Geology 101. Sarah Arnoldi

Transcription

1 Geology 101 Sarah Arnoldi Plate Tectonics Geological Time Volcanoes Minerals Igneous Rocks Sedimentary Rocks Metamorphic Rocks Groundwater Global Climate Change Mineral Resources 1

2 Contents Page Chapter Main Area 1 Plate Tectonics Sub Area Page No s. Earth Structure 4-5 Plate Tectonics Geological Time Geological Time Solar System Minerals Volcanoes Igneous Rocks Sedimentary Rocks Metamorphic Rocks Groundwater Mineral Resources

3 Chapter 1 Plate Tectonics 3

4 Earth Structure Internal structure (how do we know?) Direct samples drill cores go down 12.5 km; rocks from volcanic pipes sample maximum depths of a few hundred km Geophysics analysis of various physical properties of the Earth Study of extraterrestrial materials assuming that the formation and structure of objects in the inner Solar system is similar to that of the Earth Physical properties (magnetism) The Earth's magnetic field is tilted 11 o from the spin axis of the Earth and is superficially similar to the feld of a bar magnet For pure iron (Fe) this coordination is lost at 770 C (called the Curie point). Alternatively an electric current loop can give a field similar to that of the Earth. o The most probable way to produce such a current is to have a large volume of liquid metal (most likely Fe) Strength of magnetic feld is different in different parts of the world Magnetic minerals in mafic rocks, such as basalt, will orientate themselves within this magnetic feld and lock it into position when they cool below their Curie point (580 C for magnetite) o These rocks will then contain a record of the magnetic feld, if undisturbed. Physical properties (gravity) Earth's gravity, denoted by g, refers to the attractive force that the Earth exerts on objects on or near its surface. The apparent strength of Earth's gravity varies with latitude, altitude, and local topography and geology The latitude related variation is determined by non spherical shape of the Earth resulting from the rotation. A number of concepts related to the gravitational feld are based on the idea of isostatic balance o The vertical movement was considered to be the main type of motion possible within the Earth The isostatic concept suggests that the continents effectively float on the underlying material due to their lower density, much in the same way wood or an iceberg floats on water Thickening of continental crust results in isostatic uplift of mountain belts The same concept suggests that once the weight of large, continental glaciers has been removed, the ground surface will rebound isostatically Physical properties (Seismology) Most of what we know about the internal structure of the planet comes from the study of earthquake waves There are three main types of waves: o L wave (surface wave) 4

5 Earth Structure Summary Earth s Layers Continental Crust o Height of around 1000m o Typically 45-70km thick o 29% of crust felsic and less dense o Bulk isostasy o Most elements forming cc migrated from interior during Archean (4-2.5Ga) through differentiation o Subsequent fractionation and crystallisation led to variations in composition from mafic to silica o Average density 2.7g/cm 3 o Average geothermal gradient = 30 o C/km Oceanic Crust o Depth of -4000m o 6-10km thick o 71% of crust mafic and dense o Formed by the partial melting of mantle peridotite (high Mg and Fe) o Average density 3.0g/cm 3 Mantle o Ultramafic (peridotite) o Seismology and volcanism give clues o 0.3 o C/km is average geothermal gradient o Lithosphere Strong solid, slightly lower in density Consists of crust and uppermost mantle o Asthenosphere Composed of predominantly solid, but weak rock Similar composition to lithosphere Hotter and less rigid Material flow (crystal plastic flow, diffusion) = convection o Transition zone Contains a great deal of water Where mineral phase changes occur in olivine Core o Uses meteorites to determine the composition of the inner and outer core o Outer Core Liquid iron alloy o Inner Core Solid iron alloy (used to be molten) Not always existed formed by cooling and crystallising over time Ophiolite Sequence o Believed to be fragments of old oceanic crust Typically consists of 4 layers: Pillow basalts, sheeted dykes, gabbros and mantle peridotites Boundaries Crust/mantle: Moho discontinuity compositional o Dramatic change in seismic velocity 30-50km down related to an increase in rock density Top and bottom of mantle g/cm 3 range in density Mantle/core: Gutenberg discontinuity compositional Inner/outer core: phase liquid to solid 5

6 Physical Properties Magnetism o Magnetic field Chemical convection is responsible Occurs when differences in density are due to chemistry rather than temperature Occurs in the outer core o Magnetic field is tilted 11 o from the spin axis of the Earth Different in different parts of the world o Magnetic minerals in mafic rocks will orientate themselves within this magnetic field Lock into position when they cool before their Curie point (580 C for magnetite) o Dipolar field the magnetic field with the two poles (north and south) Geodynamo o When an iron rich fluid in the core is electrically charged and flowing o Generates the magnetic field Seismology o Tells us about the internal structure of the Earth through earthquake waves Allows us to subdivide Earth into the crust, upper and lower mantle, transition zone and core o There are three main types of waves: L wave (surface wave) P wave (compressional body wave) Can pass through liquids measured on the opposite side of the Earth As they pass through differing densities, the wave paths with refract Changes direction of waves cause P-wave shadows (some areas don t receive the waves) 5-8km/sec velocity S wave (shear body wave) 3-4km/sec velocity o Waves refract (bent) and reflect off (bounce back toward its source) a boundary between two rock types which differ in density o Combined info at stations determine epicentre of earthquake and the depth of reflecting boundary A minimum magnitude of 6 of an earthquake is needed in order to be picked up by stations o Liquid layer in the interior S waves cannot travel through liquid o Seismic tomography is a technique used to view three dimensional changes in composition and density using seismology Water content, temperature and composition will result in a seismic velocity anomaly Gravity o Force that the Earth exerts on objects o Varies with latitude, altitude and local topography Acceleration due to gravity is not the same over the planet Less at the equator than at the poles because the planet bulges out farther at Equator o Concepts isostatic balance Suggest that continents effectively float on the underlying material due to their density Suggest that when large continental glaciers have been removed, the ground surface will rebound naturally 6

