Lecture Twenty - Second review lecture

Question 1

What are the layers of the Earth in a physical sense and a behavioural sense?

Answer 1

Curst - Think veneer (8-30km) of solid rocks (granites and basalts). Mantle - Semi-solid, convecting, rich in olivine and basalt. Core - Solid inner core, liquid outer core, mainly Fe. Lithosphere - Crust + uppermost mantle. Asthenosphere - Upper mantle (nearly molten). Mesosphere/lower mantle - rest of the mantle. Outer core - Liquid Fe - 2270km. Inner core - Solid Fe - 1200km.

Question 2

What is the depth of the crust?

Answer 2

Oceanic - 8-10km - Basaltic. Continental - 15-70m - Granitic. Controlled by isostacy = The equilibrium that exists between parts of the earth’s crust, which behaves as if it consists of blocks floating on the underlying mantle, rising if material (such as an ice cap) is removed and sinking if material is deposited.

Question 3

What evidence is there for plate movement?

Answer 3

Plates move, driven by convection currents in the mantle. Evidence: Early (last 100 years) - Jigsaw fit, fossils, rock types ad climates. Recent (last 50 years) - Polar wander curves, magnetic seafloor stripes.

Question 4

Explain how plate movement generates Earth quakes.

Answer 4

P (primary) and S (secondary) waves. Focus and epicentre (focus is where the Earth quake originates in the Earth, focus is where it is expressed on the surface. Earth quakes will only occur in brittle rock, rocks too far into the mantle are too ductule and will not cause an Earth quake.

Question 5

What are the three main types of plate boundaries?

Answer 5

Divergent (rifts and ridges). Convergent (subduction zones and collisions). Strike slip (transform).

Question 6

Where does plate motion begin and end?

Answer 6

Motion begins rifts: Mid ocean rifts, African rift valley. Basaltic crust generated - records magnetic direction on cooling due to the present of magnetic crystals (magnetite). Shallow focus Earth quakes. Motion ends at collisions: Subduction zones, trenches and mountains.

Question 7

What are ocean-ocean collision zones?

Answer 7

Form trenches, island arc volcanics (andesite-dacite), shallow to deep Earthquakes. (Granite emplacements, metamorphism) Erosion and sedimentation. The oldest crust will subduct under the newer crust, because the older will be cooler and more dense.

Question 8

What are continent-ocean collision zones?

Answer 8

Form trenches, mountain building, volcanics (dacite-rhyolite), shallow to deep Earthquakes. Granite emplacements, regional and contact metamorphism, erosion and sedimentation. What is the boundary between oceanic and continental crust called when there is no plate boundary? - Passive margin. Oceanic crust will always subduct under continental crust.

Question 9

What are continent-continent collision zones?

Answer 9

Cause mountain building, shallow to deep Earthquakes. Regional metamorphism, isostasy, erosion and sedimentation. Neither plates will subduct as are both too buoyant. No volcanoes formed at these plate boundaries, as there are no melts forming without subduction.

Question 10

What is a mineral?

Answer 10

A mineral is a naturally occurring homogenous solid with a definite (but generally not fixed) chemical composition and an ordered atomic arrangement. It is usually formed by inorganic processes. Rocks are made out of minerals. Mineral stabilities and compositions are dependant on pressure, temperature and bulk composition.

Question 11

What are the major mineral groups?

Answer 11

Native elements - Gold. Oxides - Hematite. Sulfides - Pyrite. Sulphates - Gypsum. Carbonates - Calcite. Halides - Halite. Phosphates - Apatite. Silicates - most important in the crust.

Question 12

What are the major silicate groups?

Answer 12

Island silicates = olivine: High temperature, thus crystallises first. Weather and erode first. Ring/chain silicates: Pyroxene and amphibole. Phyllosilicates = sheet: Biotite and muscovite. Tectosilicates = framework: Quartz and feldspar - high temperature (NaCa-spar). Low temperature (K-spar and Qz). Weather and erode last. All these separated by how tetrahedral are bonded together.

