6.4 Tectonics

• Mantles in Earth

  • Earth has mantle parts
  • The mantle is heated from the innermost part of the Earth, the inner core
  • The inner core is estimated to be at a temperature beyond 5000 degree celsius
  • The high temperature is due to :
  • thermal energy left from the formation of the Earth
  • Friction inside Earth
  • Type of reactions that happen in the rocks
• Convection current in the mantle

  • The inner part of the mantle get thermal energy from the core
  • The fluid in the mantle will expand when heated and becomes less dense
  • This hotter and less dense fluid will rise towards the crust, cools and sinks again, resulting in convection current
• More about convection current

  • The mantle is a very thick fluid and does not flow easily, so the convection current moves slower
  • As the convection current move across underneath the crust, the tectonic plates that make up the crust are pulled along
  • As the convection current is slow, the movement of tectonic plates also slow, varying between 0.6 & 10 cm per year
• Evidence for tectonic plates

  • The continents on Earth could fit together like a giant jigsaw puzzle
  • Evidence 1: the continental jigsaw appearance
  • Hypothesis: there was once only one large continent that eventually separated
  • The separate parts became some of the tectonic plates
  • The convection current drove their movement
• Evidence for tectonic plates

  • Evidence 2: the fossil record
  • It is the collection of thousands of fossils that provide information about the time before humans came to Earth
  • Are the remains of dead animals and plants
  • Example 1: fossils of an extinct reptile called Mesosaurus, found in parts of Africa & South America
  • It is similar to crocodile, lives in the coast & shallow water
• Evidence for tectonic plates

  • Example 2: fossils of the plant called Glossopteris, found in Antartica, India, Australia, Africa & South America
• Has the magnetic field change?

  • 780 000 years ago, the compass needle pointed South
  • But today, the same compass needle pointed towards North
  • It shows that the Earth’s magnetic poles have swapped positions almost 200 times in the last 100 million years 
• Magnetic crystals

  • Magnetic crystals in molten rocks line up to point North the same way as compass needle
  • When the rock solidifies, they use the crystals to tell the direction of Earth’s magnetic field
  • This is called alignment( the direction that the crystals are pointing)
• Mid-oceanic ridges

  • Here, magma is coming up from mantle and solidify to form new rocks
  • This pushes the continents away from each other
  • The magnetic crystals always have an alignment to north
  • Rocks further away from these ridges contains magenic crystals with the opposite alignment
  • This suggests the rocks are much older
  • So, if the hypothesis is correct, more earthquakes & volcanoes can happen at the tectonic plate boundaries