Lesson #8 Volcanic Cones and Eruptions
Molten rock on the surface of the Earth.
- Pyroclasts (Pyroclastic Rock)-
Pyro is Greek for fire and clastic means rock.
Put them together and it translates into "Rock broken by fire". Pyroclasts are formed
from the eruption of a volcano. Pyroclasts range in size from very small pieces
of dust to ash to lapilli to bombs and block.
A Hawaiian term for lava that has a smooth and ropey surface. Pahoehoe
forms when the flow is slow and cools slowly.
A Hawaiian term for lava that is rough and fragmented.
Aa lava forms when the lava flow is faster and the outside cools quickly causing the
outside to become rough and fragmented.
The resistance of flow in a liquid. Lava/Magma that is thick and pasty
is said to have a high viscosity. High viscosity magma can hold a large amount of
gas. This lava/magma usually will erupt violently when the gas that is dissolved
in the magma escapes rapidly. Lava/magma that is thin and runny is said to have a low
viscosity. These lava/magma will usually not erupt very violently. These eruptions
will produce large amounts of lava and little pyroclastic material.
A tunnel formed when the surface of a lava flow cools and hardens, while
the still molten and flowing interior drains away.
The smallest of the pyroclasts. Dust from volcanic eruptions have been known
to stay floating in the atmosphere for years.
Pyroclasts that are larger than dust. Very fine particles of exploded rock
that can drift in the atmosphere for days.
Angular pieces of pyroclastic rock that is exploded from a volcano during
Rounded pieces of pyroclastic material that are exploded during an eruption.
These pyroclasts are in semi-plastic state and take their shape as they fly through
- Pyroclastic Flows-
Very hot turbulent gases, ash, and pyroclasts that are heavier
than air and will flow down the side of a mountain at high speeds. These flows have
killed thousands of people in some famous eruptions such as Vesuvius in 79 A.D.,
and Pele on the island of Martinique in 1902.
Pyroclastic rock that is in a semi-plastic state as it is shot through
the air. The rock is full of gases that escape as the rock hardens. This rock is
so full of holes that it floats on water.
Lava rock that hardens very quickly. It can cool when it hits water
or flowing down the side of a mountain. This rock is natures glass. It usually
is dark green to black in color. Native peoples throughout the world have used it
to make arrowheads, spears, and knives. It can be chipped to a very sharp edge.
Lesson #9 Hot Spots-Hawaii and Yellowstone
- Three Volcanic Cone Shapes-
- Cinder Cone-
Formed from eruptions of pumice and cinders. These cones rarely become
more than 1000 feet tall. They are formed from very violent eruptions and can produce
large amounts of dangerous gases.
- Shield Cone-
The largest of the cone types. These cones are formed from many eruptions
of runny lava through the main vent and also through fissures on the flanks of the
mountain. The largest volcano in the world, Mauna Loa, is a shield cone along with the rest of the Hawaiian Islands.
The most dangerous and beautiful of the volcanic cones. It is produced
from the alternating eruptions of ash and lava. Some of the most famous volcanoes
in the world are stratovolcanoes. Mt. Fujiama in Japan, Mt. Ranier and Mt. St. Helens in Washington, Mt. Etna in Sicily, and Mt. Vesuvius in Italy are all stratovolcanoes.
- Eruption Types-
These eruptions are produced from many long cracks in the Earth called
fissures. They are sometimes called flood eruptions because of the amount of lava
produced. The magma is thin and runny and pours out of these fissures in great quantities. The great Columbian Plateau of Washington and Idaho were produced from Icelandic
Eruptions. The lava that cover the Columbia Plateau is over a mile thick in places.
They usually form shield cones.
Very similar to Icelandic eruptions, the difference lies in the fact
that the majority of the lava flows from the main vent in Hawaiian eruptions instead
of through fissures. The lava is thin and runny and the eruptions are usually not
violent. They usually form shield cones.
Strombolian eruptions are short lived explosive eruptions that shoot very thick and pasty lava into the
air along with bursts of steam and gas. These eruptions usually produce cinder cones.
Vulcanian eruptions are more violent and explosive than strombolian eruptions. Vulcanian
eruptions contain high dark clouds of steam, ash, and gas. The ash plume builds
a cauliflower shaped head and a thinner more treetrunk-like base. When the volcano quits erupting ash and gases it then ejects thick pasty lava. Vulcanian eruptions
usually build a steep sided cone that is more symmetrical than a cinder cone called
stratovolcanoes (composite cones)
Pelean eruptions are named for the catastrophic eruption on the island of Martinique in the Caribbean
Sea in 1902. The eruption and the pyroclastic flow that followed killed 29,000 people
almost instantly. "Glowing clouds" of gas and ash flew down the mountain at over
70 miles per hour. The cloud was so full of ash that it was heavier than air and hugged
the ground as it approached the coast. The temperatures were probably around 700
degrees F. which would annihilate everything in its path.
A Plinian eruption is the most explosive of the eruption types. Mt. St. Helens eruption was a plinian
eruption. Plinian eruptions are characterized by a very high ash cloud that rise
upwards to 50,000 feet (almost 10 miles) high. Very deadly pyroclastic flows are
also part of plinian eruptions.
Mt. Vesuvius, which erupted in 79 A.D. in Italy, was a classic Plinian eruption.
Very hot ash falls killed thousands of people in the city of Pompeii. Ash falls
as high as 17 feet buried the city. Plinian eruptions were named for Pliny the Elder
of Rome who died in one of the many eruptions of Vesuvius.
- Hot Spot-
A hot spot occurs because of the intense heat of the outer core. This heat radiates
through the mantle bringing hot solid rock upward to the hot spot. These areas
of rising solid rock are called mantle plumes. Hot spots do not move, but the plates
above the hot spot moves producing island chains and the spreading of the oceans at mid-ocean
- Mantle Plume-
Mantle plumes are areas of hot solid rising rock. This rock moves
from the lower reaches of the mantle to the surface of the Earth causing the formation
A caldera is a large bowl-shaped crater that is formed by the collapse of a volcanic cone
after an eruption.
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