Formation
of a volcano
A volcano starts as magma,
which is, melted rock inside the earth’s core. In certain places the heat is
so great it nearly melts the rock inside the earth.
The magma melts it produces gas. The gas eventually becomes mixed with
magma. The magma with the gas inside it gradually rises to the surface. As the
magma is rising it melts gaps in the surrounding rock which forms a large
chamber called a magma chamber. This chamber is where all the volcanic materials
erupt.
The eruption of a volcano. The gas filled magma is under intense pressure
from the solid rock around it. This intense pressure causes the magma to melt a
channel in a weakened part of the rock. The magma rises through the channel to
the surface. The magma blasts out of the opening called the central vent. Magma
and other volcanic materials erupt out of this vent. The materials pile up to
form a volcano. When the eruption stops a crater is formed at the top of the
mountain. That is where the central vent is when a volcano has formed not all the magma from the other eruptions
reaches the surface. As the magma rises it could break through a channel wall
branch into smaller channels . The magma in these channels could escape through
a vent on the side of the mountain.
Kinds
of Volcanoes
There are three types of
volcanoes shield, cinder cones, and composite volcanoes. These three groups are
based on the shape and the material they are built of.
Shield volcanoes form when a large quantity of lava spills and spreads
widely. The volcano gradually builds up to be a low dome-shaped mountain. Mauna
Loa is a shield volcano. Thousands of overlapping lava flows formed Mauna Loa. Cinder cones form when tephra erupts from a vent and falls back to
the ground around the vent. The tephra which is generally cinders forms a
cone-shaped mountain. Paricutin in western Mexico is well known for being a
cinder cone. It began in 1943, a crack opened in a cornfield. When the eruptions
came to a stop in 1953 it was 1,345 feet.
Composite volcanoes form when lava and tephra erupt. The materials pile in
different areas. Composite volcanoes include Japan’s Mount Fuji; Mayon Volcano
located in the Philippines; and also Italy’s Vesuvius. In A.D. 79, Vesuvius
erupted destroying nearby towns of Pompeii, Herculaneum, and Stabiae under
masses of ash, dust, and cinders. Mount Saint Helens is one of the most active
volcanoes in the U.S.
Eruptions
of volcanoes
When pressure from gases within magma become too great, the gases are
pushed to the surface. Volcanic hazards include gases, lava, pyroclastic flows,
landslides, earthquakes, and dangerous eruptions. Eruptions can be very quiet,
producing lava flows that creep across the land at 2 to 10 miles per hour.
Dangerous eruptions can shoot columns of gases and rock fragments tens of miles
into the atmosphere, spreading ash hundreds of miles downwind.
The
Deadliest Volcano
Mount Saint Helens was
the deadliest volcano that ever occurred. The blast uprooted surrounding trees.
A 200-mile per hour wind caused giant rocks and destroyed the trees. The rocks
traveled so fast they started an avalanche. The avalanche hit a ridge and split
in two. One part of it poured into Spirit Lake. The main part of the avalanche
went to the valley of North Fork of the Toutle River. Minutes later Mount Saint
Helens Began to erupt. A dark column of ash rose miles into the sky. Lightening
flashed in the ash clouds and started forest fires. In Yakima, Washington 80
miles away it was so dark the street lights turned on at noon. Ash fell like
snow that wouldn’t melt. The eruption continued for 9 hours. After noon the
color of the ash column became lighter. The volcano began giving off huge flows
of pumice and ash. The material was extremely hot with temperatures of
1,000 degrees Fahrenheit, and it traveled down the mountains at speeds of 100
miles per hour. The flows went on until 5:30 P.M. They formed a wedge-shaped plain
of pumice on the side of the mountain. Two weeks later the temperatures in the
pumice were unchanged. Finally, there were mudflows, which started when the heat
from the blast melted ice and snow on the mountaintop. The water mixed ash,
pumice, dirt, and rocks of the avalanche. The result was a thick mixture that
was like wet concrete, a mudflow. The mudflows traveled fast scouring the
landscape and sweeping down the slopes into river valleys. Together their speed
and thickness did great damage. The largest mudflow was made of avalanche
material from the valley of North Fork of the Toutle River. It changed down the
river valley, tearing out steel bridges , ripping houses apart, picking up
boulders and trucks and carrying them along. Miles away it choked the
Cowlitz River and blocked shipping channels in the Columbia River. Finally, the
mudflows stopped. Mount Saint Helens was 1,300 feet shorter. The north side of
it changed from a green lovely slope to a fan-shaped wasteland. That was the end
of the eruption.
