Chapter 1 : The Earth Systems
Chapter 1 : The Earth Systems GEOL 1301 - 001
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This 10 page Class Notes was uploaded by Robin W. on Saturday October 8, 2016. The Class Notes belongs to GEOL 1301 - 001 at University of Texas at Arlington taught by John S Wickham in Fall 2016. Since its upload, it has received 28 views. For similar materials see EARTH SYSTEMS in Science at University of Texas at Arlington.
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Chapter 1 The Earth System Geology 1301-002 Professor John Wickham Purple- important people Blue-vocabulary Green-important dates/time periods • The scientific method o Geology § Greek for “earth” and “knowledge” § Coined by scientific philosophers to describe the study of rock formations and fossils § Geology: the branch of Earth science that studies all aspects of the planet: its history, its composition and internal structure, and its surface features • The goal of geology, and science, is to explain the physical universe • Scientific method: the general procedure for discovering how the universe works through systematic observations and experiments o Scientific research § Makes new discoveries § Confirms old discoveries § Hypothesis: a tentative explanation based on data collected through observations and experiments • Presented to community of scientists for criticism and repeated testing • Supported if it explains new data or predicts outcome of new experiments • Once confirmed, it gradually gains credibility § Theory: a coherent set of hypotheses that explains some aspect of nature • Supported by a substantial body of data • Survives repeated challenges • Obeys physical laws: general principles about how the universe works that can be applied in almost every situation o Ex. Newton’s law of gravity § Some hypotheses and theories have been tested so extensively that all scientists accept them as true • the longer a theory holds up to scientific challenges, the more confidently it is held • theories can never be considered completely proved o no explanation can be considered completely proved § Scientific Model: a precise representation of how a natural process operates or how a natural system behaves • Scientist combine related ideas to test consistency of knowledge and make predictions • A good model makes predictions that agree with observations • A model is often formulated as a computer program that simulates the behavior of a natural system through numerical calculations o Ex. Models used to predict weather § To encourage discussion, scientists share ideas and the data on which they were based • Ethics o Scientists must acknowledge contributors o No falsified data • Geology as a Science o Labs are insufficient for major geologic problems § They can be used for finding the ages and properties of rocks o Geology is considered an outside science § Scientist learn through experiencing o Geology is related to other sciences § Oceanography: the study of the oceans § Meteorology: the study of the atmosphere § Ecology: the abundance and distribution of life § Geophysics, geochemistry, geobiology: subfields of geology that apply the methods of physics, chemistry, and biology to geologic problems o Geology is a planetary science § Uses remote sensing devices to scan entire globe • Ex. Instruments on earth-orbiting spacecraft o Geology involves figuring out the history of the earth § Geologic record: the information preserved in the Earth’s history • Shows us that, for the most part, the processes we see in action on Earth today much in the same way throughout the geologic past o This is known as Principle of uniformitarianism § does not mean that all geologic phenomena proceed at the same gradual pace • some of the most important geological process happen as sudden events o Ex. A meteorite hitting earth § Does not mean that we have to observe an even to know its importance § Stated as a hypothesis by British geologist Charles Lyell • They decipher this record by combining info from many kinds of work through: o Examination of rocks in the field o Careful mapping of positions relative to older and younger rock formations o Determination of the ages of rocks using laboratory instruments • Earth’s shape and surface o The scientific method has roots in geodesy: a very old branch of earth science that studies earth’s shape and surface § The concept that earth is spherical (instead of flat) was developed by Greek and Indian philosophers in the 6 cen. B.C. • This was the basis of Aristotle’s theory of earth in his book Meteorologica (the first Earth science textbook) § Eratosthenes measured earth’s radius in the third century B.C. • 6, 370 km § Precise measurements have shown that earth is not a perfect sphere • b/c of daily rotation, it bulges slightly at the equator and is squashed at the poles • Earth’s smooth curvature is broken by mountains and valleys o The elevation of Earth’s surface varies by nearly 20 km § from its highest point (Mount Everest in the Himalaya at 8850 m above sea level) to its lowest point (Challenger Deep in the Marianas Trench in the Pacific Ocean at 11,030 m below sea level) • Peeling the Onion: The Discovery of a layered earth o Geologist made discoveries about the interior of the Earth by using seismic waves: a ground vibration produced by an earthquake § Greek for earthquake (seismos) § Seismometers allow geologists to locate earthquakes and also take pictures of the earth’s interior • When the first seismometers were installed at the end of the 19 h cen. Geologists learned that Earth’s interior was divided into concentric layers of different composition o Each layer is separated