A
volcano
is a
mountain
that has
lava
(hot, liquid rock) coming out from a
magma
chamber under the ground, or did have in the past. Volcanoes
[1]
are formed by the movement of
tectonic plates
.
The Earth's crust has 17 major, rigid
tectonic plates
. These float on a hotter, softer layer in its
mantle
.
[2]
Volcanoes are often found where tectonic plates are
coming together
.
Volcanoes can also form where there is stretching and thinning of the crust's plates, e.g., in the
East African Rift
.
[3]
Volcanoes are usually not found where two tectonic plates slide past one another.
Volcanism away from plate boundaries is caused by
mantle plumes
. These so-called "
hotspots
", for example
Hawaii
, are thought to arise from upwelling magma from the
core?mantle boundary
, 3,000 km deep in the Earth.
Most volcanoes have a
volcanic crater
at the top. When a volcano is
active
, materials come out of it. The materials include lava,
steam
, gaseous
sulfur compounds
,
ash
and broken
rock
pieces.
When there is enough pressure, the volcano erupts. Some
volcanic eruptions
blow off the top of the volcano. Sometimes, the magma comes out quickly and sometimes it comes slowly. Some eruptions come out at a side instead of the top.
Volcanoes are found on planets other than
Earth
. An example is
Olympus Mons
on
Mars
.
Volcanologists are scientists who study volcanoes using methods from geology, chemistry, geography, mineralogy, physics and sociology.
The world's biggest volcano is named
Mauna Loa
in Hawaii. Mauna Loa is part of the five volcanoes on Hawaii's 'Big Island'. The most recent time this volcano erupted was in 1984. It erupted 33 times in the last 170 years. Like all the other Hawaiian volcanoes, Mauna Loa was created by the movement of the
Pacific tectonic plate
which moved over the Hawaii
hotspot
in the Earth's mantle. Mauna Loa is 4,196 meters tall. It is a
shield volcano
. The largest recent eruption from Mauna Loa left a lava trail 51 kilometres (32 miles) long.
The
lava
and pyroclastic
material
(clouds of ash, lava fragments and vapor) that comes out from volcanoes can make many different kinds of land shapes. There are two basic kinds of volcanoes.
These volcanoes are formed by fluid low-
silica
mafic
lava.
Shield volcanoes
are built out of layers of
lava
from continual eruptions (without explosions). Because the lava is so fluid, it spreads out, often over a wide area. Shield volcanoes do not grow to a great height, and the layers of lava spread out to give the volcano gently
sloping sides
. Shield volcanoes can produce huge areas of
basalt
, which is usually what lava is when cooled.
The base of the volcano increases in size over successive eruptions where solidified lava spreads out and accumulates. Some of the world's largest volcanoes are shield volcanoes.
Even though their sides are not very
steep
, shield volcanoes can be huge.
Mauna Kea
in
Hawaii
is the biggest mountain on Earth if it is measured from its base on the floor of the sea.
[4]
A
stratovolcano
, also known as a
composite volcano
,
[5]
is a tall,
conical
volcano. It is built up of many layers of hardened
lava
,
tephra
,
pumice
, and
volcanic ash
.
Unlike shield volcanoes, stratovolcanoes have a steep profile and periodic eruptions. The lava that flows from stratovolcanoes cools and hardens before spreading far. It is sticky, that is, it has high
viscosity
. The
magma
forming this
lava
is often
felsic
, with high-to-intermediate levels of
silica
, and less
mafic
magma. Big felsic lava flows are uncommon, but have travelled as far as 15 km (9.3 mi).
[4]
[6]
Two famous stratovolcanoes are
Japan
's
Mount Fuji
, and
Vesuvius
. Both have big bases and steep sides that get steeper and steeper as it goes near the top. Vesuvius is famous for its destruction of the towns
Pompeii
and
Herculaneum
in 79 AD, killing thousands.
A
caldera
is a basin-like feature formed by collapse of land after a volcanic eruption. This happens after a huge stratovolcano blows its top off. The base of the crater then sinks, leaving a caldera where the top of the volcano was before.
Krakatoa
, best known for its catastrophic eruption in 1883, is much smaller now.
[4]
There are two main processes.
Volcanoes are made when two
tectonic plates
come together. When these two plates meet, one of them (usually the oceanic plate) goes under the continental plate. This is the process of
subduction
. Afterwards, it melts and makes
magma
(inside the magma chamber), and the
pressure
builds up until the magma bursts through the
Earth's crust
.
