Volcanoes (Sample Lesson)
A volcano occurs when hot material from inside the Earth erupts to the surface. For billions of years, volcanoes have been shaping and changing our planet. On Earth there are over 1,500 active volcanoes on land and below the sea, with more than 50 eruptions in the average year.
Volcanoes are a result of the tectonic activity taking place below the surface of Earth, as the huge plates of the Earth’s crust shift, move apart, or collide. Hot molten rock is known as magma, or lava once it reaches the Earth’s surface, and this magma can be found in pockets of the Earth’s crust or in the hot mantle below. Volcanoes are usually found at destructive (or convergent) boundaries where one tectonic plate is forced below another, and there is heating and melting of the rocky crust, such as the ‘Ring of Fire’ around the edges of the Pacific Ocean.
Volcanoes are described as being ‘active’ if they are known to have recently erupted, ‘dormant’ if they haven’t erupted recently but could do at some point, or ‘extinct’ when they will not erupt again. This might be because their source of hot magma has been lost; for example, because they have moved away from a ‘hot spot’ through tectonic motion, or because the magma has all fully cooled below the volcano.
⚠ MISSION OBJECTIVE
Thanks to Avanti’s World, you can safely visit the site of an erupting volcano! Once there, your task is to make observations around three themes:
- Volcanic Eruption: On the Ground
- Volcanic Eruption: In the Air
- Volcanic Eruption: Under the Surface
- Impact on Humans
On Earth we find different types of volcanoes as well as different types of eruptions. The characteristics of the magma, such as how thick (viscous) it is or how much gas is trapped inside, determine the types of eruptions and the shapes of the volcanoes that form.
Take a look at Mount Yasur Volcano in 360 below:
Shield volcanoes tend to have more frequent eruptions of lava which steadily flows down the sides of the volcano to slowly build up a gently-sloping volcano shape. In contrast, steep-sided composite volcanoes are formed when the thick, viscous lava flows more slowly before solidifying, and so slopes are built into a steep shape. These volcanoes can have explosive eruptions, which are less frequent but more destructive than the eruptions of shield volcanoes.
Volcanic eruptions are often accompanied by the release of gases and ash clouds into the air. The plumes of ash can sometimes stretch several kilometres up into the atmosphere, before travelling on the wind currents, and eventually the tiny grains of dust fall down to the ground. In the explosive eruptions from composite volcanoes, sometimes dangerous pyroclastic clouds of scorching hot gas and dust can flow down the mountain slopes, causing widespread damage and destruction.
What exactly is happening inside this volcano that causes such a violent eruption? Find the cutaway section of the volcano and look closely at what you can see inside.
- What is happening in the centre of the volcano?
- What might be producing enough force to push magma out from under the Earth’s crust?
As you saw in the scene, magma is forced up from under the Earth’s crust and emerges from the volcano. This can happen at destructive boundaries between tectonic plates, like the one shown in the diagram above. Here, an oceanic plate is being forced (subducted) underneath a denser continental plate, where molten rock wells up and forms volcanoes.
Volcanoes can also form at constructive boundaries, where two tectonic plates are moving away from one another. In this situation, the magma rises up between the gap between the plates.
Over long periods of time, the lava which flowed from previous volcanic eruptions will weather down into very fertile soils, making the areas surrounding volcanoes ideal for growing crops. In some parts of the world, volcanoes are a pull for tourism, and associated businesses, such as tours and accommodation, can grow around these amazing natural attractions.
We should always be mindful that volcanoes can be unpredictable and destructive, and so the human cost of living in close proximity can be devastatingly high. An unexpected eruption can destroy homes, villages, and even cause the loss of lives.
Explore the scene and look for evidence of human activity in the area, from buildings to agriculture.
- What are the risks of living in close proximity to volcanoes?
- Could there be any benefits to living in an area like this?