Geology Unit 1 , Mod 1- Earth Structure

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Geology Unit 1 , Mod 1- Earth Structure by Mind Map: Geology Unit 1 , Mod 1- Earth Structure

1. 1. Planetary Geology

1.1. The order of the Planets

1.1.1. My Very Easy Method Just Speed Up Names.

1.2. Origins of The Solar System

1.2.1. The Solar System formed 4500 Ma ago, when a giant molecular gas cloud of gas and dust collapsed (THE NEBULA HYPOTHESIS) - possibly when it was hit by a shock wave from a nearby supernova. Material eventually was drawn together into a rotating disc triggering nuclear reactions resulting in the formation of the sun, and other materials within the disc stuck together forming planets by the process called ACCRETION.

1.2.2. Definitions The solar System-the sun, planets , their moons, comets and asteroids. Planet-sizable object orbiting a star. Moon-natural satellite orbiting a planet. Asteroids-rocky objects which failed to form a planet. Meteorites- fragments of rock which fall to Earth. Comet- composed of ice and dust. The outer layer melts into water vapor as it nears the sun. The Terrestrial planets (inner)-Mercury, Venus Earth and Mars. The Gas Giants (outer)-Jupiter, Saturn, Uranus, Saturn. Asteroid Belt-region of space between the orbits of Mars and Jupiter, where most of the asteroids in the solar system are formed.

1.3. Space Exploration

1.3.1. Earth's Moon (Lunar) 4400 Ma-moon rocks collected by Apollo Solid Crust mantle and core The Maria-dark, composed of basalt, lava flows generated by impacts of meteorites The Highlands-light coloured areas, plagioclase-rich rock anorthosite

1.3.2. Mars Marines 3-9, space probes took photos identifying volcanoes including Olympus Mons, the largest volcano in the solar system(1960s) Mars Reconnaissance Orbiter, took photos of objects just 10 cm acros (2005) Mars science laboratory, Curiosity discovers geological evidence of habitable land and rounded pebbles provide evidence that there was once flowing water on the surface

1.3.3. Venus Size, composition and distance from Sun similar to Earth Space craft have landed and mapped the surface using radar No oceans and covered by thick clouds that trap surface heat -green house effect

2. 2. Meteorites and Volcanism in The Solar System

2.1. Meteorites

2.1.1. Mostly from the Asteroid Belt, Sometimes from the moon or Mars

2.1.2. Types Iron-6%, alloy of iron and nickel, core of small planet like object Stony-93%, silicate minerals (Olivine, Pyroxene, Plagioclase, Feldspars), core of small planet like object Carbonaceous chondrites-types of stony meteorite, contains water and organic compounds, similar to the sun (with fewer volatiles)

2.1.3. Impact Craters Occur on many planets and moons Distinctive shape-circular depression, with rim of broken rock Ejecta-material such as glass and fragmented rock thrown out of an impact crater during formation The impact causes: material to be ejected and quartz grains to be violently shocked and even melted rock strata to be tilted material at depth to be brecciated (broken up) the ejecta material falls back to the surface but the sequence of rocks is inverted because material closer to the surface is ejected first and falls back to the surface earlier

2.2. Volcanic Activity

2.2.1. Terrestrial planets-heat from the core allows material to push through the rocky mantle

2.2.2. Gas giants-exploration in 1980s reveals volcanism on some moons Io-too small for a heat source, but uses tidal heating from Jupiter's gravity field

2.3. Dating the planets

2.3.1. Radiometric dating-technique used to date materials such as rocks or carbon based substances on the comparison of the abundance of certain radioactive isotopes of carbon with known half lives (usually <1% uncertainty)

3. 3. Inside the Earth - 1

3.1. Crust

3.1.1. Oceanic Composition: Rich in Fe and MG, Basalt (pillow lavas), Dolerite (dykes). Gabro in layers Density: 2.9 g/cm3 Age: Oldest oceanic crust is 200 Ma Thickness: 5-10km average 7km

3.1.2. Continental Composition: Rich in Al and Si, Granite rocks, Igneous, Metamorphic and sedimentary rocks - deformed Density: Average 2.7 g.cm3 Age: Up to 4000 Ma Thickness: Up to 90 km, average 35 km

3.2. Moho Discontinuity

3.2.1. Up to 75 km below the surface

3.2.2. Separates crust from mantle

3.2.3. Markes change in velocity of S and P waves

3.3. Upper Mantle

3.3.1. Solid, able to flow over millions of years- Rheid

3.3.2. Mainly peridotite

3.3.3. 75-250- 650 km, temp > 1300 C - 5% crystals melt giving plasticity

3.3.4. P and S waves slow down - called 'low velocity layer'

3.4. Lower Mantle

3.4.1. 2900 to 700 km depth

3.4.2. Solid because S waves travel through it

3.4.3. P waves increase in velocity as the increasing pressure causes the rocks to become more rigid - less compressible

3.4.4. Silicate material - same as stony meteorites

3.5. Gutenberg Discontinuity

3.5.1. At 2900 km depth

3.5.2. Distinct clear boundary, rocks change

3.5.3. Liquid iron and nickel in outer core and solid stony silicate material in lower mantle

3.5.4. P wave velocity decreases and S waves stop

3.6. Outer Core

3.6.1. 5100 to 2900 km depth

3.6.2. Liquid iron and nickel

3.6.3. S waves do not pass through the outer core- indicates it is liquid

3.6.4. P waves slow down in the outer core - reduced rigidity

3.7. Lehmann Discontinuity

3.7.1. 5100 km depth

3.7.2. Phase boundary separating materials in different states and is not distinct

3.7.3. 100 km thick zone

3.7.4. Rocks change - solid inner core, solid liquid mix in the boundary zone and liquid in the outer core

3.8. Inner Core

3.8.1. 6371 to 5100 km depth

3.8.2. solid material

3.8.3. Pressure of 3.6 million atmospheres

3.8.4. Temperature >5000 C

3.8.5. P waves move through the core generating S waves

3.8.6. Fe and Ni composition similar to meteorites and with a density > 12g/cm3

4. 4. Evidence for the Structure of the Earth

4.1. Mines, Oil wells, and Boreholes

4.1.1. Western deep levels South Africa - 3586 km

4.1.2. deepest oil well, gulf of Mexico - 10.7 km

4.1.3. Siberian borehole - 13 km

4.2. Volcanoes and mid-ocean flows

4.2.1. magma and mantle fragments (Xenoliths)

