Geology: Global tectonics

The order of the planets:

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune,(Pluto)

Mnemonic: My, very, educated, mother, just, served, us, nine, (pizzas)

And where does the Asteroid Belt fit in?

Inbetween Mars and Jupiter.

Gas Giants

• They are the four planets furthest away from the Sun
• They have a gaseous surface
• They are considerably less dense than the terrestrial planets due to their gaseous composition

Terrestrial Planets

• They are the four planets closest to the Sun
• They have a solid surface
• They are very dense due to being made of rocks and metals- They have a dense inner core.

The solar system is believed to have formed around 4500 million years ago.

• The solar system formed when a giant molecular cloud of gas and dust collapsed (the nebula hypothesis) – possibly, when it was hit by a shockwave from a nearby exploding star (a supernova).
• The material eventually formed a rotating disc and as material was drawn to the centre, it triggered nuclear reactions, which resulted in the formation of the sun.
• Other material in the disc of dust particles began to stick together by a process called accretion. This formed progressively larger objects, finally resulting in the formation of planets.

Earth’s moon: The first moon landing were in 1969 (Apollo), and brought back 20kg of rock to be analysed- these rocks were much older than expected. The oldest rocks dating as far back as 4400ma. The moon has a solid crust, mantle and core, and the surface is mad up of:

-The Maria: dark areas composed of basalt lava flows, generated by impacts of meteorites

-The Highlands: Light coloured areas composed of a plagioclase-rich rock anorthosite.

Mars:

So far, the exploration of Mars has occurred in three stages:

-in the 1960s, the space probes (Mariners 3-7) flew by mars, taking as many pictures as possible- they identified volcanic activity, including Olympus mons, the largest volcanic structure in the solar system.

- As technology advanced, space crafts were sent for longer periods of time, managing to picture more of the surface due to higher quality cameras.

-In 2007, a spacecraft landed on the surface to move around and explore- this will tell us more about the geology of the planet, the presence of water, and maybe even the potential for a habitat for life.

Venus:

Venus is similar in size, mass, composition and distance from the sun as Earth. Spacecrafts have landed on Venus, and mapped it using radar – so any data on temperature of pressure are actual measurements. Venus has no oceans, and is covered by thick, rapidly spinning clouds that trap surface heat, creating a scorched greenhouse-like world with temperatures high enough to melt lead. The clouds reflect light, making Venus the brightest planet in our sky.

The Asteroid Belt:

Asteroids are large, rocky objects left over from the formation of the Solar System and lies between Mars and Jupiter. They are thought to be the remains of a planet that failed to form when the rest o the solar system was created. While most asteroids may be the size of pebbles, Ceres, the largest has a diameter of roughly 914km. Collisions between asteroids result in fragments being broken off. These fragments then travel through the solar system, some are captured by Earth’s gravity and fall to Earth’s surface as meteorites

Iron Meteorites

•  Composed of an alloy of iron and nickel, 6% of known meteorites are this type
• Thought to represent the core of a small planet like object, which formed early in the history of the Solar System.

 Stony Meteorites

• Composed of silicate materials including Olivine, Pyroxene and plagioclase feldspar. 93% of known meteorites are this type.

• Thought to represent the mantle of a small, planet like object which formed early in the history of the Solar System.

Carbonaceous Chondrites

• A type of stony meteorites which contains water and organic compounds.
• Similar in composition to the sun, but with fewer volatiles.

There is clear evidence that asteroids have collided with many planets and moons in the solar system as the impact forms distinctive craters.

The impact causes:

• Ejection of material and quartz grains to be violently shocked (shocked quartz) and even melted
• Rock strata to be tilted
• material at depth to be brecciated (broken up)
• Ejected 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 (basically flipped)

Impact craters are easier to spot on other planets or moons due to the fact that the Earth's crust is subjected to weathering , erosion and long-term recycling by plate tectonics, which destroys craters.

It was identified when the flybys of the outer Gas Giants happened in the 1980s, Io was discovered to have been extremely volcanic due to evidence of lava flows covering craters formed early on in it's history, also plumes were seen on camera-indicating volcanic activty.

Rock samples are used to determine the age of the planets through a process called radiometric dating. It uses the decay of radioactive isotopes. To know the age of an object, we need to know two important values:

• the rate at which the isotope decays-the half life- which can be determined in a laboratory
• The amount of radioactive isotope compared to the amount of isotope it decays to.

Oceanic Crust: 5-10 km, although average is 7km

Continental Crust: up to 90km under highest mountains, although average is 35km

The Asthenosphere: Roughly between 80-300km

The Upper Mantle: 35-700km

The Lower Mantle: 700-2900km

The Outer Core: 2900-5100km

The Inner Core: 5100-6371km

Crust is considerably thicker beneath continents than oceans, continental crust averaging at 35km, and Ocenic at 5-10km, it is not as dense (O=2.9g/cm3, C=2.7g/cm3), however it is much, much older( O=roughly200Ma, C= oldest is 4000Ma)

Moho=35km, At the Moho there is a change in composition from solid, to solid with a rheid flow. There is also a sudden increase in seismic velocities.

Gutenberg=2900km, At the Gutenberg there is a change of composition from solid with rheid flow, to liquid. There is a sudden increase in density, a sudden decrease in seismic velocities (P waves are refracted and S waves absorbed.)

Lehmann=5100km, At the Lehmann Discontinuity there  is a phase change solid-liquid, a steady increase in density, a stead increase in P wave velocity and Swaves are regenerated.

