the lithosphere. The plates are moving, but their rates and directions of movement vary. Plate Movements Plate movement is due to convectional flow (circular movement of the asthenosphere due to hot material rising and cooler material sinking). The plates only move a few millimeters per year, about the rate at which your fingernails grow. Types of plate boundaries: • Divergent - where plates move apart from one another. • Convergent - where plates move toward one another. • Transform - where two plates slide past one another Major Themes in Earth History 1. Deep time 2. Plate tectonics 3. Evolution of life Evolution of Life In biology, evolution is the "great unifying theory" for understanding the history of life. Evolution of Life As a result of evolution, plants and animals living today are different from their ancestors. They differ in appearance, genetic characteristics, body chemistry, and in the way they function. These differences appear to be a response to changes in the environment and competition for food. Fossils record the changes in organisms over time. Natural Selection Charles Darwin and Alfred Wallace were the first scientists to assemble a large body of convincing observational evidence in support of evolution. They proposed a mechanism for evolution which Darwin called natural selection. Natural selection is based on the following observations: • Any given species produces more offspring than can survive to maturity. • Variations exist among the offspring. • Offspring must compete with one another for food and habitat. • Offspring with the most favorable characteristics are more likely to survive to reproduce. • Beneficial traits are passed on to the next generation. Lines of evidence for evolution cited by Darwin • Fossils provide direct evidence for changes in life in rocks of different ages. • Certain organs or structures are present in a variety of species, but they are modified to function differently (homologous structures). • Modern organisms contain vestigial organs that appear to have little or no use. These structures had a useful function in ancestral species. • Animals that are very different, had similarlooking embryos. Other lines of evidence for evolution come from the fields of: • Genetics (DNA molecule) • Biochemistry (Biochemistry of closelyrelated organism is similar, but very different from more distantly related organisms). • Molecular biology (sequences of amino acids in proteins) Organic Evolution These discoveries indicate that plants and animals of each geologic era arose from earlier species by the process we call "organic evolution". Organic evolution refers to changes that have occurred in organisms with the passage of time.