Download What Causes Geohazards

What Causes Geohazards
Published on LEARNZ (http://www.learnz.org.nz)
Listen: Listen to this page [1]
Listen to this page [2]
Geohazards such as earthquakes and volcanoes would not happen if the Earth's
crust was solid and did not move. The theory of continental drift explains how the
continents have moved over time due to plate tectonics.
We would not have geohazards if the Earth's crust was solid and did not move. The
theory of continental drift explains how the land has moved over time due to plate
tectonics.
Continental drift
The German scientist Alfred Wegener, proposed in 1912 a theory of continental drift
to explain how the Earth's land masses came to be where they are now. The theory
was based on the Earth’s continents [3]once fitting together like a giant jigsaw
puzzle.
Continental drift explains how similar animals and plants could have lived at the
same time on continents that are now widely separated by ocean, and how identical
mountain ranges in different continents were once joined.
250 million years ago the Earth's continents were once joined together in one giant
supercontinent called Pangaea. Slowly, over millions of years Pangaea [4]broke into
Laurasia [5]and Gondwana [6]. These supercontinents have drifted apart forming
the smaller continents of today.
Plate tectonics
Since the 1960s, detailed geological studies of the Earth’s crust have greatly
increased our understanding of how the continents move. The theory of continental
drift has been replaced with the term ‘plate tectonics’.
On the Earth today there are seven large plates and many smaller ones. They are
made of blocks of continental and oceanic lithosphere (crust and upper mantle).
Oceanic crust is usually about 10km thick while continental crust is about 30–50km
thick. With crust and upper mantle combined, the slabs of lithosphere are between
40 and 200km thick.
The plates move in slow motion and are constantly changing shape. It is thought
that convection currents in the mantle of the Earth provide the energy to move the
tectonic plates from a few millimetres to a maximum of about 15cm per year.
Plate boundaries
Page 1 of 8
What Causes Geohazards
Published on LEARNZ (http://www.learnz.org.nz)
At the edge of the tectonic [7]plates one of three processes can occur:
Divergent boundary
Also known as a spreading boundary, where two plates move apart allowing magma
[8], or molten rock, to rise from inside the Earth to fill in the gap. The two plates
move away from each other like two conveyor belts moving in opposite directions.
This can create rift valleys on land or ocean ridges on the seafloor (for example the
Atlantic Ocean).
Convergent boundary
Where two plates are colliding. Different things will happen depending on what type
of plates are colliding:
1. If the plates are continental plates they are of the same density so neither
plate can over-ride the other and the landmasses buckle and fold, creating
mountain ranges.
2. If the plates are both oceanic plates then island arcs or basins can form.
3. If an oceanic plate collides with a continental plate the denser oceanic plate
will be subducted under the continental plate.
Transform boundary
A transform boundary occurs where two plates slide against each other in a shear
movement. But rather than sliding smoothly, the plates build up tension then
release the tension with a burst of movement. This movement is felt as an
earthquake.
Plate Tectonics in New Zealand
Page 2 of 8
What Causes Geohazards
Published on LEARNZ (http://www.learnz.org.nz)
New Zealand is located on the edge of two tectonic plates, the Indo-Australian and
the Pacific plates. This position makes New Zealand geologically active with
frequent earthquakes, geothermal areas and volcanoes. This plate boundary has
shaped New Zealand:
to the east of the North Island the Pacific plate is being forced under the
Indo-Australian plate (convergent boundary - subduction)
in the South Island the two plates push past each other sideways (transform
boundary)
to the south of New Zealand the Indo-Australian plate is being forced under
the Pacific plate (convergent boundary - subduction)
The movement of these two tectonic plates forms the New Zealand landscape that
we know and love.
Watch the GNS Science animation [9] (519k), showing the future shape and
deformation of New Zealand if the movement measured between 1994-1998 were
to continue unchanged.
Plate tectonics
Plate tectonics is a theory which explains how the Earth's surface is broken into
large plates which move. 250 million years ago the Earth’s continents [3] were
joined together in one giant landmass called Pangaea [4]. Slowly, over millions of
years, Pangaea broke in two and drifted apart forming the smaller continents of
today.
Plate tectonics explains how similar animals and plants live in countries which are
now a long way from each other, and how mountain ranges in different continents
Page 3 of 8
What Causes Geohazards
Published on LEARNZ (http://www.learnz.org.nz)
were once joined.
On the Earth today there are seven large plates and many smaller ones. The plates
move in slow motion and are always changing shape. It is thought that heat inside
the Earth creates convection currents and provides the energy to move the plates
from a few millimetres up to about 15cm per year.
Plate boundaries
At the edge of the tectonic [7]plates one of three things can happen:
Types of Plate Boundary. Image: GNS Science.
Spreading boundary (divergent)
A spreading boundary is where two plates move away from each other. The plates
move apart so magma, or molten rock, can rise from inside the Earth to fill in the
gap. This can form rift valleys on land or ocean ridges on the seafloor (for example
the Atlantic Ocean).
