Where did we come from and how did we get here?

Transcription

1 Where did we come from and how did we get here? The Universe formed about 14 billion years ago The Solar System and Earth formed about 4.6 billion years ago We are a planet, revolving around a star that is one of about four hundred billion stars in a galaxy (The Milky Way), that is one of more than 80 billion galaxies in the observable Universe. Our Star, the Sun, like most stars is composed of mostly Hydrogen, with some Helium The outer planets (Jupiter, Saturn, Neptune and Uranus) are gas giants (H and He) The Earth is one of four terrestrial planets (Mercury, Venus, Earth and Mars) in the inner Solar System, and like those planets, is composed largely of a silicate (SiO 2 ) and Iron (Fe) Unlike most of the other terrestrial planets, the Earth is a dynamic planet, with a constantly changing lithosphere (rocks), atmosphere (wind), hydrosphere (water), and biosphere (life). That is why the surface of the Earth is largely free of meteorite impact craters. Much of this dynamism is due to heat-driven convection

2 ~14 GA (GigaAnnum, i.e, Billion Years) But first things first today

3 Typical spiral galaxy. Similar to our Milk Way Galaxy

4 We are not alone. About 80 billion galaxies in the observable universe. About 400 billion stars in the Milky Way galaxy (but that may be a bit larger than average) Many (most?) of those probably have planets. How many of those planets are terrestrial (Earth-like?) How many have life?

5 The Crab Nebula in Lyra Remnants of a supernova

6 The Surface of our Sun ( a very close star)

7 SUN Hydrogen (74%), some helium (24%) Rocky inner planets Silicates with Iron/Nickel cores The giant Gas planets of the outer solar system Hydrogen, Helium, methane, water, ammonia plus small icy planets like Titan

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9 Gaseous Outer Giant

10 terrestrial planets Mars

11 terrestrial planets The surface of Mars close up

12 terrestrial planets The surface of Idaho close up

13 Earth

14 Our moon: Luna

15 Close up of Tycho

16 Earth s Outermost Layers The most dynamic portion of the Earth Atmosphere Thin gaseous envelope surrounding Earth Hydrosphere Water layer dominated by the oceans Biosphere All living things on the planet Lithosphere Rocky outer shell

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19 Heat driven convection 1. Bottom water is warmed 2. It expands an is therefore less dense 3. It rises to the surface and then spreads out 4. Cooler water at the sides descends to fill the void

20 A convective thunderstorm

21 Atoms and Minerals

22 What are we (the Earth) made of? All matter is composed of atoms, which consist of a nucleus with protons and neutrons, and electrons which orbit the nucleus Bonds are formed between the valence electrons of atoms to form molecules Minerals are naturally occurring inorganic solid that has an exact (or clearly defined range) chemical composition with an orderly internal arrangement of atoms generally formed by inorganic processes. The nature of the bonds results in the physical properties of minerals, including crystal form, cleavage, fracture, hardness, density, color, luster, streak, etc. Rocks are formed of minerals The rock-forming minerals include silicates, carbonates, evaporites and secondary minerals such as clays Rocks are formed of minerals Most rocks are silicates and are composed of cations linked by silicate tetrahedra, chains, sheets and solids

23 Matter Atoms The smallest unit of an element that retain its properties Molecules - a small orderly group of atoms that possess specific properties - H 2 O Small nucleus surrounded by a cloud of electrons The nucleus contains protons and neutrons

24 Bonding Atoms are stable when their outmost electron shell is filled Atoms lose, gain or share electrons to achieve a noble gas structure Types or bonds Ionic Covalent Metallic

25 The Nature of Minerals Mineral A naturally occurring inorganic solid that has an exact (or clearly defined range) chemical composition with an orderly internal arrangement of atoms generally formed by inorganic processes.

26 Physical Properties Crystal Form Cleavage and Fracture Hardness Density Color Luster: Metallic vs Non-metallic Streak Taste, magnetism, etc.