4.3. Ophiolite sites

4.3.1. Ocean crust thrust above continental plate

4.3.2. Ocean Crust on land

4.4. Geothermal gradient

4.4.1. depends on the conductivity of the rock

4.4.2. 25 (typical) -50 (volcanic areas) C/Km

4.4.3. gradient higher in crust than in mantle

4.4.4. radioactive heat generation concentrated in crust

4.4.5. heat transfer mechanisms different

4.4.6. conduction in lithosphere plates

4.4.7. conduction in asthenosphere and upper mantle

5. 5. Indirect Evidence for the Structure of the Earth

5.1. Seismic (Earthquake) Waves

5.1.1. Density and waves travel faster in rigid incompressible rocks travel slower in more dense rock slows in asthenosphere due to partial melting speed up in mantle due to increased rigidity Gutenberg discontinuity slows P and stops S waves P waves speed up at Lehmann discontinuity S waves propagated at right angles in core

5.1.2. Finding the Moho Both P and S waves skow in asthenosphere, 1-5% partial melting reduces rigidity Both P and S waves speed up through the mantle as the pressure increases and the rock becomes more incompressible

5.1.3. Where P and S waves cannot passs - the shadow Zone seismic waves are refracted at the Gutenberg Discontinuity Beyond the shadow zone, S waves are not recorded and P waves arrive late as they travel more slowly through it.

5.1.4. Iron meteorites (density: 7.0 to 8.0 g/cm3, composition: iron and nickel with some sulfur and silicon materials) are similar to Earths core

5.1.5. Less dense stony meteorites (Density: 3.0 - 3.7 g/cm3, composition: silicate materials similar to peridotite) are similar to Earth's mantle Chondrites are stony meteorites with small globules of olivine and a little carbon; they probably represent an overall composition of the Earth

6. 6. Magnetic Earth

6.1. Definitions

6.1.1. Remnant magnetism - the magnetism shown by rocks due to the alignment of their magnetic minerals according to the Earth's magnetic field at the time of formation

6.1.2. A magnetic anomaly - a value for the Earth's magnetic field that is different from the expected value

6.1.3. Magnetic inclination - the angle of dip of the line of a magnetic filed. It is the angle of with the horizontal made by a compass needle

6.1.4. Palaeomagnetism - ancient magnetism stored in rocks

6.2. Variation of magnetic inclination with latitude

6.2.1. The intensity of the magnetic field is greatest near the poles, and weaker near the equator

6.2.2. A map of the intensity contours is called an isodynamic chart A min intensity occurs over South America A max intensity occurs over Northern Canada, Siberia and Australia

6.3. How some rocks retain magnetism permanently

6.3.1. Igneous rocks containing iron minerals such as magnetite are subject to Earth's magnetic field when they cool

6.3.2. The iron minerals align parallel to the magnetic field

6.4. Magnetic reversals and their casues

6.4.1. Magnetic reversals are when he north and south poles of the Earth's magnetic field reverse due to changes in the convection currents in the liquid metal outer core

6.5. How magnetic reversals can provide evidence for sea floor spreading

6.5.1. magnetic reversals produce stripes of alternating normal and reversed polarity in rocks of the ocean floor

6.5.2. these stripes are parallel and symmetric to the mid ocean ridge suggesting that new rocks are formed at the ridge and carried laterally away as new rocks form

6.6. The probable origin of the Earth's magnetic field

6.6.1. The Earth's magnetic field acts as a self exciting dynamo

6.6.2. Probably a magnetic flare from the sun initiated the field and is maintained by the iron in the core and the differential movement between the liquid outer core and the solid inner core

6.6.3. Convection currents caused by heat flow and chandler's wobble

6.7. Polar wandering

6.7.1. A polar wandering curve can be drawn with records from rocks pf different ages

6.7.2. this can be compared to a curve for a different continent

6.7.3. Apparent polar wandering measured in Europe and North America was different. Rocks of the same age in North America and Europe suggested that the North pole was in two locations at once.

6.7.4. The Earth's geographical and magnetic north pole are close so it is likely that the magnetic pole has always been near the north pole.

6.7.5. Therefore the continents must have moved on the Surface of the Earth

7. 3. Inside the Earth - 2

7.1. Diagram. 1. continental crust, 2. oceanic crust, 3. upper mantle, 4. lower mantle, 5. outer core, 6. inner core, A: Mohorovičić discontinuity, B: Gutenberg Discontinuity, C: Lehmann discontinuity.

7.2. Asthenosphere

7.2.1. Section of the upper mantle that is more plastic but still solid

7.3. Lithosphere

7.3.1. top of the mantle with crust- always rigid and brittle

7.4. Cross section through the upper mantle and crust. (O : g/cm3)

7.5. Definitions

7.5.1. Rheid- solid material that flows

7.5.2. Peridotite- an ultramafic igneous rock composed of materials olivine and pyroxene with a course crystal size

7.5.3. Olivine- dense, ferromagnesian silicate mineral

7.5.4. Partial melting- where a proportion of the minerals will have a lower melting point, allowing them to melt while the rest remains solid