by shape, nearly spherical boundaries (refer to fig. 1.9 in textbook) o Earth’s density § Emil Wiechert • At the end of the 19 cen. he proposed that earth’s interior was layered o This was before seismic data had become available o Wanted to know why the Earth was so dense § What made a planet composed of normal rocks have such a high density? § Was aware of the growing pressure, the deeper you travel within the earth • Found the effect of pressure was too small for the density he calculated § Eratosthenes • measured Earth’s volume in 250 B.C. § Isaac Newton • Measured the gravitational force that pulls objects to Earth’s surface in 1680 § Henry Cavendish • Calculated earth’s average density to be about 5.5 g/cm3 o The Mantle and the Core § Wiechert turned to meteors to give clues about Earth’s interior • He knew meteors were made of alloys: a mixture of two heavy metals o Meteors contained an alloy of iron and nickel and have densities as high as 8 g/cm3 § He knew these two elements were relatively abundant in the solar system -----> proposed a hypothesis in 1896 that iron and nickel dropped to the earth’s center due to the force of gravity • This created a dense core: the dense central part of the earth o This was surrounded by a silicate- rich shell he called the mantle: the region that forms the main bulk of the earth between the crust and the core § German for the word “coat” • From this hypothesis he created an earth model that agreed with Cavendish’s value for Earth’s average density o Could explain for the existence of iron-nickel meteorites o Used seismic waves to test hypothesis § Compressional waves: waves which expand and compress the material they move through as they travel through solid, liquid, or gas § Shear waves: move material from side-to-side and can only go through solids • In 1906, Robert Oldham, a British seismologist, sorted out the paths traveled by the different waves and discovered that shear waves did not travel through the core o Thus, the core must have been liquid § Due to iron melting at lower temps. than silicates • In 1914, Beno Gutenberg confirmed Oldham’s observations o Determined that the depth of the core-mantle boundary was about 2890 km o The Crust § A Croatian scientist detected another boundary called the Moho for short • Separates a crust of lower-density silicates from higher-density silicates • Global feature • Substantially shallower beneath oceans than beneath continents o Rocks in oceanic crust contain more iron, so oceanic crust is denser than continental crust o The Inner Core § B/c the mantle is solid and the outer part of the core is liquid, the core- mantle boundary reflects seismic waves § In 1936 Inge Lehmann, a Danish seismologist, discovered a sharp spherical boundary at a depth of 5150 km • This indicates a central mass with a higher density than the liquid core • Inner core: a solid metallic sphere suspended within a liquid outer core the layer of the earth extending from the core-mantle boundary to the inner core o Chemical composition of Earth’s major layers th § By the mid-20 cen. geologists discovered earth’s major layers • Crust, mantle, outer core, and inner core • Discovered the earth’s mantle is split up into the upper mantle and the lower mantle o Separated by a transition zone where rock density increases by a series of steps • Showed that earth’s outer core can’t be made up of pure-iron nickel alloy, because these metals are denser than the outer core o About 10% of the earth’s outer core is made up of lighter elements § Ex. Oxygen and sulfur • Only 8 out of more than a hundred elements make up 99% of earth’s mass o 90% of earth’s mass consists of iron, oxygen, silicon, and magnesium § iron and oxygen are the most abundant elements § iron is concentrated in the core § oxygen is concentrated is the crust and mantle § the crust contains more silica than the mantle and the core has almost no silica • all of this confirms Wiechert’s hypothesis that the different compositions of earth’s layers are due to gravity o Earth as a system of interacting components § Earth is powered by an internal and external heat engine • A heat engine transforms heat into mechanical motion or work o Earth’s internal heat engine is powered by the energy within earth’s interior and drives the movement of the mantle and the core o Earth’s external heat engine is powered by the sun and drives earth’s climate and weather § Earth system: all parts of our planet and all their interactions • Open system: exchanges energy and mass with its surroundings o It interacts with other systems as well o The climate is controlled by solar energy coming in and heat energy radiating out o The exchange of mass between earth and space is relatively small • We focus on components of the whole system o Hydrosphere: earth’s surface and ground waters o Cryosphere: earth’s ice caps, glaciers, and snowfields o Biosphere: earth’s living organisms • Geosystems: specialized subsystems that produce specific types of activity o Ex. Climate change or mountain building § The climate system • Weather: term to describe temperature, precipitation, cloud coverage, and winds observed at a particular location and time on earth’s surface o Extremely variable o governed primarily by the changes in solar energy input on seasonal and daily cycles o climate: averaging temperatures and other variables over many years of observation § produced by weather cycles § A complete description of climate also includes measures of how variable the weather has been • Ex. Highest and lowest temperatures ever recorded on a given day § Climate system: all the earth system components that determine climate on a global scale