27 Rock-Forming Minerals About 20 common minerals make up most rocks Silicates dominate Quartz, Feldspars, Mica, Amphiboles, Pyroxenes Carbonates are common Evaporite minerals Secondary minerals formed during weathering

28 Silicate Minerals Silica tetrahedron may polymerize to form a variety of geometric structures, alone or in combination with other cations Isolated tetrahedron Single chains Double chains 2-D sheet 3-D frameworks

29 Silica Tetrahedron

30 Silicate Structures Isolated Single chain Double chain Sheet Solid

31 Nonsilicate Minerals Carbonates (biologic) Calcite - Ca CO 3 Dolomite - CaMg(CO 3 ) 2 Evaporite Minerals (seawater evaporation) Gypsum - CaSO 4-2H 2 O Halite NaCl Clays and Oxides (rust and weathering) Hematite Bentonite, Kaolinite

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33 Rocks

34 Imagine the first rock and the cycles that it has been through.

35 Igneous Rocks

36 Igneous Rocks Form from Magma (hot, liquid rock) Cool and solidify underground (plutonic) or as lavas above ground (volcanic) Most properties are controlled by silica (SiO 2 ) content: classification, melting point, minerals, appearance, etc. Viscosity of magma is controlled by temperature, silica content, and to a lesser extent, water. Silica-rich magmas are more likely to erupt explosively than are mafic magmas, which are runny Texture (size and shape of xtals) is controlled by the rate cooling history of the rock.

37 Igneous Rocks (cont) Faster cooling results in finer-grained crystals Common textures include aphanitic (fine-grained), phaneritic (coarse-grained), porphyritic (big xtals in a fine-grained matrix), pyroclastic (explosive) and glassy The kind of volcanism depends upon the viscosity of magma Plutonic bodies include plutons, batholiths, sills, dikes etc. Magmas originate in the upper Mantle Magmas differentiate (change composition) through mixing, melting of country rock, and partial melting The Bowen s Reaction series describes the order in which silicate minerals solidify in a magma

38 Mafic (Fe,Mg rich) Magmas Silica content of ~ 50% High concentrations of Fe, Mg and Ca High temperature of molten magma 1000 o to 1200 o C Major minerals Olivine - Ca-rich Plagioclase Pyroxene

39 Felsic (Si,Al-rich) Magma Silica content of 65-77% High concentrations of Al, Na and K Lower temperature magmas Less than 850 o C Major minerals Feldspars - Micas Quartz

40 Magma Viscosity Controlled by silica temperature As magma cools, silica tetrahedron form links Similar to polymers - e.g., nylon Increasing linkages Higher silica & lower temp Linkages increase viscosity Note: this is just like oils, fats and other organic compounds used in the household

41 Igneous Textures Texture - the size, shape and relationship of mineral crystals in the rock Reflects cooling history of the magma or lava Slow cooling rate Fast cooling rate Very fast cooling rate >> Big crystals >> Small crystals >> glass

42 Glassy texture in obsidian

43 Aphanitic Texture Fine grained texture Few crystals visible in hand specimen Relatively rapid rate of cooling

44 Aphanitic texture in rhyolite

45 Phaneritic Texture Coarse grained texture Relatively slow rate of cooling Equigranular, interlocking crystals Slow cooling = crystallization at depth Pegmatites - very coarse grained texture

46 Phaneritic texture in granite

47 Porphyritic Texture Well formed crystals (phenocrysts) Fine grained matrix (groundmass) Complex cooling history Initial stage of slow cooling Large, well formed crystals form Later stage of rapid cooling Remaining magma crystallizes more rapidly

48 Porphyritic igneous rock: Big xtals in a fine grain matrix

49 Pyroclastic Texture Produced by explosive volcanic eruptions May appear porphyritic with visible crystals Crystals show breakage or distortion Matrix may be dominated by glassy fragments Fragments also show distortion Hot fragments may weld together

50 Concept Art, p. 105

51 Fine grained Coarse grained

52 Classification of common igneous rocks

53 Volcanic Eruptions Basaltic eruptions are runny Low Silica + High T = Low Viscosity Produce Lava Flows - Pahoehoe or Aa Flood basalts Shield Volcanoes Pillow lavas

54 Fig. 4-1, p. 102

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56 Flood basalts with several thick and thin layers. Each layer represents a separate eruption.

