Crustal Plates
• Crustal Plates The Earth's crust is divided into a series of plates:
Eurasian Plate African Plate Antarctic Plate IndianAustralian 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 supercontinent 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 supercontinent 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, landslides 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.