What is Lava?
Lava is melted rock that has reached the Earth's surface through a
volcano's main vent or through side vents and fissures.
Some volcanoes produce little or no lava. Some volcanoes eject
pyroclasts, which are fragmented or broken rock. The word pyroclastic
comes from a Greek word that means "Rock broken by fire".
When volcanoes do produce lava flows they are classified as either
Pahoehoe or Aa. The lava is identical in both pahoehoe and aa lava
flows, the difference comes from the amount of lava erupted and the speed of
cooling. Pahoehoe lava flows are produced from a small amount of lava that moves
slowly, while aa flows usually are associated with a large volume of lava that
moves swiftly. Aa flows are generally 6-15 feet thick and pahoehoe flows are
usually 1-3 feet thick.
If the lava is very hot and has a low viscosity (runny with a
low gas and silica content) the lava flow is called Pahoehoe. If, on the other
hand, the lava has a high viscosity (thick and pasty with a high
gas and silica content) it is called Aa.
Silica is a white or colourless crystal that is present in sand and quartz.
It is one of the most abundant compounds in the Earth's crust.
Pahoehoe (Pa-Hoy-Hoy) lava flows are very hot,
thin and runny. When it cools is has a smooth to ropey texture because of the
low silica content which makes it cool quickly.
Pahoehoe flows creep along generally at less than 3 feet per minute but some
flows have been measured at over 20 miles per hour. The terms pahoehoe and aa
are from the native Hawaiian language and are now used by geologists the world
over.
This pahoehoe flow is advancing on the skeleton of a large mammal perhaps a
horse or a cow. The lava will engulf the animal and may fossilize the remains!!
Notice how the flow advances in globs of lava. These globs of lava are
called lobes.
Aa lava flows are formed when the lava is produced in a manner that allows it to cool quickly. When a fire fountain shoots the lava high into the air it cools somewhat before it can flow after landing on the surface. Aa lava also forms when there is a huge amount of lava produced or a steep slope moves the lava at high speeds. These high speeds put the lava in greater contact with the air, which makes it cool more quickly.
A lava tube forms when the lava on the outer surface of the flow cools much faster than the inside of the flow. The outside becomes cooled hardened lava rock while the inside stays molten and also keeps flowing. If something happens to stop the flowing lava there will be nothing to fill the void and a tube is the result.
When Mt. St. Helens erupted in 1980 the ash cloud rose to an altitude of
over 50,000 feet, that is almost 10 miles high! The mountain kept spewing ash
for another nine hours on May 18th. The ash deposits were many inches deep in
many cities in Washington. This ash choked humans and animals. People were
forced to wear gas masks so they could go outside of their homes.
Pyroclasts are particles that are ejected during a volcanic
eruption. They range in size from very small particles called dust to ash (1/10
of an inch) to lapilli ("little stones" 1/10 of an inch to 2 inches ) to the
largest of the pyroclasts, blocks and bombs (2 inches to many feet in diameter).
Volcanic Ash is any very fine grained material erupted from a
volcano that is less than 1/10 of an inch (2 millimeters) in diameter. This is
very fine material and was given the name ash because it resembles ashes from
the burning of wood or coal.
Volcanic ash is rock that has been exploded and shattered by steam inside
the volcano. Ash and lava flows build stratovolcanoes into mountains with
repeated eruptions.
Pyroclastic flows are spinning mixtures of pyroclasts (small
pieces of obsidian, ash, pumice, and cinders) and very hot gases. They flow
down the side of the volcano at speeds up to 100 miles per hour and at
temperatures sometimes over 700 degrees Fahrenheit!! With temperatures that high
pyroclastic flows kill everything it their path.
There were two pyroclastic flows from Mt. St. Helens main eruption in 1980.
The first flow was called the "stone wind" and it annihilated everything in its
path. Huge trees over one hundred feet tall were snappped and splintered like
twigs. Temperatures of over 700 degrees ate up all the oxygen in the area. All
animal life in its path was destroyed in seconds including 57 humans. Later in
the day another pyroclastic flow piled pumice and ash in thick deposits for many
miles around the mountain.
The photo on the left is a pyroclastic flow down the north flank of Mt. St.
Helens.
Pumice is a very light coloured, frothy volcanic rock. Pumice
is formed from lava that is full of gas. The lava is ejected and shot through
the air during an eruption. As the lava hurtles through the air it cools and the
gases escape leaving the rock full of holes.
Pumice is so light that it actually floats on water. Huge pumice blocks have
been seen floating on the ocean after large eruptions. Some lava blocks are
large enough to carry small animals.
Pumice is ground up and used today in soaps, abrasive cleansers, and also in
polishes.
Bombs and blocks are the largest of the pyroclasts.
Blocks are angular chunks of rock that has been ejected from a
volcano during an eruption.
A bomb is formed as lava hurtles through the air, cooling and
forming a hardened lava rock. A bomb's shape is usually more rounded or
streamlined
Answer the following:
a) High viscosity
b) Low viscosity