Chapter 2 Internal Energy and Plate Tectonics

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1 Natural Disasters Tenth Edition Chapter 2 Internal Energy and Plate Tectonics Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-1

2 Origin of the Sun and Planets (1 of 3) Solar system began as rotating spherical cloud of gas, ice, dust and debris Gravitational attraction brought particles together into bigger and bigger particles Cloud contracted, sped up and flattened into disk Formation of Sun Greatest accumulation of matter (H and He) at center of disk Temperature at center increased to 1 million degrees centigrade Nuclear fusion of hydrogen (H) and helium (He) began, producing solar radiation Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-2

3 Origin of the Sun and Planets (2 of 3) Formation of planets Rings of concentrated matter formed within disk Particles within rings continued to collide to form planets Inner planets (Mercury, Venus, Earth and Mars) lost much gas and liquid to solar radiation, becoming rocky (terrestrial) Outer planets retained gas and liquid, as giant gas planets Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-3

4 Origin of the Sun and Planets (3 of 3) Impact Origin of the Moon Early impact of Mars-sized body with Earth Impact generated massive cloud of dust (from Earth s crust and mantle) and gas which condensed to form Moon Lightweight gases and liquids lost to space Lesser abundance of iron (from Earth s core) in Moon Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-4

5 Figure 2.1 Origin of the Sun and Planets Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-5

6 Earth History (1 of 2) Earth began as aggregating mass of particles and gases Aggregation took 30 to 100 million years Occurred nearly 4.6 billion years ago Processes of planet formation created huge amounts of heat Impact energy Decay of radioactive elements Gravitational energy Differentiation into layers Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-6

7 Earth History (2 of 2) Differentiation into layers As temperature rose above 1,000 centigrade, iron melted Liquid iron is denser than remaining rock, so sank toward center of Earth to form inner and outer core Release of gravitational energy produced additional heat Remaining rock melted, allowing low-density material to rise Low-density material formed crust, oceans and atmosphere 4.4 billion years ago: large oceans, small continents 3.5 billion years ago: life (photosynthetic bacteria) 2.5 billion years ago: large continent 1.5 billion years ago: plate tectonics Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-7

8 Figure 2.2 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-8

9 Side Note: Mother Earth Analogy with Mother Earth, 46-year-old woman: (1 Mother Earth year = 100 million geologic years) First seven years unaccounted for 42 years old: life appeared on continents 45 years old: flowering plants 8 months ago: dinosaurs died out Last week: human ancestors evolved Yesterday: humans evolved Last hour: discovered agriculture, settled down 1 minute ago: Industrial Revolution Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-9

10 The Layered Earth (1 of 2) Differentiated into layers of increasing density Center of Earth: Iron-rich core 7,000 km in diameter Inner core is solid and 2,450 km in diameter Outer core is liquid and has viscous convection currents, responsible for Earth s magnetic field Surrounding core is Earth s mantle, 2,900 km thick Stony in composition (like chondritic meteorites) 83% of Earth s volume, 67% of Earth s mass Low-density elements have been sweated out of the mantle to form the crust, atmosphere and ocean Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-10

11 The Layered Earth (2 of 2) Layers can be described in terms of Different density (different chemical and mineral compositions) Crust overlies mantle Or different strength Lithosphere overlies asthenosphere Lithosphere is rigid (solid rock) Asthenosphere is fluid-like (plastic rock) Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-11

12 Figure 2.3 The Layered Earth Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-12

13 Figure 2.4 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-13

14 Side Note: Volcanoes and the Origin of the Ocean, Atmosphere and Life (1 of 2) Volcanic gases: Hydrogen (H), oxygen (O), carbon (C), sulfur (S), chlorine (Cl), nitrogen (N) Combine to make: Water (H 2 O), carbon dioxide (CO 2 ), sulfur dioxide (SO 2 ), hydrogen sulfide (H 2 S), carbon monoxide (CO), nitrogen (N 2 ), hydrogen (H 2 ), hydrochloric acid (HCl), methane (CH 4 ), and others Dominant volcanic gas is water vapor more than 90% Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-14

