Slide 2 Slide 3 Basaltic volcanoes are the most common volcanic
landmasses on the continents. They are found where the mantle rises
up to form new plates. Basaltic volcanoes are rarely found above
the sea surface. Basaltic lava flows freely. Its cooling surface
forms a skin, which wrinkles and puckers with the movement beneath.
This ropey lava is known by its Hawaiian name pahoehoe. If this
surface breaks up, it form blocks of lava with rough surfaces, also
known as aa An active andesitic volcano is extremely violent. The
eruptions can happen at any time and the eruptions can cause
considerable damage.This type of eruption can also send huge clouds
of hot ash and dust over great distances. Andesitic volcanoes are
more dangerous than basaltic volcanoes because the lava in
andesitic volcanoes is more viscous. This causes the lava to get
clogged up in the volcano, then when it does erupt the lava
suddenly breaks through causing a great explosion. Slide 4 A
volcano is formed when two tectonic plates hit each other. One of
the plates goes under the other. This is where the earth heats rock
into magma and gases. This can happen as deep as 150 kilometres
below the earths crust. Magma settles in pools or chambers near the
surface. This is where the eruption takes place. Volcanoes can also
be formed when the plates push away from each other and the magma
pushes up and lava forms. These types of volcanoes are found on the
ocean floor. Cross section of a volcano. Cross section of a volcano
erupting. Cross section of a volcano. Slide 5 Deep in the earth
mantle there are areas that are very turbulent and hot. These are
known as hot spots, they create the right conditions for basaltic
volcanoes to form. When a plate is continuously moving it can
creates a string of volcanoes. When a tectonic plate moves over a
hot spot, a volcano forms on the surface as rising material breaks
through. This happens over and over again and eventually there is a
chain of extinct basaltic volcanoes that stretches across the
ocean. Ring of fire. Cross section of a hot spot volcano World map
of hot spots Diagram showing how hot spot volcanoes are formed
Slide 6 Most volcanoes are found under the seabed. Because the
ocean floor is very thin, it can be easily pierced by the magma
which lies beneath especially on the plate margins. When the
tectonic plates move apart, the rising magma seeps up into the gap,
and hardens into a new strip of crust. This type of plate margin is
present in the Atlantic Ocean, making the Atlantic wider and
forcing Europe and America apart. Active underwater volcanoes
usually erupt very violently. Hot molten lava explodes when it hits
cold seawater and produces thick black clouds of steam and ash.
Under water volcano erupting. Diagram of an underwater volcano
erupting. Underwater volcano erupting. Diagram of underwater
volcanoes. Slide 7 There are thousand of volcanoes on Venus. That
is why the planet has such a thick atmosphere. Scientist dont know
if any of the volcanoes are still active. Mars is only half the
size of earth but the volcanoes there dwarf the tallest mountains
on earth. However, all the volcanoes on mars are extinct. Jupiter's
moon is the most volcanically active planet or moon in the solar
system. A noxious mixture of molten and gaseous sulfur dioxide is
spewed from its surface. Volcanoes are not always hot. A space
probe that passed Triton in 1989 discovered large geysers on
Neptune's moon Triton that erupt with super cold liquid nitrogen.
Billions of years ago the Earths moon was volcanically active. The
dark patches we can see on the surface are lakes of solidified
lava. Venus Mars LoTritonThe Moon Slide 8 Mt. St. Helens is located
in Skamania county in Washington. It erupted on the 18 th of May
1980 after lying dormant for 120 years. Scientists had been
watching the volcano for months. There was a huge bulge in the
volcanos side because the lava was moving upward and therefore an
eruption was imminent. The blast ripped a huge hole in the once
cone shaped volcano, flattened forests for miles around and rained
ash over cities as far away as Oklahoma. Fifty seven people died
when Mt. St. Helens erupted. BeforeAfter The crater in the cone of
Mt. St. Helens. Mt. St. Helens erupting. Diagram of the stages in
which Mt. St. Helens erupted. Slide 9 Krakatau erupted on the 27 of
August 1883. It was an ultra plinian eruption, 20 cubic kilometres
of tephra, ash and lava was blasted out of the volcano. The bang
that Krakatau made when it erupted is said to be the loudest sound
humans have ever heard. The eruption obliterated the volcanic cone
of Krakatau and destroyed the island in which it sat. 36,417 people
died including the victims of the tsunamis that were generated by
hot pyroclastic flows entering the ocean. Now a new volcanic cone,
Anak Krakatau, has risen from the seafloor where Krakatau once
stood. Anak Krakatau Lava erupting from Krakatau. Photo taken of
Krakatau erupting in 1883. Slide 10 Mt. Vesuvius is located in
Italy near Pompeii. It has erupted a few times, the most famous of
those eruptions was in 79 AD on the 26 th of May. Eight cubic
kilometres of ash, dust and rocks was blasted out of the volcano
and covered Pompeii. In 1743 the exploration of the ancient site
began. When Mt. Vesuvius erupted, over 2,000 people were covered by
lava and hot ash. When the lava set and the bodies of those people
decayed a hole was left in the shape and position that the people
died in. A plaster cast of a dog found in Pompeii Recreation of Mt.
Vesuvius erupting 79 AD. Plaster casts of people found in Pompeii A
2 nd recreation of Mt. Vesuvius erupting 79 AD. Slide 11 Toba is
found on the Indonesian island of Sumatra. It erupted approximately
73,500 years ago. The eruption was ultra plinian. There was an
eruptive volume of about 2,800 cubic kilometres. After the eruption
volcanic ash and sulfur dioxide aerosols injected into the
stratosphere and formed a thin layer surrounding the globe that
partially blocked the suns rays. This caused a long cold volcanic
winter. Some scientists have speculated that this eruption brought
humans to the brink of extinction. But Toba wasnt the biggest
volcanic eruption ever, the eruption of La Garita in Western United
States approximately 27,000,000 years ago, was almost twice as big.
The Plinian column. Toba as it is today Slide 12 Tell me one fact
that you find interesting. (I hope youve been listening!) Slide 13
To find all the information for my PowerPoint presentation on
volcanoes, I used many books they are: Volcanoes and Earthquakes by
Terry Jennings Volcanoes and Thermal Springs by Josh Wilson
Earthquakes and Ancient Volcanoes by Bruce Mclish Earth By Susanna
van rose My Best Book on Volcanoes by Simon Adams Volcano by Rosie
Nelson Earthquakes and Volcanoes by Brian Knapp Volcano evacuation
by Dougal Dixon Encyclopedia of Science Susan Mckeever I also used
lots of websites to find my information including:
www.bbc.co.uk/history/ancient/romans/pompeii_rediscovery_01.
shtml#top www.volcanoelive.com/pliny
www.volcanolive.com/krakatau.html
www.enchantedlearning.com/subjects/volcano/ Slide 14 Thank you for
watching my slideshow, I hope that you enjoyed it, and that you
learnt something new about volcanoes.
