Movement of Tectonic Plates

crust- continental and oceanic                                                                                                (mantle-crust boundary is marked by the Moho discontinuity                                          continental- thickness = 35km-70km, density = 2.6-2.2g/cm cubed, mineral comp = silicon, basalt and aluminium                                                                                                              oceanic - thickness = 5km-10km, density = 2.7g/cm cubed, mineral comp = silicon, basalt + Mg

Lithosphere- rigid, lies immediately above asthenosphere, varies in thickness

asthenosphere- semi Molton, thickness = 80-200km and flows slowly

Mantle- makes up 80% of the earth total vol, depth = 2600km, density of 3.3kg/m cubed at Moho and 5.6 kg/m cubed at the core, mineral comp = Mg and iron

outer core- liquid, 2260km thick, made of iron and nickel                                                              inner core- solid, 1220km thick, iron-rich

convection currents-  occur within asthenosphere, caused by vast amounts of heat generated deep in the mantle, as a result, the asthenosphere carries the lithosphere with it

Seafloor spreading

• lateral movement of new oceanic crust away from the mid-Atlantic ridge (constructive plate boundary)
• plates moved from mid-ocean ridges to subduction at ocean trenches

Paleomagnetism 

• traces of change in earth magnetic field in the alignment of magnetic minerals in the sedimentary and igneous rock
• iron particles in lava are aligned with the earth's magnetic field
• every 400,000 to 500,000 years the polarity reverses; this results in a series of magnetic stripes with sea-floor rocks aligned alternately towards north and south poles
• the stripped pattern suggests it's slowly moving away from the boundary

• The fit of shorelines of continents the appearance of same rock type and mountain ranges of the same age on continents ow widely separated 
• continental fit- reconstructions using the latest ocean basin data confirmed the close fit between continents when they're reassembled to form Pangea
• A good example of ‘continental fit’ is shown by the similarities in shape of the coastlines of South America and Africa 
• Coastlines of the Earth’s major continents appear to fit together like a jigsaw. 

• Higher Viscosity magma (acid, high silica content)
• Rhyolitic and andesite lava
• the violent bursting of gas bubbles when magma reaches the surface
• vent and top of cone often shattered
• Material erupted: Gas, Dust, Ash, Lava bombs, Tephra
• long periods with no activity
• steep-sided stratovolcano (made up of layers of ash and lava); caldera

stratovolcano

• complex internal networks of lava flows which form sills and dykes
• vents of stratovolcanoes fill with a mass of solidified magma which acts as a plug  and prevents magma rising freely
• enormous pressure builds up until eventually, it erupts

caldera

• volcanic craters- more than 2km in diameter 
• develop when eruption destroys most of the cone and the underlying magma chamber is largely emptied. The side of the volcano collapses to form a caldera

• lower viscosity magma  (basic lava- low silica content, higher temperature at eruption)
• basaltic lava
• gas bubbles expand freely; limited explosive force
• Material: gas and lava
• frequent eruption

Lava Plateaux

• basic magma erupts from multiple fissures, vast areas covered by magma (flood basalts)
• e.g. Deccan Plateaux which covers more than 500,000km squared

shield volcano

• Gentle sloping sides result from basic lava 
• if successive flows accumulate for long enough, huge volcanos extending horizontally for 10's of km

• Hawaiian chain of the island (formed as Pacific plate slowly moved NW over Hawaiian hotspot, vast amounts of basaltic have accumulated) lies at the center of the Pacific Plate, thousands of km away from a plate boundary. 
• As Pacific plate continues to move away from Hotspot, volcanoes loose source of magma and become extinct
• Hotspot- a fixed area of intense volcanic activity where magma from a rising mantle plume reaches the Earth's surface
• Running through East Africa is a 4000km long rift valley containing several active volcanoes
• over the past 30million years, the crust has been stretched, causing tension within local rocks
• resulting in rifting, with magma forcing itself to the surface and creating a line of active volcanoes
• This is how Mount Kilimanjaro was formed

• A volcano that erupts more than 1000km cubed of material in a single eruption
• Giant calderas
• Yellowstone has a caldera measuring 75km in diameter
• Toba in Indonesia erupted 75,000 years ago
• Most resent = Taupo, North Island, New Zealand 25,000 years ago

Focus- Origin of the earthquake

Epicenter- Point on earth surface located directly above the focus

Shallow-Focus

• 70km below the surface
• They occur in cold, brittle rocks resulting from fracturing of rock due to stress within the crust
• Release low levels of energy, although high-energy shallow quakes are capable of causing severe impacts
• Majority at convergent place boundaries 

• 70-700km below the surface
• with increasing depth pressure and temperature increases to very high levels 
• Minerals change type and volume 

 Benioff zones are dipping, roughly planar zones of increased earthquake activity produced by the interaction of a downgoing oceanic crustal plate with an overriding continental or oceanic plate. They occur at boundaries of crustal plates called subduction zones.