15 Side Note: Volcanoes and the Origin of the Ocean, Atmosphere and Life (2 of 2) Volcanic rocks: Oxygen (O), silicon (Si), aluminum (Al), iron (Fe), calcium (Ca), magnesium (Mg), sodium (Na), potassium (K) 4.5 billion years of volcanism has brought light weight elements to the surface to make up Continents Oceans Atmosphere CHON (carbon, hydrogen, oxygen, nitrogen) elements of life Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-15

16 The Layered Earth: Behavior of Materials (1 of 3) Behavior of Materials Gas, solid and liquid are obvious terms, but should be considered with respect to time Over longer time periods, solids may behave as liquids Glacier: solid ice, yet flows (ultrahigh viscosity liquid) over years Elastic deformation is recoverable object returns to original shape Ductile deformation is permanent stress applied over long time or at high temperatures Brittle deformation is permanent stress applied very quickly to shatter or break object Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-16

17 The Layered Earth: Behavior of Materials (2 of 3) Permanent stress occurs when yield stress is reached Rocks at Earth s surface (low temperature, low pressure): brittle Rocks in asthenosphere: soft plastic Rocks in deeper mantle: stiff plastic Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-17

18 The Layered Earth: Behavior of Materials (3 of 3) Asthenosphere is plastic About 250 km thick Comes to surface at mid-ocean ridges Lies more than 100 km below surface elsewhere Allows Earth to be oblate spheroid (flattened during rotation; like Solar System during formation) Allows continents to float atop the mantle, by principles of isostasy Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-18

19 Figure 2.5 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-19

20 Isostasy (1 of 2) Isostasy: Less dense materials float on top of more dense materials (i.e. iceberg floating in ocean); buoyancy principle Earth is a series of density-stratified layers Core densities up to 16 gm/cm 3 Mantle densities from 5.7 to 3.3 gm/cm 3 Continents (crust) densities around 2.7 gm/cm 3 Oceans densities around 1.03 gm/cm 3 Atmosphere least dense Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-20

21 Isostasy (2 of 2) Examples of isostasy: Impoundment of water in Lake Mead behind Hoover Dam caused area to sink 175 mm over 15 years Scandinavia is currently rising (about 200 m so far) Had been depressed under weight of ice sheets during last Ice Age (since 10,000 years ago) Ground ruptures and earthquakes are present Viking ship buried in the harbor mud of Stockholm was lifted above sea level Another 200 m of uplift is likely Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-21

22 Figure 2.6 Isostatic Rebound Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-22

23 Internal Sources of Energy (1 of 5) Impact energy Tremendous numbers of smaller bodies hit the Earth early after its formation, converting energy of motion to heat Gravitational energy As Earth pulled to smaller and denser mass, gravitational energy was released as heat Heat from both of these early sources is still flowing to the surface today, as heat conducts very slowly through rock Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-23

24 Internal Sources of Energy (2 of 5) Radioactive isotopes Unstable radioactive atoms (isotopes) decay and release heat Early Earth had much larger amount of shortlived radioactive elements and therefore much greater heat production than now Radioactive decay process: Measured by half-life: time for half the atoms of a radioactive element (parent) to disintegrate to decay (daughter) product Half-lives against time is negative exponential curve Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-24

25 Internal Sources of Energy (3 of 5) Sum of internal energy from impacts, gravity and radioactive elements (plus tidal friction energy) is very large Internal temperatures have been declining since early Earth maximum, but still significant enough to cause plate tectonics, earthquakes and volcanic eruptions Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-25

26 Internal Sources of Energy (4 of 5) Age of Earth Oldest Solar System materials are 4.57 billion years old Measured using radioactive elements in Moon rocks and meteorites Oldest Earth rocks (found in northwest Canada) are billion years old Oldest Earth materials (zircon grains from Australian sandstone) are 4.37 billion years old Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-26