and how climate changes with time • Involves behavior of atmosphere as well as interactions with hydrosphere, lithosphere, biosphere and cryosphere § Greenhouse effect • The suns heat is trapped by water vapor, carbon dioxide, and other gases in the earth’s atmosphere o Greenhouse gases, mostly carbon dioxide, play a role in regulating earth’s climate • The behavior of the atmosphere is also regulated by interactions with the lithosphere o Ex. Carbon dioxide coming out from a volcanic eruption § Scientists build models to understand climate systems • Some argue that these models are unreliable because they lack features of a real earth system • A major problem analyzed by the models is anthropogenic (human-generated) emission of carbon dioxide and other greenhouse gases § The Plate tectonic system • Cooling at the surface forms a strong lithosphere: strong outer shell of the earth that comprises of the crust and upper mantle o Broken into about a dozen large plates that move across the earth a few centimeters every year § Each plate is rigid and rides on the asthenosphere o Greek for stone “lithos” o This encases the asthenosphere: weak ductile layer of rock the comprises of the upper mantle § Greek for weak § Moldable • Convection: the general process of hot material rising to the top and cooler material sinking to the bottom o Hot material rises at a boundary, cools as it moves away, and sinks back down by the force of gravity to be heated up again in the mantle § Very slow process o Plate tectonic system: global geosystem that includes the mantle (and its convection) as well as the lithospheric plates § The geodynamo • Produces a magnetic field inside earth’s liquid outer core o When rocks form, they become slightly magnetized by this field § Helps decipher geologic record and understand how things behaved in the past o Earth’s rotational axis goes through the north and south poles § The magnetic pole is about 11 degrees from this rotational axis § At any place on Earth (except near the magnetic poles), a compass needle that is free to swing under the influence of the magnetic field will rotate into a position parallel to the local line of force, approximately in the north-south direction § Scientists theorize that heat from the earth’s core causes convection that generalizes and maintains the magnetic field § Geodynamo: the global geosystem that produces earth’s magnetic field, driven by convection in earth’s outer core • Magnetic reversal occurs at irregular intervals • Models show sporadic reversals as a result of interactions with earth’s core § Interactions among geosystems support life • Habitats are largely controlled by the climate system • The biosphere regulates things like the amount of carbon dioxide, methane and other greenhouse gasses o Also determines the surface temp. of the planet • Plate tectonics produce volcanoes, which resupply the atmosphere and oceans with water and gases from Earth’s deep interior • The atmosphere, cryosphere and hydrosphere create a variety of habitats that enrich the biosphere • The magnetic field reaches outward to space to form a barrier against solar wind o This protects us from harmful radiation from the sun, which would kill many aspects of the biosphere o An overview of geologic time § The origin of earth and its global geosystems • geologists have been able to show that Earth and the other planets of the solar system formed about 4.56 billion years ago through the rapid condensation of a dust cloud that circulated around the young Sun • within 100 million years the moon formed and earth’s core separated from its mantle • the geologic “dark ages” o the next several hundred million years, very little is known about the development of earth as very little survived the bombardment of large meteorites • the oldest rocks are over 4 billion years old o rocks from about 3.8 billion years ago show evidence of erosion, indicating a hydrosphere and climate not too different from the one we have today o about 3.5 billion years ago there is evidence of a magnetic field as strong as the one we have today, showing evidence of a geodynamo o about 2.5 billion years ago, there is evidence of low— density crust large enough to form continents on the earth’s surface § the evolution of life • fossils: traces of organisms preserved in geologic record o fossils from primitive bacteria found in rocks date back to 3.5 billion years ago o oxygen buildup in the atmosphere occurred about 2.7 billion years ago § probably rose to modern levels around 2 million years ago o life consisted of small, single-celled organisms § these didn’t become more complex until between 1 and 2 billion years ago § the first animals appeared around 600 million years ago • evolving in waves § In a period starting 542 million years ago and probably lasting less than 10 million years, eight entirely new branches of the animal kingdom were established, including the ancestors of nearly all animals inhabiting Earth today • During the evolutionary “big bang” animals that were in shells first began to leave them behind § The evolution was punctuated by brief periods of mass extinction when many animals died off at once • The most recent one occurred 65 million years ago when the dinosaurs died off • Causes of others are still being debated • The largest mass extinction took place 251 million years ago when 95% of the animal population died off • Climate change can also cause mass extinction • Sometimes mass extinctions reduce the number of competing species and foster the evolution and growth of other species o Ex. The evolution of mammals which led to early hominoids which eventually led to modern-day humans