Ocanic crust:Basaltic- (pillow lavas), Dolerite Dykes, and Sheeted Gabbro.

Continental crust:Granitic- all three types of rocks(igneous, sedimentary and metamorphic), deformed.

Asthenosphere: Peridotite rock, with 5% partial melt

Mantle: Made of peridotite, and solid silicates (more dense in the lower mantle)

Outer core: Liquid Iron Nickel

Inner core: Solid iron nickel due to high pressure.

It is a rheid, plastic layer with 1-5% partial melting.

In the asthenosphere, rigidity is lost, and therefore P and S waves will slow down, which is why it is given the name 'the low velocity layer'. This is how it is identified.

 The Asthenosphere helps in the process of Plate tectonics, it serves as a medium for flow and movement of the lithosphere- which controls a large proportion of the tectonic processes.

A rigid, brittle layer made of part of the crust and upper mantle, which is divided into plates.

we have access to higher levels of crust by means of mines for coal, metals, ores and diamonds- which means we can collect rock samples from lower depths that have been brought up, which provide us with evidence of the Earth’s structure.

The magma that feeds volcanoes through vents originates in the lower crust /upper mantle, and so carries up samples of rocks from these layers- such as kimberlite pipes which carry xenoliths of peridotite. Once analysed, they provide direct evidence for the structure of the Earth due to the fact they originate form there!

• During the collisions of plates, sections of the oceanic crust may be broken off a descending oceanic plate and thrust onto the edge of the continental plate, instead of being carried down into the mantle.
• They may be exposed by erosion. This section of oceanic crust can be examined on land without the need to drill a borehole through the ocean floor. This reveals that if an ophiolite sequence is returned to it's original, undeformed sequence- the oceanic crusts original thickness, structure and composition can be seen.

P and S waves travel through the layers of the Earth, so are called body waves.

• They travel faster if the rock becomes more rigid and incompressible.
• They travel slower if the rock becomes more dense.
• P and S waves slow down in the asthenosphere due to the fact the partial melt reduces the rigidity.
• Both P and S waves speed up through the mantle as the pressure increases and the rock becomes more incompressible
• P waves suddenely slow down at the Gutenberg discontinuity as they enter the liquid outer core where the rigidity is low. S waves stop completely as they cannot be transmitted in a liquid.
• P waves start to speed up at the Lehmann discontinuity as they enter the solid inner core. S waves are propagated at 90 degrees to the P waves.

• Seismic waves are refracted at the Gutenberg discontinuity , which marks the edges of Earth's core.
• The wave reaching the surface at 102 degrees from the focus is not affected.
• The waves that should have arrived between 103 and 142 degrees are refracted at the discontinuity creating a shadow zone where there are no P and S waves.
• Beyond the shadow zone, from 142 degrees, S waves are not recieved, and P waves arrive late as they travel more slowly through it.

Due to the fact that their properties allow us to recognise depth, and state. S waves are unable to travel in liquid, so the fact they stop at 2900km, and start again at 5100km suggests to us that the Outer core is at this depth, and also that it is liquid in state. Not only this, but the fact they restart after 5100km suggests to us the inner core is in fact solid, compared with liquid. Likely due to the high pressure.

The average density of the Earth is 5.5g/cm3, the density of the rocks making up the continental crust is 2.7g/cm3 and that of the oceanic crust 2.9g/cm3. The density of the core must therefore be high. There is a change of density with depth, with a clear increase at the core boundary where the composition is belived to become iron and nickel.

The Earth shares a common origin with the other planets and the debris left over from the formation of the solar system. Frequently, small fragments of debris fall to Earth as meteorites, although most burn up as they pass through the atmosphere. The meteorites are not the same as our crustal rocks, which have been influenced by weathering, metamorphism and other changes, but they could be similar in composition to the mantle and the core of the Earth.

Of the two main types of meteorite, the more dense iron meteorites are likely to be similar to Earth's core. The less dense, more stony meteorite, is likely to be similar to Earth's mantle.

• A convecting mass of molten iron will generate elctricity, the generation of electricity induces magnetism , which generates more and more electricity.
• The balance between generation and destructionaallows the Earth to show a continuous, if weak, magnetic field. (self-exciting dynamo)
• The outer core temperature is well above the Curie Point, at which materials lose their magnetism.
• So, the Earth's magnetism cannot be permanent and must be constantly generated.

The iron-rich magnetic minerals in lavas align themselves with the Earth's magnetic field, and as they cool through the Curie point, this magnetism is permanently fixed.

They act like frozen compasses, showing the direction to the poles at the time of their formation.

The 'self-exciting dynamo' sometimes runs down as convection in the core changes. The magnetic field gradually fades away over a period of several thousand years. It then increases again, but witht the poles the opposite way around, The evidence for these reversals is found in the rocks as remenant magnetism.

• We can also find the latitude of a volcano at the time that it erupted its lavas by using the magnetic inclination of the frozen compasses.
• Following the lines of the Earth's magnetic filed, a freely suspended compass needle lies exactly vertical at the magnetic poles. This is how the magnetic poles are located .
• At the magnetic equator , the needle is exactly horizontal .
• We can now measure the inclination of a rock of known age to find its  latitude when formed.

Geologists studying palaeomagnetic pole positions haave found evidence suggesting that the magnetic poles have wandered all over the globe. This is called apparent polar wandering.

Evidence of this can be seen in the rocks of the same age in North America and Europe, in two completely different positions, suggesting wandering.