Unit 6-3: Types of Volcanoes

What is a volcano? A volcano is a spot in the Earth’s crust where magma can reach the surface.  It is named from the Roman island of Vulcano.  The island of Vulcano was in turn named after Vulcan, the Roman god of fire. There are many different types of volcanoes depending on their magma types, eruption styles and structure.

Shield Volcanoes Easily identified by its gently sweeping sides.  These volcanoes do not look like the common idea of a volcano which has steep sides. These volcanoes form from repeated eruptions of highly basaltic lava. Remember, basaltic lava has a very smooth flow and doesn’t explode when erupting.

Shield Volcanoes Different lava flows over time build up the volcano. As time progresses, the volcano slowly builds upwards and outwards

Shield Volcanoes All of the volcanoes on Hawaii are shield volcanoes. These volcanoes are commonly formed due to hot spots. These volcanoes can sometimes have a collapsed caldera.

Shield Volcanoes

Caldera When the magma chamber below a volcano empties, the rock of the volcano may not be strong enough to support itself.  A roughly circular fracture, called a ring fault, develops around the edges of the magma chamber.  Once the rock begins to break, the volcano collapses into the empty chamber.

Caldera A caldera collapse may occur due to one single powerful eruption, or due to many small eruptions. The caldera itself is the collapsed area of a volcano that could not support its weight. Caldera can sometimes form lakes when there is nowhere for water to escape.

Caldera

Cinder Cones A cinder cone volcano is identified by its steep sides that are made up of ejected volcanic fragments. This type of volcano is common at subduction boundaries, or on the sides of shield volcanoes.

Cinder Cones Lava flows from the top of the cinder cone are rare, as the tephra that makes up the sides of the volcano cannot support a lava flow. If the side of a cinder cone is breached, then lava can flow down the side. Paricutin in Mexico is a famous example of a cinder cone.

Cinder Cones

Stratovolcano A stratovolcano is a “combination” of a shield volcano and a cinder cone. It is made up of layers of lava flow with lava fragments in between. These volcanoes are characterized by their steep sides and periodic, explosive eruptions.

Stratovolcano Alternating layers of tephra and lava flows

Stratovolcano These volcanoes are common near subduction zones These volcanoes are also the most common type of volcano. In this type of volcano, magma steadily builds up in the magma chamber.  Once the magma chamber has reached its critical mass, the volcano explodes violently.

Stratovolcano These volcanoes are among the most famous, and dangerous, volcanoes. Mount St. Helens, Mount Pinatubo, Mount Tambora and Mount Vesuvius are some examples of incredibly explosive volcanoes. Why are these volcanoes so dangerous? What about them is different?

Stratovolcano

Effects of volcanoes Gas builds up in highly viscous magma. As the magma approaches the critical mass, it tends to force the gas out of the magma. Once the gas is out of the magma, it pushes with incredible force. If it breaches the surface, you have an incredible explosion.

Effects of volcanoes A stratovolcano’s explosive eruption will pour tons of gas into the atmosphere and force a powerful pyroclastic flow into the air. Sulfur dioxide is one type of gas that will enter the atmosphere and can cause acid rain to fall near the volcano.

Effects of volcanoes Pyroclastic Flow:  The flow can travel incredibly fast  It can level forests  It can bury cities  12+ inches of ash (tiny lava fragments) on a building can be enough to cause collapse.

Effects of volcanoes The explosive eruption with its tephra and gas expulsion can also affect the global climate! The year without a summer:  Mount Tambora in Indonesia was the most powerful eruption in recorded history.  It put so much material in the atmosphere from the volcanic cloud that it lowered the global temperature by 3°C! (that’s about 5°F)