tectonics

• location of earthquakes

  • mostly found along plate margins

  - eg: ring of fire - pacific plate
• what are intra-plate earthquakes ?

  Earthquakes occurring within tectonic plates away from boundaries
• locations of volcanoes

  • close to plate boundaries

  • over hotspots

  • distribution is clearer than earthquakes
• lithosphere

  • basically the earths crust

  • approx 100km deep

  • broken into 7 major tectonic plates and lots of minor ones
• asthenosphere

  • top of the mantle where the plates float and move around on
• who proposed the idea of Pangea and tectonic plates ?

  alfred wegner
• alfred wegner

  • german scientist

  • proposed Pangea and tectonic plate movement in 1912
• what is Pangea ? 


  • theory that all the continents were joined together in a large mass called Pangea until about 200 million years ago
• who discovered the Mid Atlantic Ridge ?

  Harry Hess
• Harry Hess 


  • professor of geology

  • 1962 (cold war) he mapped the ocean floor using sonar

  • discovered the ridges in the middle of the atlantic ocean
• structure of the earth

  made up of 4 layers

  1. inner core
  2. outer core
  3. mantle
  4. crust
• inner core

  • hottest part of the earth

  • approx 6000°C

  • mostly consists of iron
• outer core

  • about 4500°C - 6000°C

  • semi molten iron and nickel
• mantle

  • upper part is solid

  • middle part is semi molten
• crust

  • outer layer of the earth

  • oceanic crust is about 6 - 10 km deep and made of basalt

  • continental crust is about 45 - 50 km deep and made of granite
• how much can tectonic plates move per year ?

  about 2 - 15 cm per year
• why do plates move ?

  • heat is produced by decaying radioactive elements

  • heats the lower mantle causing convection currents
• convection currents

  • move in circles in the upper mantle, causing plates to move
• slab pull

  newly formed oceanic crust at mid ocean ridges is denser and thicker,

  when it cools and sinks into the mantle, it takes the plate with it
• subduction

  when two plates move towards each, one slides under the mantle and melts into the subduction zone

  • destructive plate boundaries
• sea floor spreading

  as new crust is formed at the mid atlantic ridge, it pushes the plates apart to make room for the new crust in the middle
• paleomagnetism

  study of the earths changing magnetic field
• how often does the earths polarity change ?

  every 400,000 years or so
• evidencing paleomagnetism

  • as the sea floor spreads, the lava cools and turns into rock,

  • and the minerals in this rock line up with the earths polarity

  • you can find the same patterns of magnetic direction on both sides of the ridge
• boundary when two plates move away from each other

  divergent or constructive
• divergent plate boundaries

  • magma rises to fill the gap when two plates move apart

  • this creates shield volcanoes

  • magma escapes easily so the eruption isn’t as powerful

  • earthquakes will be shallow and low magnitude
• example of divergent boundaries

  • mid atlantic ridge

  • african rift valley
• mid atlantic ridge

  • about 16,000km long

  • created about years ago

  • valley about 80 - 120km wide in the middle of the ridge
• east african rift system

  • rift began about 65 million years ago (tertiary period)

  • 3 plates moving away from each other

  • resulting in a scar across the earth
• how big is the rift system‘s valley

  400km
• the eastern rift passes thorough

  Kenya and Ethiopia
• the western rift passes through

  Uganda and Malawi
• east african rift system

  • as the plates pull apart, faults develop on both sides and the central floor sinks

  • lakes fill up the valley as the earths crust stretches
• what is a fault scarp ?

  when 2 blocks move away from each other, one slips down
• what is a hanging wall ?

  the block that slips down in a fault scarp
• what is a graben ?

  the dropped down area in between the two plates
• what is a horst ?

  raised block of central ground between the two plates
• CASE STUDY: ICELAND on a divergent boundary
• iceland

  • straddles a divergent boundary

  • basaltic magma fills the gap

  • prone to shield volcanoes

  •- iceland is also on a hotspot, causing some composite volcanoes
• shield volcanoes

  • layers of ash and hardens lava

  • viscous lava

  • flatter shape

  • not as effusive