7 Chapter 3 Minerals 7

8 Defining a mineral 1. Naturally occurring o True minerals are naturally occurring whereas synthetic gemstones are generated by artificial/ human-made processes and not minerals 2. Inorganic 3. Solid substance 4. Crystalline structure o Atoms are arranged in an orderly, repetitive manner 5. Definite chemical composition that allows for variation Definitions Rock: solid mass of any mineral, or mineral-like matter that occurs naturally o Some rocks can be made entirely of one mineral. Eg limestone calcite Aggregate: minerals are joined in such a way that their individual properties are retained Atoms and bonding Atoms: smallest particles that cannot be chemically split. Contain protons and neutrons in a central nucleus that is surrounded by electrons Protons and electrons have electrical charge Protons (charge +1) Neutrons (charge 0) Electrons (charge -1) Atoms bond under the conditions (temperatures and pressures) that occur on Earth Chemical bond: transfer or sharing of electrons that allows each atom to attain a full valence shell of electrons Ionic bonds: o Forms ions positively and negatively charged atoms and symmetrically distributed o Bond is the attraction of oppositely charged ions to one another o Complete transfer of electrons between atoms Covalent bonds: o A chemical bond formed by the sharing of one or more valence electrons between a pair of atoms o Strongest type of bond Metallic bonds: o Valence electrons are free to move from one atom to another so all atoms share valence electrons. Relatively weak o Accounts for the high electrical conductivity of metals o Found in copper, gold, aluminium and silver Hybrid bonds: o Incomplete transfer of electrons that results in atoms partially transferring and partially sharing electrons 8

9 Mineral Colour/Streak Lustre S.G Cleavage Fracture Hardness Other Group and structure Olivine Green No Streak Vitreous Medium None Conchoidal Mafic Silicate Single/ Independent tetrahedral Amphibole (hornblende) Brown, Greenish Black Grey-White Streak Vitreous to dull Medium 2 At 60 and 120 Irregular 5-6 Mafic Silicate Double chain Pyroxene (augite) Brown, Black White Streak (rare) Vitreous to dull Medium 2 At 90 o Irregular Mafic Silicate Single chain Biotite Mica Brown, Black, Transparent Grey Streak Vitreous to sub metallic Medium 1 At 180 o Irregular 2.5 Mafic Silicate Sheet Quartz White, Grey No Streak Vitreous to waxy Medium None Conchoidal 7 Felsic Hexagonal crystals Silicate Threedimensional framework 9

10 Mineral Colour/ Streak Lustre S.G Cleavage Fracture Hardness Other Group and structure Picture Fluorite Clear light green / white Vitreous Medium 4 good to perfect Octahedral Irregular 4 -Fluorescent under UV -Cubic or octahedral Halide Pyrite Brass-yellow / greenishblack Metallic Heavy Very Heavy N/A Irregular ~6-6.5 Opaque Sulphide Galena Silver-grey / silver-grey Metallic Very Heavy 3 90 Irregular ~2.5 Opaque Sulphide Magnetite Earthy black to reddish brown / black Dull-earthy to submetallic Heavy-Very Heavy None Irregular Octahedral or granular habit, magnetic Oxide Haematite Silvery-grey / reddishbrown Metallic Heavy-Very Heavy None Irregular to subconchoidal Reinform, botryoidal or massive habit Oxide 10

11 Chapter 5 Igneous Rocks 11

12 Rock Name Andesite Photo Texture and form Fine grained Porphyritic Extrusive Intermediate Minerals Sodium and calcium Plagioclase feldspar Amphibole Biotite Formation Volcanic Arcs systems (convergent plate). Forms composite volcanos Basalt Fine grained Aphanitic Extrusive Mafic Calcium rich feld. Pyroxene Olivine Mid ocean ridges (divergent plate boundaries). Form when mantle peridotes partially melt Diorite Coarse grained Phaneritic Intrusive Intermediate Sodium and calcium Plagioclase feldspar Amphibole Biotite Volcanic arcs and mountain building systems (subduction along the edge of a continent-andes Mountain). Form batholiths Dolerite Intrusive Mafic Found as sill or dyke type intrusions Medium grained equivalent of basalt Gabbro Coarse grained Phaneritic Intrusive Mafic Calcium rich feld. Pyroxene Olivine Found on continents in dykes, sills from slow cooling of mafic magma. Most derived from hot spots Granite Obsidian Coarse grained Phaneritic Intrusive Felsic Fine grained Extrusive Glassy Felsic Sodium rich feld. Potassium feld. Quartz Muscovite Formed from partial melting of continental crust: high in silica Rapid cooling after an eruption. Forms compact glass. Does not have minerals as the chemical ions did not have a chance to arrange in crystals Pegmatite Extremely coarse grained Intrusive Quartz Feldspar Formed in the fluid-rich late stages of magma crystallisation Pumice And Tuff Rhyolite Fine grained Extrusive Vesicular Felsic Fine grained Extrusive Aphanitic Felsic Forms when a lot of gas escapes with a volcanic eruption. Lavas are stiff and high in silica, which holds the volcanic gases in bubbles rather than letting it escape Sodium rich feld. Potassium feld. Quartz Muscovite At convergent plate boundaries when there is partial melting of the lower continental crust of the overriding plate 12