What is a volcano?

A volcano is an opening in the Earth’s crust where magma – a mixture of red-hot liquid rock, mineral crystals, rock fragments and dissolved gases from inside the planet erupts onto the surface.

Are all volcanoes active?

Volcanoes are found in three states – extinct, dormant and active. An extinct volcano will never erupt again. A dormant volcano has not erupted in 2000 years. An active volcano has erupted recently and is likely to erupt again.

What are the main features of a volcano?

Are all volcanoes the same?

Composite volcanoes

Composite volcanoes, sometimes known as strato volcanoes, are steep sided cones formed from layers of ash and [lava] flows. The eruptions from these volcanoes may be a pyroclastic flow rather than a flow of lava. A pyroclastic flow is a superheated mixture of hot steam, ash, rock and dust. A pyroclastic flow can travel down the side of a volcano at very high speeds with temperatures over 400 degrees celsius. Composite volcanoes can rise over 8000 feet.

When composite volcanoes erupt they are explosive and pose a threat to nearby life and property. Eruptions are explosive due to the thick, highly viscous lava that is produced by composite cone volcanoes. This viscous lava has a lot to do with why they are shaped the way they are. The thick lava cannot travel far down the slope of the volcano before it cools.

Composite volcanoes are usually found at destructive plate margins. Examples of composite volcanoes include Mount Fuji (Japan), Mount St Helens (USA) and Mount Pinatubo (Philippines).

Shield volcanoes

Shield volcanoes are low with gently sloping sides and are formed from layers of lava. Eruptions are typically non-explosive. Shield volcanoes produce fast flowing fluid [lava] that can flow for many miles. Eruptions tend to be frequent but relatively gentle. Although these eruptions destroy property, death or injury to humans rarely occurs.

Shield volcanoes are usually found at constructive boundaries and sometimes at volcanic hotspots. Examples of shield volcanoes include Mount Kilauea and Maunaloa on Hawaii.

Dome (acid cone) volcanoes

Acid lava is much more viscous than lava which flows from shield volcanoes. Dome volcanoes have much steeper sides than shield volcanoes. This is because the lava is thick and sticky. It cannot flow very far before it cools and hardens. An example is Puy de Dome in the Auvergne region of France which last erupted over 1 million years ago.

Are all volcanic eruptions the same?

The simple answer is no! Volcanic eruptions are often thought of as cataclysmic explosions that produce vast quantities of lava, ash and other volcanic materials. However, volcanoes can actually erupt in a range of different ways. A volcano can erupt in a range of different ways during different eruptions and even during different stages in the same eruption.

Plinian eruptions

These the most explosive and violent of volcanic eruptions. They produce huge plumes of ash and gas that typically takes the shape of a huge mushroom cloud. Planian eruptions are also known as Vesuvian eruptions due to their similarity to the eruption of Mount Vesuvius in 79BC.

In Plinian Eruptions the magma has high silica content. They are highly explosive and the AD79 eruption that buried Pompeii and Herculaneum was one of these. Plinian eruptions are started by highly viscous magma that has high gas content. As the magma emerges it depressurizes and this allows the gas to expand, propelling pyroclastic material as high as 45 km in the air, at hundreds of feet per second, up and out of the Troposphere. These eruptions can last for days and create a sustained and tall eruption plume, which drops huge amount of tephra, fallen volcanic material, on surrounding areas. Additionally, a Plinian eruption can produce extremely fast moving lava flows that destroy everything in their path.

Features of a Plinian eruption

Hawaiian eruptions

In Hawaiian Eruptions the lava is more basic and basaltic, with low gas pressures and low silica content. This means the lava is very runny. These eruptions are generally not explosive or destructive and do not throw huge amounts of Tephra or pyroclastic material in the air. Instead they produce low-viscosity, low-gas-content lava that flows over large areas producing gently sloping shield volcanoes and lava plateaus. Eruptions can form fire fountains, Lava thrust up to 50m in the air for many hours. The general eruption style is a steady lava flow from a central vent, which can produce wide lava lakes, ponds of lava forming in craters or other depressions.

Strombolian eruptions

Strombolian eruptions are named after Stromboli in Italy. The effects are impressive but not particularly dangerous. They eject short bursts of lava 15 to 90 meters in the air. The lava has a fairly high viscosity (it’s quite thick due to its high silica content), so gas pressure builds up before material can be ejected from the volcano. These regular explosions can produce impressive booming sounds, however the eruptions are relatively small. Lava flows from Strombolian eruptions are not common though they do produce small amounts of ash.

Stromboli eruption

How do we measure the size of an eruption?

The Volcanic Explosivity Index (see table below) is the volcanic equivalent to the Richter Scale. The VEI is used to identify the magnitude of a volcanic eruption. The VEI is based on the height of the ash plume and an estimate of the volume of material that is expelled. Eruptions with small VEI numbers are far more common than those with a high VEI. Strombolian eruptions usually have a VEI of 1 or 2 whereas Plinian eruptions have a VEI of 5 or 6.

source: www.coolgeography.co.uk

Lava, it’s all the same, right?

Surprisingly it’s not! Molten rock that erupts onto the Earth’s surface is known as lava. The temperature of lava ranges between 700 and 1200°C. Lava flows under the influence of gravity. However, as it cools it forms solid (igneous) rock.

A basaltic lava flow from Mount Kilauea, Hawaii

Different volcanoes produce different types of lava. Lava composition varies in terms of temperature and composition, particularly their silica content. The hottest lavas with the lower silica content are quite runny. These are known as basaltic lavas. They are able to travel over long distances even over relief that is relatively gently sloping, before cooling and solidifying as basalt. At the other extreme lava that is relatively cool (around 700°C) with a high silica content form only slow moving flows. Rhyolite is an example of this type of lava.

Source: www.coolgeography.co.uk

What is a pyroclastic flow?

A pyroclastic flow is a fast-moving current of hot gas and rock (collectively known as tephra), which reaches speeds moving away from a volcano of up to 700 km/h (450 mph). The gas can reach temperatures of about 1,000 °C. Pyroclastic flows normally hug the ground and travel downhill, or spread laterally under gravity. Their speed depends upon the density of the current, the volcanic output rate, and the gradient of the slope. They are a common and devastating result of certain explosive volcanic eruptions.