27 Internal Sources of Energy (5 of 5) Age of Earth Earth must be younger than 4.57 billion years old materials that formed the planet Seems that Earth has existed as coherent mass since about 4.54 billion years ago Probably took 30 million years (0.03 billion years) for Earth to form Collision that formed the Moon seems to have occurred between and billion years ago Earth must be older than 4.4 billion years old zircons Conclusion: Earth has existed about 4.5 billion years Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-27

28 Figure 2.8 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-28

29 Figure 2.9 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-29

30 In Greater Depth: Radioactive Isotopes (1 of 2) Elements defined by number of positively charged protons Isotopes are different forms of the same element with different numbers of neutrons Radioactive isotopes are unstable and release energy through their decay process to more stable isotopes Knowing the half-life of radioactive isotopes allows us to use their quantities as clocks to date rocks Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-30

31 In Greater Depth: Radioactive Isotopes (2 of 2) Nuclear fission: Parent atom sheds particles to become smaller daughter atom Alpha particle: two protons and two neutrons (helium atom) Beta particle: electron Gamma radiation: lowers energy level of nucleus Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-31

32 Figure 2.10 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-32

33 In Greater Depth: Radioactivity Disasters (1 of 2) Chernobyl disaster of 1986, in Ukraine Explosion released 185 million curies of radioactivity, affecting much of Europe 50 deaths in area, with many more later deaths from cancer Possibly caused by misinterpretation of small earthquake Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-33

34 In Greater Depth: Radioactivity Disasters (2 of 2) Can such a thing occur in nature? Depends on relative amounts of U-238 and U-235: Most uranium ore is U-238, about 0.7% is U-235 Uranium ore used in reactors is enriched to 2-4% U-235 Because U-235 decays more rapidly than U-238, at some point in the past all uranium ore would have had about 2-4% U-235 Sites in West Africa were natural nuclear reactors about 2.1 billion years ago, at about 400 degrees centigrade temperatures Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-34

35 Plate Tectonics (1 of 2) Tectonic cycle: Melted asthenosphere flows upward as magma Cools to form new ocean floor (lithosphere) New oceanic lithosphere (slab) diverges from zone of formation atop asthenosphere (seafloor spreading) When slab of oceanic lithosphere collides with another slab, older, colder, denser slab sub ducts under younger, hotter, less dense slab Sub ducted slab is reabsorbed into the mantle Cycle takes on order of 250 million years Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-35

36 Plate Tectonics (2 of 2) Lithosphere of Earth is broken into plates Plate Tectonics: Study of movement and interaction of plates Zones of plate-edge interactions are responsible for most earthquakes, volcanoes and mountains Divergence zones Plates pull apart during seafloor spreading Transform faults Plates slide past one another Convergence zones Plates collide with one another Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-36

37 Figure 2.12 Plate Tectonics: Tectonic cycle Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-37

38 Figure 2.13 Plate Tectonics Lithosphere of Earth is broken into plates separated by: divergence zones, transform faults, convergence zones Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-38

39 Development of the Plate Tectonics Concept (1 of 2) 1620: Francis Bacon noted parallelism of Atlantic coastlines of Africa and South America Late 1800s: Eduard Suess suggests ancient supercontinent Gondwanaland (South America, Africa, Antarctica, Australia, India and New Zealand) 1915: Alfred Wegener s book supports theory of continental drift all the continents had once been supercontinent Pangaea, and had since drifted apart Theory of continental drift was rejected because mechanism for movement of continents could not, at the time, be visualized Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-39

40 Development of the Plate Tectonics Concept (2 of 2) 20 th century: study of ocean floors provided wealth of new data and breakthroughs in understanding Lithosphere moves laterally Continents are set within oceanic crust and ride along plates Theory of plate tectonics was developed and widely accepted Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-40

