Earthquakes make the Earth ring! Rocks really do bend Blame the plates by alendar


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									E    qua kes
 arth fact-ite
                         G E O fAc t S A N d Ac t i v i t i E S f O r t h E c l A S S rO O M

Earthquakes make
the Earth ring!
  Earthquakes are a seriously good name       Even though surface rocks appear hard           The continent of Australia sits well
for these massive events. Not only does       and brittle they do deform elastically. All   inside the Australian Plate and all the
the earth really quake when these events      the energy of the compressive forces they     earthquakes that occur under continental
occur but really big earthquakes make         are under is stored up in the deformed        Australia are intraplate earthquakes.
the whole planet ring like a bell and can     rock minerals. When the rocks finally do      Residents of Darwin often feel plate
be detected on the opposite side of the       slip past each other the energy stored        margin earthquakes that occur because
planet! However, not all earthquakes are      in the deformed minerals is suddenly          the Australian plate is colliding with the
the caused by exactly the same process        released as they return to their initial      Eurasian and Pacific plates in a broad
or have the same intensity. Earthquakes       shape. The energy released in this elastic    zone under Indonesia and Papua New
are usually the result of two pieces of the   rebound can be massive and is the likely      Guinea. Likewise, the Alpine Fault running
Earth’s crust rubbing past each other but     cause of the huge damage generated by         through the South Island of New Zealand
the amount of energy involved can vary        some large earthquakes.                       is the boundary where the Australian and
greatly, resulting in small, medium or                                                      Pacific plates slide past each other and
large earthquakes. Most people wouldn’t       Blame the plates                              is a zone of high earthquake frequency
notice the small earthquakes that happen                                                    and intensity. By contrast, there are many
                                                Nearly 80% of all earthquakes occur
all the time, even in Australia.                                                            submarine earthquakes under a mountain
                                              on plate boundaries, places where one
                                                                                            range on the ocean floor between
                                              tectonic plate is moving past, over,
Rocks really do bend                          under or into another plate. The other
                                                                                            Australia and Antarctica. This rocky ridge
                                                                                            is the line along which the Australian and
   All earthquakes are the result of energy   20% occur away from plate boundaries
                                                                                            Antarctic plates are moving apart. The
being released as blocks of rock slide past   as intraplate earthquakes. These
                                                                                            earthquakes here are rarely very large.
each other. Sometimes the movement            earthquakes occur because the brittle
is horizontal, sometimes vertical and         rocks of the upper crust are broken into        Earthquakes are felt over large areas
sometimes a combination of both vectors.      smaller pieces by faults. The plates are      but the position on the surface above the
If the movements always happened a            constantly being pulled apart from each       point where most energy is released is
little bit at a time all the energy would     other or are crashing into each other,        called the epicentre. The depth to the
be dissipated in small earthquakes and        placing the intraplate regions under          most intense point of energy release can
no-one would even know they existed.          extensional or compressional forces.          vary considerably and this is known as
Unfortunately, rock fractures are not         To accommodate these forces rocks             the focus. Shallow focus earthquakes
nicely lubricated most of the time and        either side of the intraplate faults          can be particularly damaging and can
the two blocks will not necessarily slide     move from time to time and intraplate         cause significant fault scarps to form at
past each other despite being pushed by       earthquakes occur. Most are small but         the surface. These sudden changes in the
massive tectonic forces.                      some, such as the Newcastle earthquake        landscape can disrupt river flows, break
                                              of 1989, are large enough to cause death      pipes, ruin roads and cause massive
                                              and destruction.                              damage to cities. On the sea floor they
                                                                                            can displace large volumes of water and
                                                                                            cause tsunamis.

                Acknowledgement: Greg McNamara, Education and Outreach, Geological Society of Australia
E    qua kes
 arth fact-ite
                         G E O fAc t S A N d Ac t i v i t i E S f O r t h E c l A S S rO O M

Shaky stuff
1. Examine the map below.                                          5. The Indian Plate is moving northwards and has
   The Australian Plate is moving northwards.                         been ever since it broke away from Gondwana
   Is the South Island of New Zealand                                 over 100 million years ago.
   moving northwards too?                                             How does this help explain the world’s
   Explain your answer                                                highest mountains?

2. The Australian Plate is moving northwards and                   6. The Pacific Plate and the Nazca Plate are moving
   colliding with the Eurasian Plate. The edge of the                 away from each other. The boundary between
   Australian Plate is plunging under the Eurasian                    the two plates is a zone of upwelling magma that
   Plate into a subduction zone. In this zone lots of                 continually adds new rock to each plate as it moves.
   earthquakes happen and as the plate moves                          Assuming the plates have each been spreading
   deeper some of it melts to form magma that                         apart at the same rate where will you find the oldest
   erupts as violent volcanoes. These volcanoes                       rocks in the Pacific?
   form the islands of Indonesia.                                     Explain your reasoning.
   What is happening to form the volcanoes of
   north eastern Papua New Guinea?

3. The Pacific Plate is moving
   in a north westerly direction.
   What earthquake prone area on the
   North American Plate has a boundary
   with the Pacific Plate that also has
   a north westerly aspect and how
   could this be used to explain the
   earthquakes of this region?

4. What is the name commonly given
   to the boundary between these two
   plates in this area?

                                             IMAGE SUPPLIED By THE USGS
                Acknowledgement: Greg McNamara, Education and Outreach, Geological Society of Australia

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