Volcanoes

Introduction

This chapter looks at different types of volcanoes, their history and structure. It also covers volcanic eruptions and their impact on our planet.

What is a volcano?

According to ancient Roman mythology, Vulcan was the god of fire. The Romans believed that when he was angry he could make mountains 'spit fire'. A few thousand years of scientific research later people have realised that some mountains 'spit fire' because of plate tectonics and not because of the mood swings of some cranky god, but they kept the name of the volcano anyway.

A volcano is a mountain where hot gases and molten rocks come through the Earth's crust. This process is called volcanic eruption. See image 1.

Volcanic eruptions

Volcanic eruptions usually happen in the same places. If we look at the map of the Earth's tectonic plates we will see that the majority of volcanic eruptions occur on the boundaries of these plates. That happens because eruptions are caused by plate tectonics. When tectonic plates collide or pull apart they break and change the inside layers of the Earth. When the Earth's crust breaks, the hot liquid rock of the mantle escapes to the surface. The mixture of molten rock, crystals and dissolved gases from the mantle is called magma. When magma erupts to the Earth's surface, it is called lava.

Not all volcanic eruptions happen near the boundaries of tectonic plates. Some eruptions happen in the middle of the plates. These eruptions are caused by mantle streams called plumes. Mantle plumes are formed when hot magma rises beneath the thinner, or weaker, areas of the Earth's crust, creating 'hot spots'. Volcanic islands are formed when mantle plumes form at the bottom of the ocean.
Not all volcanic eruptions are strong. Many volcanoes just gently ooze lava without blowing up. Sudden eruptions happen when magma is released after being trapped underground for a while. Some volcanoes are very active, erupting year after year. Others are 'asleep' or dormant. Dormant volcanoes are volcanoes that have not erupted for a long time. Some volcanoes can become extinct, which means they stop erupting altogether and become just 'peaceful' mountains.

Volcanic structure

A volcano consists of a several main parts: vent, pipe, crater and cone. See image 2. The vent is an opening in the Earth's crust. Magma comes out through the vent. The pipe is a 'corridor' through which magma rises to the surface during an eruption. The crater is a bowl-shaped hole at the top of the volcano. Volcanic materials like ash, lava, cinders and different gases form the volcanic crater. Some gases are toxic and they can suffocate people. Ash, lava and cinders are different forms of the Earth's magma and rocks that have been broken into pieces. When they cool down and harden they form the volcanic cone. Volcanic lava and ash are mixed, forming the volcanic cone.

Types of volcanoes

There are several different types of volcanoes. See image 3. They are:

Shield volcanoes

Large, slowly erupting volcanoes are called shield volcanoes that are built by many layers of lava. Hot, liquid lava spills out of the central vent or group of vents and forms a broad-shaped, gently sloping cone. Shield volcanoes are usually formed at the hot spots of the Earth's crust which are away from the edges of tectonic plates.

Submarine volcanoes

Underwater volcanoes are called submarine volcanoes. The eruption of a submarine volcano is spotted when steam and rocks come out of the water above the volcano. Sometimes these eruptions go unnoticed because the volcano is deep in the ocean. Pillow lava is formed under water after submarine volcanic eruptions. Pillow lava consists of big, round lumps of rocks that look like huge pillows.

Cinder cone

A cinder cone is a steep, conical hill formed above the volcanic vent. Cinder cones are very common volcanic landforms.They are built from small, light lava pieces called cinders.

Strato volcanoes

A large, steep-sided volcano made from many layers of lava and rock pieces is called a strato volcano. Sometimes these volcanoes are also known as composite volcanoes. Strato volcanoes often form snow-capped peaks that can be as high as 2500 metres. Between eruptions, strato volcanoes are often so quiet they seem extinct.

Subglacial volcanoes

Subglacial volcanoes form underneath the ice caps. When the ice melts it breaks the volcanic lava top leaving the volcano flat-topped. A few sub-glacial volcanoes can be found in Iceland. These volcanoes are also called table volcanoes, or mobergs.

Volcanic eruptions and the universe

The universe is made of all matter, space, energy and time. Everything around us is part of the universe. Volcanic eruptions have a great impact on Earth and its different systems.

Atmosphere

Gases surrounding a celestial body make up its atmosphere. Volcanic eruptions affect the Earth's climate by throwing gases and dust particles into the atmosphere which may warm or cool the Earth's surface, depending on how sunlight interacts with the volcanic material.

Hydrosphere

The part of the Earth composed of water is called the hydrosphere. Submarine volcanic eruptions cause the ocean's water temperature to rise. Volcanic gas and ash cause the water temperature to fall. Volcanic eruptions melt mountain ice caps and interfere with atmospheric clouds and seawater levels.

Lithosphere

The Earth's crust and the upper layer of its mantle make up the system called the lithosphere. The powerful forces of volcanic eruptions can change, destroy or create new types of rocks and landforms.

Biosphere

The portion of the Earth and its atmosphere that can support life is called the biosphere. Most volcanic eruptions destroy most forms of life around them by hot lava and poisonous gases.