41 In Greater Depth: Earth s Magnetic Field (1 of 2) Earth s magnetic field acts like giant bar magnet, with north end near the North Pole and south end near the South Pole Magnetic pole axis is now inclined 11 o from vertical (tilt has varied with time) so that magnetic poles do not coincide with geographic poles (but are always near each other) Inclination of magnetic lines can also be used to determine at what latitude the rock formed Magnetic field is caused by dynamo in liquid outer core: Movements of iron-rich fluid create electric currents that generate magnetic field Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-41

42 In Greater Depth: Earth s Magnetic Field (2 of 2) Problematic details of Earth s magnetic field await resolution: Strength of magnetic field waxes and wanes Magnetic pole moves about geographic pole irregularly, crossing 5 o to 10 o of latitude each century Magnetic polarity reverses Every several thousand to tens of millions of years Orientation of magnetic field switches from north (normal) polarity to south (reverse) polarity Reverses take few thousand years to complete, with complex magnetic field present during switch Changes in magnetic field are preserved in most volcanic and some sedimentary rocks Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-42

43 Figure 2.14 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-43

44 Magnetization of Volcanic Rocks (1 of 3) Magnetic patterns of ocean floor first observed in mid 20 th century very important to theory of plate tectonics Why does the ocean floor have a magnetic pattern? When lava cools to below 550 o C (Curie point), atoms in iron-bearing minerals line up in direction (polarity) of Earth s magnetic field Polarity of Earth s magnetic field can be either to north or to south and depends on time in Earth s history Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-44

45 Magnetization of Volcanic Rocks (2 of 3) Successive lava flows stack up one on top of another, each lava flow recording Earth s polarity at time it formed Each lava flow can also Be dated using radioactive elements in rock to give its age Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-45

46 Magnetization of Volcanic Rocks (3 of 3) Magnetic patterns of ocean floor What does magnetic polarity of lava flows tell us? Plotting the polarity of different lava flows against their ages gives us a record of the Earth s polarity at different times in the past Timing of polarity reversals (north to south; south to north) seems random Reversals probably caused by changes in the flow of iron-rich liquid in the Earth s outer core Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-46

47 Figure 2.15 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-47

48 Figure 2.16 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-48

49 Magnetization Patterns on the Seafloors (1 of 2) Atlantic Ocean floor is striped by parallel bands of magnetized rock with alternating polarities Stripes are parallel to mid-ocean ridges, and pattern of stripes is symmetrical across midocean ridges (pattern on one side of ridge has mirror opposite on other side) Pattern of alternating polarity stripes is same as pattern of length of time between successive reversals of Earth s magnetic field Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-49

50 Magnetization Patterns on the Seafloors (2 of 2) Magma is injected into the ocean ridges; it cools and forms new rock imprinted with the Earth s magnetic field Seafloor is then pulled away from ocean ridge like two large conveyor belts going in opposite directions seafloor spreading Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-50

51 Figure 2.17 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-51

52 Figure 2.18 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-52

53 Other Evidence of Plate Tectonics: Epicenters Earthquake epicenters outline plate boundaries Map of earthquake epicenters around the world shows not a random pattern, but lines of earthquake activity that define edges of tectonic plates Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-53

54 Figure 2.19 Global Earthquakes Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-54

55 Other Evidence of Plate Tectonics: Deep Earthquakes Deep earthquakes Most earthquakes occur at depths less than 25 km Next to deep-ocean trenches, earthquakes occur along inclined planes to depths up to 700 km Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-55

56 Figure 2.20 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-56

57 Other Evidence of Plate Tectonics: Ages From Ocean Basins Oldest rocks on ocean floor are ~200 million years old (<5% Earth s age) Ocean basins are young features continually being formed (at mid ocean ridges) and destroyed (at subduction zones) Hot spots in the mantle (plumes) cause volcanoes on overlying plate, which form in a line as plate moves over hot spot, getting older in direction of plate movement Sediment on the seafloor is very thin at mid ocean ridges (where seafloor is very young) and thicker near trenches (where seafloor is oldest) Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-57

