Volcanoes

What is a volcano?

A volcano is a vent or chimney that provides an opening in the earth's surface through which molten rock called magma, can escape. This is often accompanied by gas, ash and pieces of rock. The shape of a volcano is made up of layers of hardened lava and ash that build up with each eruption.

Volcanoes come in different shapes and sizes, due to the rate of flow of the lava. Slow moving lava that travels a long way will produce a gently sloping, shield volcano. However, lava that hardens before it can flow very far builds up high layers, like Mount Fuji, in Japan.

There are several different types of eruptions. These can cause huge devastation to a landscape, destroy farmland and may lead to loss of life. Lava pouring into the ocean can cause beaches to disappear and new coastlines to form.

Volcanoes however, do have a positive side. Lava breaking down over the years can provide areas with rich, fertile volcanic soil. Volcanic rock may also contain precious stones such as diamonds and opals. Hot springs are a source of geothermal energy.

When a volcano erupts

Magma from deep inside the earth's mantle rises to the surface and collects in a magma chamber where it mixes with gases and water. Pressure builds up and forces its way through a weak spot in the earth's crust. When this pressure pack of magma finds a vent or chimney, it surges up releasing red-hot lava, steam, gases and ash through the crater at the top of the vent.

Types of volcanoes

Shield volcanoes

Shield volcanoes are large volcanoes that are built almost entirely of fluid lava flows. It has broad sloping sides and is usually surrounded by gently sloping hills in a circular or fan shaped pattern that looks like a warrior's shield.

The volcano is produced by the action of the gas (steam or water vapour) with heat from the earth's core. This action melts rock turning it into magma. The pressure from the heat of the gas pushes the magma upwards till it explodes. Molten magma shoots upward from deep below the ocean floor and breaks through the drifting plates to form shield volcanoes. Lava flows gently and continuously out of the central volcanic vent or group of vents. This lava is very runny, and can't be piled up into steep mounds. It gradually accumulates and cools around the volcano. The eruptions are usually not very violent and lava ‘oozes’ out of the cone

These types of volcanoes are built up slowly. The lava usually spreads widely over great distances then cools as thin gently dipping sheets. Lavas can also erupt from vents along fractures (rift zones) that form on the sides of the cone. Some of the largest volcanoes in the world are Shield volcanoes.

The picture shows White Island volcano in the Bay of Plenty. (Google Images)

Shield volcanoes may be produced by hot spots which lay far away from the edges of tectonic plates. Shields also occur along the mid-oceanic ridge, where sea floor spreading is in progress and along subduction related volcanic arcs.

Cinder cone volcanoes

Cinder cone volcanoes are the most common kind of volcanoes. They have steep sided cones of basaltic fragments and are smaller and simpler than composite volcanoes. Streaming gases carry liquid lava blobs into the atmosphere that fall back to earth around a single vent to form the cone. The volcano forms when ash, cinders and bombs pile up around the vent to form a circular or oval cone.

Cinders are melted volcanic rock that cooled and formed pebble-sized pieces when it was thrown out into the air. They are ejected from a single vent and accumulate around the vent when they fall back to earth.

Bombs are melted volcanic rock that cooled and formed large pieces of rock when it was thrown out into the air before landing on the ground

Cinder cones are chiefly formed by Strombolian eruptions. They grow rapidly and soon reach their maximum size. Cinder cones can occur alone or in small to large groups or fields. Most have a bowl-shaped crater at the summit. The longer the eruption, the higher the cone. They rarely exceed 250 meters in height and 500 meters in diameter, although some may rise to as high as 650 meters or more. If gas pressure drops, the final stage cinder cone construction may be a lava flow that breaks through the base of the cone. If a lot of water in the environment has access to the molten magma, their interaction may result in a maar volcano rather than a cinder cone. The shape of a cinder cone can be modified during its life. When the position of the vent alters, aligned twin cones develop. Nested, buried or breached cones are formed when the power of the eruption varies.

Mt Taranaki, New Zealand's second largest volcano is a good example of the cinder cone volcano. (Google Images)

Stratovolcano or Composite volcano

A stratovolcano, also known as a composite volcano is usually a tall, cone-shaped (conical) volcano that has been built up by many layers (strata) of hardened lava.Unlike shield volcanoes, stratovolcanoes are usually very steep. When they erupt they can be very explosive.

The lava that flows from stratovolcanoes usually cools and hardens before it spreads too far. This is because of its high viscosity.

Stratovolcanoes are sometimes called "composite volcanoes" because of they build up after successive eruptions and lava flows.They are among the most common types of volcanoes.

The lava often solidifies in the central pipe and this blocks the passage of the magma. It also traps the rising gas and prevents it from escaping. The pressure builds up which often leads to a violent explosion of ash and cinders at first, which will drop back around the vent. Lava then escapes and flows over the ash and cinders. Viscous lava solidies in the pipe once again, and this process is repeated again. Violent eruptions of ash and cinders followed by lava forms a composite volcano.

Mt Ruapehu our largest volcano – North Island

Ring of Fire

What is a geyser?

There are three requirements for a geyser: water, heat, and a special plumbing system below the surface. Yellowstone is one of only a handful of places in the world which meet these conditions. Its unique thermal features are a result of its volcanic origins.

The average continental crust is 30 kilometers thick. But, at Yellowstone, there is a hot spot where the crust is thinner, only about 10 kilometers thick. This causes volcanic eruptions. The last was was just over 600,000 years ago. Water enters the ground and enters the region above the hot spot. As it picks up heat, it heads towards the surface.

The plumbing system of the geyser connects it to the water reservoir. The boiling, highly pressurized water flows up the plumbing system of the geyser. Eventually it encounters the cooler water that is up at the surface of the geyser. As more hot water fills the geyser, more and more steam rises to the surface. Constrictions within the plumbing system and cooler water just below the surface block the hot water from traveling further upwards. When pressure from the hot water sufficiently builds up, the hot water and steam are forced up to the surface.

The release of some of the hot water causes the pressure to decrease, and more hot water is converted to steam. This steam expands up to 1500 times its original liquid volume and the geyser erupts. After the eruption, the pressure is released, and the cycle begins again with the geyser recharging the supply of hot water