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Earth Movements


THE INTERIOR STRUCTURE OF THE EARTH

The Crust

  • The crust on which we are is approximately 64 km thick under land areas to as little as 12 km thick under sea areas.
  • The upper layer of the crust is made mostly of silicon and aluminium, and is rigid.
  • The lower layer of the crust is made of silicon and magnesium, and is plastic.

The Mantle

  • The mantle is beneath the crust, and is about 3000 km thick.
  • It is made of mostly iron and magnesium.
  • It is plastic like putty, and continually changes shape due to heat and pressure.

The Core

  • The core extends from 3000 km beneath the Earth's surface to the centre of the Earth.
  • The outer layer of the core is liquid molten rock, and is about 2000 km thick.
  • The inner layer of the core is solid rock, and is about 1500 km thick.
  • The core is mostly iron, nickel and cobalt.

CRUSTAL PLATES

  • Crustal Plates - The Earth's crust is divided into a series of plates:
Eurasian Plate
African Plate
Antarctic Plate
Indian-Australian Plate
Pacific Plate
South American Plate
North American Plate
  • Convergent Plate Boundaries

Where plates move towards each other or past each other, great tension results. This tension builds up until the plates slide past or under each other, during an Earthquake.

  • Divergent Plate Boundaries

Where plates move away from each other, molten rock (magma) from deep beneath the crust, oozes out through the space between the plates forming a Volcano.


THEORY OF PLATE TECTONICS

  • Theory of Continental Drift - In 1912, Alfred Wegener noticed the similarities between the coastline shapes and the fossils of continents such as Africa and South America. He suggested that the continents may have once been joined together in a large super-continent, and had since drifted to their present positions.
  • Theory of Plate Tectonics - This is the present well-accepted idea that where the plates move towards each other, earthquakes occur, and where the plates move away from each other, volcanoes occur. For these to occur, there are slow-moving convection currents within the mantle. This is supported by exploration of the Atlantic Ocean floor, and laser measurements showing the distances between continents changing by a few centimetres a year.
  • Pangaea - Pangaea was the name given to the one large super-continent that may have existed up till 200 million years ago before dinosaurs roamed the Earth.
  • Laurasia and Gondwana - About 200 million years ago, Pangaea may have split into two smaller super-continents. The northern hemisphere super-continent with Europe, northern Asia and North America was called Laurasia. The southern hemisphere super-continent with Africa, southern Asia, Australia, Antarctica and South America was called Gondwana. Then, many millions of years later, the continents as we know them, drifted to their present positions.

VOLCANOES

  • Volcanoes are mountains or hills formed around a vent in the Earth's crust through which hot molten rock (magma) is expelled. When the magma reaches the Earth's surface, it is called lava. The lava cools, solidifies and accumulates around the vent, building a cone.
  • 3 Igneous Rocks
  1. Granite - volcanic rock with large crystals formed by slow cooling of magma beneath the Earth's surface
  2. Basalt - volcanic rock with small crystals formed by rapid cooling of lava on the Earth's surface
  3. Pumicestone - volcanic rock filled with air bubbles formed by the ejection and rapid cooling of lava in the air
  • After the Eruption
  1. A crater lake may form when rain fills the cooled volcanic crater.
  2. If the volcano sides erode over time leaving the solid vent, mountains such as the Glasshouse Mountains are formed.
  3. If a volcano has not erupted in a long time but may erupt again, it is said to be dormant (sleeping).
  4. If a volcano has not erupted in a very long time and probably will not erupt again, it is called extinct.
  • Types of Volcanoes
  1. Shield Cone - Wide at the base with gentle slopes, formed by gentle volcanoes
  2. Cinder Cone - Narrow at the base, steep slopes built up by angular interlocking cinders (ash), formed by explosive volcanoes
  3. Composite Cone - Intermediate between the shield and cinder cones, built up from alternating layers of lava and cinder
  4. Dome Mountain - Magma pushes up through sedimentary rock

EARTHQUAKES

  • Earthquakes are shocks caused by moving plates. Shock waves are produced from a stress point deep beneath the Earth's surface and become less intense the further they travel.
  • Focus - the point deep beneath the Earth's surface where an earthquake starts
  • Epicentre - the point directly above the focus on the Earth's surface, where most damage occurs
  • Seismograph - an instrument to detect and record the vibrations of the Earth's crust
  • 3 Types of Vibrations or Shock Waves
  1. P Waves - Primary or push waves that travel deep beneath the Earth's surface at about 5.5 to 13.5 km/s
  2. S Waves - Secondary or shake waves that travel deep beneath the Earth's surface at about 3.7 to 7 km/s
  3. L Waves - Longitudinal waves that travel along the Earth's surface at about 3.2 km/s, causing greatest damage to buildings, land slides and tsunamis (tidal waves)
  • The Richter Scale - This scale was devised in 1935 to measure the magnitude of an earthquake on a scale from 0 (almost unnoticed quake) to 10 (HUGE quake).
  • Each number on the scale has 10 times more intensity than the previous one.
  • The Modified Mercalli Scale - This scale describes an earthquake's damage from 1 to 12.

FOLDING

  • Folding is the buckling of rock layers due to lateral earth forces.
  • Anticlines are the crests or tops of folds.
  • Synclines are the troughs or bottoms of folds.

FAULTING

  • Faulting occurs when intense stretching or pushing forces occur along a break in the Earth's crust.
  • The break is called a fault.

UNCONFORMITY

  • An unconformity is a surface that separates two layers or strata, and represents an interval of time in which deposition stopped, erosion removed some sediments and rock, and then deposition resumed.