Question 13

What are the three main types of rocks?

Answer 13

Igneous - Formed from molten rock. Sedimentary - Formed from recycled rock. Metamorphic- Changed rocks.

Question 14

Describe the characteristics of igneous rocks.

Answer 14

Rocks that solidify from a molten magma. Source of magma comes from: Subduction related melting of hydrated crust (this is the most common). Breaks in the crust allow mantle-sourced magma to escape - spreading ridges. Hotspot activity. They types of igneous rock tell the magma source, cooling history and tectonic activity.

Question 15

What igneous rocks are formed from the different magma sources?

Answer 15

Felsic (quartz rich) igneous rocks (rhyolites, granites, granodiorites) are from continental sources. Mafic (no-quartz) igneous rocks (basalts, gabbros) are from oceanic crust sources and hot sports. Intermediates rock (andersites and dolerites) are from early stages of island formation or from early stages of rifting.

Question 16

What does the cooling history indicate about a rock?

Answer 16

Course grained rocks cool slowly within the curst - igneous intrusions, plutons/batholiths. Fine grained rocks cool rapidly at the surface - lavas.

Question 17

What are the characteristics of volcanoes?

Answer 17

Shape of volcano is controlled by the quartz content. On continents: Quartz rich to quarts poor volcanics from mis of sediments (quartz rich) dragged flow with pate and plate components (basaltic) melting at depth. Viscous Lavas. Explosive. On oceans: Island are building, eventually leafs to continents (cratons). - Mid range lavas. - Medium hazard. At rifts - Fire fountains. - Basaltic lavas, low quartz. - Pillow lavas, smokers. At hotspots - Fire fountains. - Basaltic, rich in olivine, Hawaiian Chain, shield volcanos.

Question 18

What are clastic (epiclastic) sediments?

Answer 18

Eroded by the wind, water, chemicals and gravity. Transported by wind, water, ice and gravity. Deposited by a drop in energy. Rocks made (diagenesis) by compaction and/or cementation. Rounding = time of transport. Sorting - time and energy of transport.

Question 19

What are the different types of sedimentary rocks?

Answer 19

Chemical sediments are: The result of precipitation of dissolved chemicals in water. Limestone, gypsum and salt. Organic/biochemical sediments are: Formed by compaction of organic-rich sands and shales. Coal, oil and gas. Pyroclastic sediments are; Clastic sediments from volcanic sources. Residual sediments are: What is left behind when water has removed everything else. Bauxite and laterite.

Question 20

What is metamorphism?

Answer 20

Changes in rocks resulting from high temperature and/or pressure. Two main types: Contact and regional. Contact - high temperature and unchanged pressure. Regional - increasing temperature and pressure. New minerals form as old minerals partially melt or change state - result in change in rocks. Rocks become harder, and the types of new minerals tell you how much pressure and temperature they’ve been under. Pressure causes long or flat minerals (biotite, muscovite, amphibole and pyroxine) to align at right angles to the direction of pressure (stress).

Question 21

What is an example of how contact and regional metamorphism affect the same original rock in different ways?

Answer 21

E.g. Mudstones - quartz and mica rich sediments (pelites). Regional (increasing T and P). Mudstone -> Slate -> Phyllite -> Schist -> Gneiss. Contact (increasing T, same P). Mudstone -> Hornfels.

Question 22

What is mountain building?

Answer 22

Igneous intrusions - Granite emplacements (plutons/batholiths). Contact metamorphism around plutons. Regional metamorphism in depths - deformation of sediments. The higher the mountain, the faster it erodes. Rate depends on erosional forces - wind, water, chemical and gravity. Sediments form sedimentary rock. Erosion results in isostatic uplift.

Question 23

What is Bowen’s Reaction Cycle?

Answer 23