The second way is when a tectonic plate moves over a
hot spot
in the Earth's crust. The hot spot works its way through the crust until it breaks through. The
caldera
of
Yellowstone Park
was formed in that way; so were the
Hawaiian Islands
.
A
traditional
way to
classify
or
identify
volcanoes is by its pattern of
eruptions
. Those volcanoes which may erupt again at any time are called
active
. Those that are now quiet are called
dormant
(inactive). Those volcanoes which have not erupted in historical times are called
extinct
.
An active volcano is currently erupting, or it has erupted in the last 10,000 years. An example of an active volcano is
Mount St. Helens
in the
United States
(US).
[7]
A dormant volcano is "sleeping," but it could awaken in the future.
Mount Rainier
in the
United States
is considered dormant.
[7]
An extinct volcano has not erupted in the past 10,000 years.
[7]
Edinburgh Castle
in Scotland sits on top of an extinct volcano.
[8]
The Earth's largest volcano has been discovered.
[9]
[10]
It is 2 km below the sea on an underwater
plateau
known as the
Shatsky Rise
. This is about 1,600 km east of
Japan
. The previous record-holder,
Mauna Loa
in
Hawaii
, is still the largest volcano on land.
The 310,000 km
2
(119,000 sq mi) volcano,
Tamu Massif
, is comparable in size to
Mars
' vast
Olympus Mons
volcano, which is the largest known volcano in the
Solar System
. It was formed about 145 million years ago when massive lava flows erupted from the centre of the volcano to form a broad, shield-like feature. That suggests the volcano produced a
flood basalt
eruption.
The Tamu Massif extends some 30 km (18 miles) into the
Earth's crust
. The researchers doubted the submerged volcano's peak ever rose above sea level during its lifetime and say it is unlikely to erupt again.
- "The bottom line is that we think that Tamu Massif was built in a short (geologically speaking) time of one to several million years and it has been extinct since", co-author William Sager of the
University of Houston
told the AFP
news agency
.
- "There were lots of oceanic plateaus (that) erupted during the
Cretaceous
period (145-65 million years ago) but we don't see them since. Scientists would like to know why... The biggest oceanic plateau is
Ontong Java Plateau
, near the equator in the
Pacific
, east of the
Solomon Islands
. It is much bigger than Tamu ? it's the size of France".
[9]
- ↑
The plural of volcano can be
either
volcanos or volcanoes. Both are correct and it is not a matter of British vs US spelling.
Oxford English Dictionary
.
- ↑
NSTA Press / Archive.Org (2007).
"Earthquakes, Volcanoes, and Tsunamis"
(PDF)
.
Resources for Environmental Literacy
. Archived from
the original
(PDF)
on September 13, 2012
. Retrieved
April 22,
2014
.
- ↑
Foulger, Gillian R. (2010).
Plates vs. Plumes: a geological controversy
. Wiley-Blackwell.
ISBN
978-1-4051-6148-0
.
- ↑
4.0
4.1
4.2
Earth Science
. Austin,
Texas
: Holt, Rinehart and Winston. 2001.
ISBN
0-03-055667-8
.
- ↑
"Principal types of volcanoes. USGS"
.
Archived
from the original on 2013-05-09
. Retrieved
2013-07-02
.
- ↑
"Garibaldi volcanic belt: Garibaldi Lake volcanic field"
.
Catalogue of Canadian volcanoes
. Geological Survey of Canada. 2009-04-01. Archived from
the original
on 2009-06-26
. Retrieved
2010-06-27
.
- ↑
7.0
7.1
7.2
Ball, Jessica (September 8, 2010).
"Voices: Dead or alive ... or neither? Why a dormant volcano is not a dead one"
.
Earth Magazine
.
Archived
from the original on 28 January 2013
. Retrieved
14 August
2012
.
- ↑
University of Edinburgh,
"Holyrood Park Geology"
Archived
2007-08-30 at the
Wayback Machine
; retrieved 2012-8-2.
- ↑
9.0
9.1
World's largest volcano discovered beneath Pacific.
BBC Science & Environment
.
[1]
Archived
2019-12-19 at the
Wayback Machine
- ↑
Witze, Alexandra 2013. Underwater volcano is Earth's biggest:
Tamu Massif
rivals the size of Olympus Mons on Mars.
[2]
Archived
2014-07-06 at the
Wayback Machine
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Volcanoes
.