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Chapter 1 An Introduction to Geology The Science of Geology • Geology - the science that pursues an understanding of planet Earth “If there is an interesting place you want to go, there is interesting geology that you can study there” (Cappadocia, Central Turkey). The Science of Geology • Physical geology - examines the materials composing Earth and seeks to understand the many processes that operate beneath and upon its surface • Historical geology - seeks an understanding of the origin of Earth and its development through time Mersin ophiolite, Turkey The Science of Geology 1.3: satellite image of Mt. Vesuvius, Italy. • Geology, people, and the environment • Many important relationships exist between people and the natural environment Problems and issues addressed by geology include • Natural hazards, resources, world population growth, and environmental issues • Some historical views of the Earth • Aristotle, 300 BC; • James Ussher, ca. 1600, ‘Earth was created in 4004 BC;’ • Catastrophism • Earth’s features formed through sudden and violent changes. ‘The Dog of Pompeii’ Dwelling in Goreme, Cappadocia The Science of Geology Hutton, Kelvin, and the great Earth debates. • The beginnings of modern geology “All natural processes that affect the Earth’s crust (erosion, deposition, volcanic eruptions, faulting, glaciation etc.) operate with the same intensity and under the same set of physical constraints now as in the geologic past.” “(as to the age of Earth) we see no vestige of a beginning, no prospect of an end.” These points are incorrect - why? • ca. 1780, James Huton’s Theory of the Earth; • Uniformitarianism: “the processes that operate today have operated in the past.” • a uniformitarian view of Earth requires a vast amount of time…. James Hutton, 1726-1797 1 Hutton, Kelvin, Twain and the great Earth debates. Hutton, Lyell, Kelvin, and the great Earth debates. C. Lyell: ’Principles of Geology’ "The present is the key to the past." (T&L p. 4) Lord Kelvin, the greatest physicist of his day… “Earth cannot be more than 100 million years old [on the basis of a simple convective cooling model].” Mark Twain, satirist and writer of the time: “Scientific research has shed considerable darkness on the subject of the Age of the Earth, and if they continue at their present pace, we’ll soon know nothing about it!” Uniformitarianism today: Earth’s geological history can be interpreted through processes we see operating today. We can also understand Earth’s present by interpreting its past. Sir Charles Lyell, 1797-1875 The geologic time scale Geologic time • Relative dating and the geologic time scale • Relative dating means that dates are placed in their proper sequence or order without knowing their age in years; • Fossils of pre-existing life, such as this fish and amphibian, are important markers of relative time. The principle of faunal succession is useful for dividing Phanerozoic time. Figure 1.8 Geologic time • Geologists are now able to assign fairly accurate dates to events in Earth history (better than + 1% with ANIMAL). Auburn Noble Isotope Mass Analysis Lab ANIMAL Figure 1.8 2 Geologic time The nature of scientific inquiry • The magnitude of geologic time • Involves vast times – millions or billions of years • An appreciation for the magnitude of geologic time is important because many processes are very gradual • Science assumes the natural world is consistent and predictable • A goal of science is to discover patterns in nature and use the knowledge to make predictions • Scientists collect data through observation and measurements Figure 1.8 The nature of scientific inquiry • How or why things happen is explained using a • Hypothesis – a tentative (or untested) explanation • Theory – a well-tested and widely accepted view that the scientific community agrees best explains certain observable facts • Scientific Laws provide brief usually mathematical statements to describe nature. The nature of scientific inquiry… methods of science • The ‘Scientific method’ involves • Gathering facts through observations • Formulation of hypotheses and theories, and laws. • There is no fixed path that scientists follow that leads to scientific knowledge, yet the paths begin with inquiry and are guided by logic Scientific laws are not ‘superior’ to theories. A view of Earth • Earth’s four spheres • Hydrosphere • Atmosphere • Biosphere • Solid Earth Earth as a system Earth System Science • Aims to study Earth as a system composed of numerous interacting parts or subsystems • Employs an interdisciplinary approach to solve global environmental problems Systems have ‘negative’ and ‘positive’ feedback mechanisms (p.15) 3 The hydrologic cycle is one of Earth’s many subsystems Earth as a system • The Earth system is powered by: o Solar energy drives most external processes in the • Atmosphere • Hydrosphere • At Earth’s surface o c.f. figure 1.13 Early evolution of Earth • Origin of planet Earth • Most researchers believe that Earth and the other planets formed at essentially the same time • Nebular hypothesis – Rotating cloud called the solar nebula – Composed of hydrogen and helium – Nebula began to contract about 5 billion years ago Early evolution of Earth 1.15 *We see nebula today, and earth’s interior has the same composition as early solar system materials. *Samples of the Early Solar System • Chondrites - contain spherical aggregates of high-temperature minerals (olivine and pyroxene, in ‘chondrules’); condensed from the solar nebula with some ‘pre-solar’ grains • Fe-Ni Meteorites • Lunar samples • Martian meteorites Chondrites generally give radiometric ages of ~ 4.5 Ga Radioactive decay of isotopes (U234, U235, K40, etc.) drives processes in Earth’s interior o Lithosphere, Asthenosphere, Mesosphere, Core • Origin of planet Earth • Nebular hypothesis – Assumes a flat, disk shape with the protosun (pre-Sun) at the center – Inner planets begin to form from metallic and rocky substances – Larger outer planets began forming from fragments of ices (H2O, CO2 , and others) 1.15 Early evolution of Earth • Formation of Earth’s layered structure • Metals (especially Fe and Ni) sank to the center • Molten rock rose to produce a primitive crust • Chemical segregation established the three basic divisions of Earth’s interior: core, mantle, and crust; • Primitive atmosphere evolved from gases in Earth’s interior 1 mm Chondrule 4 Prior to 2005, scientists felt that the bulk composition of rocks in Earth’s interior was the same; • using spectrometers more precise than previously available, in 2005 scientists discovered important chemical differences between the bulk Earth and the chondrites. Earth’s Structure Main Divisions: Crust; Mantle; Core. Geophysical Divisions: Lithosphere; Asthenosphere; Mesosphere; Outer Core; Inner Core. the simple model that Earth separated from a uniform solar nebula and then changed only by simple ‘segregation’ is incorrect. c.f. 1.16 1 mm Chondrule Earth’s internal structure • Layers defined by composition • Crust • Mantle • Core • Layers defined by physical properties • • • • Lithosphere Asthenosphere Mesosphere Inner and Outer Core How do we know anything about Earth’s interior? “We have never sampled the mantle directly…. “We have been able to examine slivers of the uppermost mantle and overlying oceanic crust … in Cyprus, (Turkey,) Newfoundland and Oman (p. 23).” c.f. 1.16 Oceanic Crust section, Antakya, Turkey (Syrian Antioch) The face of Earth - continents Note the shields… platforms… mountain belts 1.18 5 The face of Earth ocean basins 1.17 James Hutton, ~1780 ‘Theory of the Earth’ Note abyssal plains, spreading ridges, seamounts, and trenches… The rock cycle c.f. 1.23 Rocks and the rock cycle • Basic rock types • Igneous rocks • Cooling and solidification of magma (molten rock) • Examples include granite and basalt Rocks and the rock cycle • Basic rock types • Sedimentary rocks • Accumulate in layers at Earth’s surface • Sediments are derived from weathering of preexisting rocks • Examples include sandstone and limestone Rocks and the rock cycle • Basic rock types • Metamorphic rocks • Formed by “changing” preexisting igneous, sedimentary or other metamorphic rocks • Driving forces are increased heat and pressure • Examples include gneiss and marble Plate Tectonics (Chapter 2) 6