Colliding boundary (convergent)
This is where two plates push together. Different things will happen depending on
what type of plates are colliding:
If the plates are continental plates they are the same weight so the plates cannot
over-ride each other and the land buckles and folds, creating mountain ranges.
If the plates are both oceanic plates then island arcs or basins can form.
If an oceanic plate collides with a continental plate the heavier oceanic plate will
Page 4 of 8
What Causes Geohazards
Published on LEARNZ (http://www.learnz.org.nz)
sink (subduct) under the continental plate, this can lead to volcanic activity.
Sliding boundary (transform)
This is when two plates slide against each other, but rather than sliding smoothly,
the plates can get stuck and build up stress. This stress is then released as a burst
of movement and is felt as an earthquake.
Plate tectonics in New Zealand
New Zealand sits on the edge of two tectonic plates, the Indo-Australian and the
Pacific plates. This makes New Zealand geologically active with many earthquakes,
geothermal areas and volcanoes.
The movement of these two tectonic plates shapes the New Zealand landscape that
we know and love.
Watch the GNS Science animation [9] (519k), showing the future shape of New
Zealand if the current movement continues.
Page 5 of 8
What Causes Geohazards
Published on LEARNZ (http://www.learnz.org.nz)
Māori keywords:
rū whenua
puia
whenua
kōwhatu
rohe
shaking of the land,
earthquake
volcano
land
rock
boundary
Audio Maori keywords:
rū whenua - shaking of the land, earthquake [10]
puia - volcano [11]
whenua - land [12]
kōwhatu - rock [13]
rohe - boundary [14]
Complete the 'Pangaea to the Present' activity [15] on continental drift.
Complete the 'Pangaea to the Present' activity [15] on continental drift.
[16]
Convection currents caused by heating in the Earth's mantle explain how
the continents move. Who came up with this theory and what is this theory
called? Image: Public Domain.
Page 6 of 8
What Causes Geohazards
Published on LEARNZ (http://www.learnz.org.nz)
[17]
New Zealand is located on the edge of two tectonic plates; the Indo
Australian Plate and the Pacific Plate. How do you think this affects New
Zealand? Image: United States Geological Survey.
[18]
Two tectonic plates push past each other along the Alpine Fault. What land
feature can be seen alongside the Alpine Fault? Image: GNS Science.
[19]
Page 7 of 8
What Causes Geohazards
Published on LEARNZ (http://www.learnz.org.nz)
The Indo Australian Plate is like a bulldozer pushing at the weaker Pacific
Plate causing it to rise up into the mountain range known as the Southern
Alps. What else does this plate movement cause? Image: LEARNZ.
Source URL: http://www.learnz.org.nz/geohazards163/what-causes-geohazards
Links
[1] http://www.learnz.org.nz/sites/learnz.org.nz/files/bg-what-causes-geohazards_0.
mp3?uuid=593ddb21eb9e9
[2] http://www.learnz.org.nz/sites/learnz.org.nz/files/bg-easy-what-causes-geohazar
ds_0.mp3?uuid=593ddb21eb30e
[3] http://www.learnz.org.nz/geohazards163/glossary#continent
[4] http://www.learnz.org.nz/geohazards163/glossary#pangaea
[5] http://www.learnz.org.nz/geohazards163/glossary#laurasia
[6] http://www.learnz.org.nz/geohazards163/glossary#gondwana
[7] http://www.learnz.org.nz/geohazards163/glossary#tectonic
[8] http://www.learnz.org.nz/geohazards163/glossary#magma
[9] http://www.learnz.org.nz/sites/learnz.org.nz/files/deformation_0.avi
[10] http://www.learnz.org.nz/sites/learnz.org.nz/files/ru-whenua_11.mp3?uuid=593
ddb21ebb68
[11]
http://www.learnz.org.nz/sites/learnz.org.nz/files/puia_5.mp3?uuid=593ddb21ebc83
[12] http://www.learnz.org.nz/sites/learnz.org.nz/files/whenua_12.mp3?uuid=593dd
b21ebdab
[13] http://www.learnz.org.nz/sites/learnz.org.nz/files/kowhatu_3.mp3?uuid=593ddb
21ebec7
[14]
http://www.learnz.org.nz/sites/learnz.org.nz/files/rohe_4.mp3?uuid=593ddb21ebfd7
[15] http://www.learnz.org.nz/sites/learnz.org.nz/files/pangaea-to-the-present.pdf
[16] http://www.learnz.org.nz/sites/learnz.org.nz/files/b-gh163-01-what-causes-geoh
azards.jpg
[17] http://www.learnz.org.nz/sites/learnz.org.nz/files/b-gh163-02-what-causes-geoh
azards.gif
[18] http://www.learnz.org.nz/sites/learnz.org.nz/files/b-gh163-03-what-causes-geoh
azards.jpg
[19] http://www.learnz.org.nz/sites/learnz.org.nz/files/b-gh163-04-what-causes-geoh
azards.jpg
Page 8 of 8