57 Fig. 5-12d, p. 145

58 Intermediate & Silicic Eruptions Higher Silica + Lower T = Higher Viscosity Composite or Stratovolcanos Lava Domes Ash Flow Calderas

59 Concept Art, p. 155

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61 Mt Fuji: Stratovolcano

62 Caldera Explosions: Super volcanoes

63 Fig. 5-9b, p. 142

64 Fig. 5-9c, p. 142

65 Fig. 5-9d, p. 142

66 Fig. 5-9e, p. 142

67 Basalt River Gravels Rhyolite Basalt

68 Fig. 5-21c, p. 157

69 Concept Art, p. 104

70 Plutonic Rocks Less dense magmas rise through the crust Intrusions form as magma solidifies beneath the surface

71 Figure Types of magmatic intrusions

72 Half Dome; part of the Sierra Nevada batholith

73 Sill; parallels layers in the country rock

74 Dike; cuts across layers in the country rock

75 Origin of Magmas Solid rock is at equilibrium with its surrounding Changes in the surroundings may cause solid rock magma Raising T Lowering P Changing composition

76 Magma Differentiation Magmas, and the resulting igneous rocks, show a wide range of compositions Source Rock variations cause major and minor variations in the magma Magma Mixing Assimilation

77 Bowen s Reaction Series

78 Metamorphic Rocks Rocks can be metamorphosed (changed) into other rocks when subjected to high temperatures and pressures. The presence of fluids increases the rate of metamorphism Metamorphic changes occur in the solid state The three kinds of metamorphism are Regional, Contact and Hydrothermal Regional metamorphism involves large scale pressures and temperatures caused by collision of plates in subduction zones or continental collisions Contact metamorphism involves baking of adjacent rocks by hot magma intrusions Hydrothermal alteration involves alteration of minerals through percolation of hot, mineral-rich fluids through the rock The Parent rock is an important control on the type of metamorphic rock formed Index minerals form at specific temperatures and pressures and thus record the T and P experienced by the rock Metamorphic rock textures are either foliated (layered due to directional pressure) or non-foliated

79 Metamorphic Rocks The transformation of rock by temperature and pressure Alters igneous, sedimentary and even other metamorphic rocks

80 Heat What causes metamorphism? Most important agent Heat drives recrystallization - creates new, stable minerals Pressure (stress) Increases with depth Pressure can be applied equally in all directions or differentially, i.e. directed Fluids The flow of hot mineral-rich water through the rock can have a big impact on metamorphism Referred to as hydrothermal alteration and creates specific easily identified minerals

81 Main factor affecting metamorphism Parent rock Metamorphic rocks typically have the same chemical composition as the parent rock. They contain different minerals, but the same chemicals; just rearranged. Exception: at sometimes gases like carbon dioxide (CO 2 ) and water (H 2 O) are released Examples: Quartz Sandstone Quartzite Shale Slate Schist Gneiss Granite Granite, though minerals might align

82 Source of Heat

83 Source of Fluids

84 Ocean-Continent convergence

85 Regional Metamorphism: Subduction zones.. High T Low P High P Low T High T High P

86 Why it is called regional Colors represent different levels of Temperature and Pressure as recorded in the minerals. Fig Regional Metamorphic Gradients This regional pattern was caused by the collision of two continents

87 Other minerals behave similarly Metamorphic Index Minerals

88 Index Minerals in metamorphic rocks Each of these minerals is an index of T and P

89 Metamorphic textures Foliation Foliation can form in various ways: Rotation of platy or elongated minerals Recrystallization of minerals in a preferred orientation Changing the shape of equidimensional grains into elongated and aligned shapes

90 Development of foliation due to directed pressure

91 Change in metamorphic grade with depth

92 Progressive metamorphism of a shale Shale

93 Progressive metamorphism of a shale Schist

94 Progressive metamorphism of a shale Gneiss

95 Common metamorphic rocks Nonfoliated rocks Quartzite Formed from a parent rock of quartz-rich sandstone Quartz grains are fused together Forms in intermediate T, P conditions

96 Sample of quartzite Thin section of quartzite

97 Marble (Random fabric = annealing; nonfoliated)