1. INTRODUCTION TO PHYSICAL GEGOLOGY
- Earth is a complex, dynamic planet that has changed continually for the past 4.6 billion years due to interrelated internal and external Earth subsystems.
- System – combination of related parts that interact in an organized fashion
- Earth as a system – interconnected components (or subsystem) that interact and affect each other in many ways.
Atmosphere, hydrosphere, biosphere, lithosphere, mantle, and core are interconnected such that a change in one part affects the others.
II. What is Geology? Study of earth (Greek: geo = earth and logos = study of).
Can be divided into 2 broad areas:
-Physical Geology - study of earth material and processes operating within and on earth’s surface.
-Historical Geology – origin and evolution of the earth through time. Further subdivided into many other specialties (Table 1.2).
III. Geology Affect our Everyday Lives
- Natural disasters (geologic processes) (earthquake, floods, volcanoes, landslides) occur everyday worldwide.
- Most disasters CANNOT be prevented, but the more knowledgeable we are of disasters, the better able we will be to predict or control them such that their impact is lessened (mitigation)
- A basic understanding of geology will allow better informed decisions to be made concerning things such as land use and human impact on the environment.
- Nearly everything around you is made of geological materials and powered by geological resources (most resources are nonrenewable).
- As world population increases, geologists will play an increasingly important role in locating resources and protecting the environment.
IV. Global geologic and environmental issues facing humankind
- Growing population- energy; food; land; resources; water
- Greenhouse effects
- Environmental problems
V. The origin of the Solar System
- Solar system includes: Sun, 9 planets, 61 moons, asteroids, meteorites, comets, dust, and gases (Figure).
- Interstellar material condensed, collapsed, began rotating, flattened to disk – solar nebula (Figure 1.11)
- Most material (90%) was concentrated in the center and became the Sun
- Turbulence caused localized eddies where gases and solid particles condensed and grew into planetismals and then planets.
- Origin of the Earth – accretion of material within solar system accumulated enough material in one area to form the Earth
- Initially cool (solid) and uniform in composition (Mostly O, Si, AL, Fe, Mg) and density (homogeneous) (Figure 1.12).
- Early cool Earth gradually heated by gravitational compression, meteoric impact, and radioactive decay; Earth become hot enough to melt iron and nickel; these high density elements sank to center to form core; lower density minerals flowed upward forming the mantle and crust resulted in our differentiated Earth.
VI. Why is Earth A Dynamic Planet?
- Earth has continuously changed during its 4.6 billion-years existence.
- Earth is differentiated: consists of three concentric layers (Figure 1.13) vary in composition, temperature, and pressure, and physical properties.
- Core: density 10 to 13 gm/cm3: occupies 16% of Earth’s volume: solid inner core and liquid outer portion; mostly iron.
- Mantle: density 3.3 to 5.7 gm/cm3; occupies 83% of Earth’s volume; dense igneous rock.
- Crust: outermost layer (“skin”) of earth; igneous and sedimentary rocks;
- continental crust: 20-90 km thick; density 2.7 gm/cm3
- oceanic crust: 5-10 km thick; density 3.0 gm/cm3
- Outer Earth can also be subdivided based on different physical properties (mechanical behavior):
Lithosphere- crust + upper mantle combined (cool, rigid) (Fig 1.13); “plates” (Fig 1.14)
Asthenosphere- partially melted rock near upper part of mantle; hot flows plastically.
VII. Plate Tectonics Theory
A unifying theory (perspective 1.2) that accounts for many seemingly unrelated geological phenomena (mountain ranges, oceans basins, volcanoes, earthquakes, etc.).
- lithosphere broken into several pieces called plates (Fig. 1.15).
- rigid plates slowly moving relative to one another by sliding on the flowing asthenosphere.
- powered by convection in the mantle (Fig 1.14).
- most large scale geologic phenomena occur along plate boundaries
- Plate boundaries are:
Divergent- plates move apart, magma fills crack, magma solidifies, plate grows. (Fig 1.16)
Convergent- plates collide; heavier plate (oceanic) is forced under lighter plate (continental) (called subduction), plate shrinks (Fig 1.16)
Transform- plates slide sideways past each other (Fig 1.16)
VIII. Rock Cycle
- three major groups of rocks; igneous, sedimentary, metamorphic
- interrelationship between the internal and external processes
- rock cycle help relate the 3 rock groups (Fig 1.17)
- driven by plate tectonics (Fig 1.19)
IX. Geologic Time and Uniformitarianism
-uniformitarianism- Geological processes operating now are the same that have operated in the geologic past, or "The present is the key to the past".
-(rates or intensities of geological processes may have varied through time, but physical and chemical laws of nature have remained the same and cannot be violated).
- Important because:
1. can therefore use present day observations to determine the past history of the earth.
2. requires a very old age for the Earth (most geologic processes very slow, Earth must be very old).
-if accept uniformitarianism, earth must be very old; How old?
-early geologists pieced together a sequential chronology based on fossils (Geologic time scale; fig. 1.20)
-following discovery of radioactivity (~1900), techniques for radioactive dating of minerals discovered.
-found that earth formed approximately 4.6 billion years ago (4600 MY) (fig. 1.20).