58 Figure 2.22 Hawaiian Islands and Seamount Chain and Their Ages Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-58

59 Other Evidence of Plate Tectonics: Ranges and Trenches Oceanic mountain ranges and deep trenches Ocean bottom is mostly about 3.7 km deep, with two areas of exception: Continuous mountain ranges extend more than 65,000 km along ocean floors Volcanic mountains that form at spreading centers, where plates pull apart and magma rises to fill gaps Narrow trenches extend to depths of more than 11 km Tops of sub ducting plates turn downward to enter mantle Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-59

60 Other Evidence of Plate Tectonics: Seafloor Depth Systematic increases in seafloor depth Ocean floor depths increase systematically with seafloor age, moving away from mid-ocean ridges As oceanic crust gets older, it cools and becomes denser, therefore sinking a little lower into mantle Weight of sediments on plate also cause it to sink a little into mantle Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-60

61 Figure 2.23 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-61

62 Other Evidence of Plate Tectonics: Fit of the Continents If continents on either side of the Atlantic Ocean used to be adjacent, their outlines should match up Outlines of continents at the 1,800 m water depth line match up very well 1,800 m water depth line marks boundary between lower-density continental rocks and higher-density oceanic rocks Continental masses cover 40% of Earth s surface, ocean basins cover other 60% Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-62

63 Other Evidence of Plate Tectonics: Changing Positions of the Continents (1 of 2) 220 million years ago, supercontinent Pangaea covered 40% of Earth (60% was Panthalassa, massive ocean) 180 million years ago: Pangaea had broken up into Laurasia and Gondwanaland 135 million years ago: north Atlantic Ocean was opening; India was moving toward Asia Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-63

64 Other Evidence of Plate Tectonics: Changing Positions of the Continents (2 of 2) 65 million years ago: south Atlantic Ocean was opening; Africa and Europe had collided Present: India has collided with Asia; Eurasia and North America are separate; Australia and Antarctica are far apart Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-64

65 Figure 2.24 Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-65

66 Figure 2.25a Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-66

67 Figure 2.25b Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-67

68 Figure 2.25c Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-68

69 Figure 2.25d Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-69

70 The Grand Unifying Theory (1 of 3) Tectonic cycle: Rising hot rock in mantle melts to liquid magma Buildup of magma causes overlying lithosphere to uplift and fracture; fractured lithosphere is then pulled outward and downward by gravity, aided by convection in mantle Asthenosphere melts and rises to fill fractures, creating new oceanic lithosphere New oceanic lithosphere becomes colder and denser as it gets older and farther from the ridge where it formed Eventually oceanic lithosphere collides with another plate; whichever is colder and denser will be forced underneath and pulled back down into the mantle Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-70

71 The Grand Unifying Theory (2 of 3) Tectonic cycle: When two plates collide, denser (colder, older) plate goes beneath less-dense (warmer, younger) plate in subduction Oceanic plate beneath oceanic plate: Volcanic island arc next to trench (Aleutian Islands of Alaska) Oceanic plate beneath continental plate: Volcanic arc on continent edge next to trench (Cascade Range of Oregon, Washington, British Columbia) Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-71

72 The Grand Unifying Theory (3 of 3) Plate tectonics requires time perspective of millions and billions of years Plate movement may be 1 cm/year 75 cm in human lifetime Uniformitarianism: small events over long lengths of time add up to huge results Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-72

73 Figure 2.26 Plate Tectonic Processes Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-73

74 How We Understand the Earth (1 of 2) Must think in terms of geologic time rather than human time thousands, millions and billions of years In 1788, Hutton introduced concept of geologic time: No vestige of a beginning, no prospect of an end. Everyday changes over millions of years add up to major results Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-74

75 How We Understand the Earth (2 of 2) Uniformitarianism: natural laws are uniform through time and space; the present is the key to the past Contrast to previously believed catastrophism Currently modified actualism: rates of Earth processes can vary Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2-75