Wanted: Oscar Obsidian - Auckland Museum

Background
Ruapehu, Taranaki and Ngauruhoe are typical
New Zealand examples. The Pacific Rim of Fire
consists mostly of stratovolcanoes.
Caldera - Caldera volcanoes are huge craters in
the ground and can often lack any type of built
up flanks around them. They form from gigantic
explosions of rhyolite magma. The sudden
release of large volumes of ash drains the
magma chamber and results in collapse of the
structure forming a large depression called a
caldera. Some calderas can be as much as 50
km in diameter such as in Yellowstone National
Park in the USA. Lakes Taupo and Rotorua are
typical New Zealand examples of this type of
volcano.
Dome - After an explosive dacitic or rhyolitic
eruption, some magma can remain around the
edges of the caldera. This material is very viscous
and all the gases have already escaped, so it
oozes out like toothpaste to form steep sided
domes of obsidian. A New Zealand example is
Ngongotaha on the shores of Lake Rotorua.
Eruption Styles
The driving force of an eruption is the escape of
gases which are pressurised (eg water, carbon
dioxide) underground in the magma. If the
magma is not viscous the gases escape easily and
the eruption is not violent. When the magma is
viscous the release of gas is highly explosive.
Examples:
Lava flows - When gases are released quietly, the
lava will flow along the ground. Sometimes blobs
of lava fly through the air producing aerodynamically
shaped bombs.
Fire fountaining - Spectacular fire fountaining produces
frothy gas-rich fragments of basalt called
scoria. Scoria forms steep sided cones like Mt
Eden.
Auckland Museum 9
Pyroclastic fall - In viscous magma the gases are
explosively released as a jetblast, producing an
ash column into the atmosphere. The ash can blow
hundreds of kilometres away from the volcano
and mantle the landscape. Volcanic ash is very
abrasive.
Raindrops falling through ash clouds can produce
fossil raindrops, called accretionary lapilli. Each
one grows like a snowball as ash is added onto
the surface as it falls.
Pyroclastic flows - Sometimes the eruption column
above the vent becomes so ash-laden that it collapses
back to earth and flows along the ground.
This is called a pyroclastic flow. These flows consist
of gas and fine volcanic debris that are hot
(700°C) and can travel at high speeds (100 to
700 km/h). When pyroclastic flows stop and cool,
they leave deposits called ignimbrite.
Size Does Matter
The 1995-6 eruptions from Mt Ruapehu affected
human activities such as skifields and airports. But
these events were small compared to the 1991
Pinatubo eruption in the Philippines which lowered
the average global temperature by at least 0.5
°C.
The largest eruption in modern history was when
Tambora in Indonesia erupted in April 1815
(erupting 150kms 3 of rock). The effects on the
weather were marked- Europe and North America
experienced 'a year without a summer' in 1816.
The Taupo Volcanic Zone is the most active rhyolitic
region on Earth, regularly producing many
large-scale eruptions. The most recent was the
hugely explosive Taupo eruption of AD 232
(erupting 145kms 3 of rock). However, even this is
dwarfed in size by its predecessor from the same
caldera 26 000 years ago (which erupted
800kms 3 of rock). In comparison, the largest
known volcanic eruption was 76 000 years ago
when the Indonesian island of Sumatra exploded
to form the Toba Caldera, erupting 2800 km 3 of
rock.
From right, Lake Taupo (26,000 years ago), Lake Rotorua (230,000 years ago), Tambora (1815), Lake Taupo
(232 AD), Pinatubo (1991), St Helens (1980), Tawawera (1886), Ruapehu (1995-6).