Download The crust is the outermost of several onion

The crust is the outermost of several
onion-like layers that have been
recognised to comprise the
structure of the Earth. It is much
thinner than the deeper layers being
only about 30 to 60 km thick
beneath continents and about 5 to 8
km thick beneath the oceans.
The crust and a portion of the next
layer, the 'mantle', forms a relatively
rigid shell which has been called the
'lithosphere'. This shell is broken into
fragments called tectonic plates, and
the continents are embedded in some
of these plates. Altogether there are six
large tectonic plates and a number of
smaller ones. Slow convection
processes in even deeper layers of the
Earth cause the plates to be always
slowly moving relative to each other. The movement can take
different characteristics at different places - in some places the
edges slide past each other, in others they move apart, and in
yet others they move towards each other with one plate
over-riding the other.
Where the plates are moving apart the edges almost
everywhere lie beneath the ocean and a result is that there
is a giant rift valley which almost circles the world. At
over 55,000 kilometers long, this is the greatest geological
feature on the solid surface of the planet. Along its length,
and also the length of several branch rifts, hot molten rock
from deeper inside the Earth is intermittently oozing up to
the surface. It solidifies when it meets the cold water of
the ocean, and forms new rock on either side of the rift,
called the 'mid-oceanic ridge'.
Where plates move towards each other the surface
expressions of the boundary are more diffuse but often
comprise deep oceanic trenches, and lines or arcs of
volcanoes, and where there is continental crust this tends
to become heaped up into mountain ranges.
New Zealand lies astride the boundary between the Pacific
plate to the east and the Australian-Indian plate to the west.
The constant slow movement that occurs as these plates
over-ride each other leads to earthquakes and elevated
temperatures as rock mass grinds against rock mass and heat
is transported from below.
This has resulted in the creation
of volcanoes in the North Island
and the formation of the
Southern Alps in the South
Island.
The Taupo Volcanic Zone is the
band of volcanoes over 240km
long and no more than 50km
wide, stretching from White
Island in the north to Mount
Ruapehu in the south. Within
this zone some of the most
violent eruptions the world has
seen have taken place. It
contains also more than twenty
intensely active geothermal
systems. Vast bodies of heated
semi-molten rock or 'magma'
believed to be residing at shallow
depth (perhaps only 5 to 10 km) in the Earth's crust here are
responsible for the exceptional size and violence of the many
volcanic eruptions.
The Okataina Volcanic Centre is one of five major active
areas in the Taupo Volcanic Zone and is believed to have
been the site of several enormous eruptions during the last
250,000 years. During each such eruption the surface
collapsed in the vent area after the magma had escaped. This
has resulted in the progressive formation of a large basin or
'caldera' which is roughly rectangular and measures 18km x
25km. Smaller, less violent eruptions over the last 38,000
years have partly filled the caldera with lava flows, lava
domes, pumice and basaltic ash beds, and erosion has added
sediments. Eruptions younger than 20,000 years old have
taken place along deep fracture systems across the collapsed
basin and several lineations can be recognised. This has
created the volcanoes of the Haroharo complex in the
northern part of the caldera, and Tarawera in the southern
part. There are also other smaller concentrations elsewhere.
Tarawera Volcano had four major eruptions between 18,000
and 800 years ago. Its most recent eruption was in 1886 and
was relatively small in terms of the amount of material
ejected (between one and two cubic kilometres), but even so
was by far the largest to occur in New Zealand since
European settlement. Apart from peculiar waves on Lake
Tarawera ten days before the upheaval, the first definite signs
of its coming were earthquake tremors felt at Te Wairoa and
Rotorua beginning at about 12.30am on 10 June 1886.
These increased in intensity until eruptions began from the
central dome of Tarawera at about 2.00am. During the next
half an hour craters opened along the full length of the
volcano and earthquakes occurred repeatedly. Later (see
other sheets for more detail) the eruption extended beneath
Rotomahana lake and even beyond this into the area now
known as Waimangu Volcanic Valley. In the end a rift almost
16km long had opened in the Earth's surface and was actively
erupting. The material ejected consisted of new basaltic
molten lava and much old material caught up in an explosion
of the hydrothermal system at Rotomahana.
This was showered over much of the Bay of Plenty coating
the land for miles around with mud and volcanic ash. The
eruption also destroyed the famous Pink and White Terraces
and enlarged Lake Rotomahana. About 120 people in seven
small villages in the vicinity were killed. As a geological
event the eruption was thus rather middling on the scale of
others that have occurred in New Zealand in the remote past,
nevertheless it had a profound influence on the topography
and killed all animal and plant life in the immediately
surrounding region.
Author: Professor RF Keam Physics Department University of Auckland
View of local area from Mt Tarawera soon after 1886, showing sites of some pre-European villages
Waimangu Volcanic Valley, P O Box 6141, Rotorua, New Zealand
Phone 64 7 3666 137, Fax 64 7 3666 607, Email: [email protected]
www.waimangu.com