Volcanoes
You have read that most volcanoes form
at plate boundaries. A volcano is a mountain
fromed by lava and ash. Lava is magma that
reaches Earth’s surface. Ash is small pieces of
hardened lava.
Chains of volcanoes form where a
continental plate and an oceanic plate collide.
The edge of the oeanic plate pushes under the
edge of the continental plate. The leading
edge of the oceanic plate melts as it sinks deep into the mantle. The melted rock
becomes magma that forces it ways up between the plates. The volcanoes of the
Cascades, such as Mount St. Helens, formed this way.
Sometimes volcanoes form in the middle
of plates,over unusually hot columns of magma.
The magma melts a hole in the plate and rised
through the hole, causing a volcanic eruption.
The Hawaiian Islands are the tops of a chian of
volcanoes that formed in the middle of the
Pacific plate. As the Pacific plate continues moving over this hot spot, new volcanoes
and new islands form. The big island of Hasai’I, with its active volcano, Kilauea, is the
youngest island in the chain. Kure Atoll, and ixtinct volcano 2,617 km (about 1,625
mi.) to the northwest, is the oldest.
√What is a volcano?
Five Types of Volcanoes
Shield Volcanoes
Shield volcanoes are a type of volcano given their
name for their broad gently sloping profile which
looks like a warrior's shield. They are composed of
thousands of fluid lava flows that spread great
distances, sometimes greater than 50
kilometers. The tallest volcano in the world is a
shield volcano. Mauna Kea Volcano rises 13,798 feet above sea level but extends
about 19,700 feet to the ocean floor, for a grand total of 33,500 feet above sea level,
dwarfing the world's tallest mountain, Everest, by over 4,000 feet!
Composite Volcanoes
Composite volcanoes are the result of multiple eruptions occurring over a
prolonged time period. These formations are made by alternating layers of lava and
rock fragments. They have steep sides and can usually exceed heights of 2500 m.
Most such structures have a crater at the summit with a central vent. This central
vent can be accompanied by other clusters of vents.
These types of volcanoes can be so quiet in
between eruptions that they seem almost extinct. When a
composite volcano becomes dormant, erosion begins to
destroy the cone. After many years, the great cone is
stripped away and eventually all traces of the cone are
worn down. However, they can cause quite a stir upon eruption as they detonate in
an explosive way. Usually large and conical in shape these volcanoes are constructed
along the edge of tectonic plates. Mount St Helens, Mount Fuji and Mount
Kilimanjaro are well known composite volcanoes.
Cinder Cone
Cinder Cone volcanoes are steep conical hills
formed from particles and blobs of hardened lava
above a vent. These lava blobs usually solidify in
mid air before they land on the ground around the
vent. Over passing time, these types of
volcanoes build up into rounded or oval shaped
structures. Most cinder cones have a bowl shaped
crater at the peak and rarely exceed 250 m in height and 500m in diameter.
Although the most common volcanic formations in the world, cinder cones are
the least complex type of volcano. Cinder cones can occur alone but usually grow in
groups. Cinder cones can grow rapidly and modify their shape when the position of
the vent changes.
Dome Volcanoes
Dome volcanoes are rounded formations made by
small masses of lava that are too thick to flow very far. The
viscous lava from these domes just oozes out and spills over
and around the vent. The dome grows by accumulation of
the hardening lava, and the structure grows from material
spilling off the sides of the dome. These mounds can grow for many months, even
years with the sides being very steep. Lava domes are threatening as these volcano
types can explode violently without prior warning releasing a huge amount of rock
and ash. Mount Pelee in Martinique is such an example.
Caldera
Calderas are massive crater-like depressions that can cover many tens of
square miles. These calderas form when volcanoes explode with terrible destruction,
completely obliterating the original volcano, and surrounding area. the original
volcano, and surrounding area.
One famous example of a caldera is North
America’s Crater Lake. Around 7,000 years ago a
massive volcanic peak reaching 12,000 feet in
elevation set in this location. A powerful eruption
blasted the top 4,000 feet away, leaving a deep bowl shaped caldera. Since that time,
a new dome has begun to form in the center of the caldera.
Yellowstone Caldera
The hot springs, mud pots, fumaroles and geysers found at Yellowstone
National Park bubble, pop and hiss for a really good reason. Yellowstone sits atop one
of the largest volcanoes in the world! Yellowstone Caldera measures 55 km (34 miles)
wide by 72 km (45 miles) long and caps a massive chamber of molten rock called
magma. And while Yellowstone Volcano hasn't erupted for thousands of years, three
eruptions have been supervolcano in size. An eruption 640,000 years ago produced
6,000 times the ash as Mt. St. Helens. The three supervolcanic explosions blew
enough ash and lava to fill the Grand Canyon
in Arizona! Will Yellowstone Caldera erupt in
the future? Scientists think so. But they don't
know when. And they don't know if the next
eruption will be a supervolcano blast.