Volcanoes in the Middle of Tectonic Plates

This GCSE Geography quiz will challenge you on volcanoes in the middle of tectonic plates. By now, you will no doubt have learnt that volcanoes occur in narrow belts in certain areas of the world. These belts also suffer earthquakes and they mark out the areas where the Earth's tectonic plates are coming together or moving apart. But there are some volcanoes that occur well away from the edges of the plates. These create natural hazards that are a danger to life when they erupt.

Volcanoes that form away from plate boundaries are created by hotspots. A hotspot is an area in the mantle that is believed to be somewhat hotter than the surrounding material. The rising magma from the hotspot can push its way through the crust and burst out at the surface, forming a volcano.

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As the plate moves over a hotspot, it leaves a trail of extinct volcanoes. These can help to show how far and in which direction the plate has moved in the past. Earth scientists still don't understand hotspots very well, there are some theories but nothing has been proved yet.

The usual example of volcanoes of this type are the Hawaiian Islands in the Pacific. These are a chain of volcanic islands and you can clearly see that they have formed in a line. The oldest are to the north west and are no longer volcanically active. The youngest is the biggest, and the only one that has volcanoes. There is a new volcano forming underwater about 35 km from this island, one day, it will grow enough to be above sea level. Measured from the sea bed to their summits, the volcanoes of Hawaii are some of the world's tallest mountains.

Other sites of this type of hotspot volcano are the Galapagos Islands, the Canary Islands and most of the small islands of the south Pacific. They are formed from shield volcanoes. A shield volcano erupts basaltic lava (when it solidifies it forms the igneous rock basalt). This type of lava has a low viscosity (flows easily) and so shield volcanoes have wide bases and gentle slopes. The other basic type of volcano is a composite cone. This has steeper sides and erupts a more viscous lava. Its vent can become blocked which leads to powerful explosions as the pressure builds up. The magma that feeds this type of volcano is said to be acidic and the lava cools to form igneous rocks like andesite and rhyolite.

Hotspot volcanoes occur under continental land masses as well as under oceans. You have probably heard of the Yellowstone supervolcano in the USA. Yellowstone Park lies above a large magma chamber which has erupted several times during the last few million years. A supervolcano releases much larger amounts of material (at least 1000 km3) to a 'normal' volcano. To give you an idea of how big that is, if you dug a hole 1 km wide and 1 km deep, it would need to be 1000 km long to have the same volume as the smallest amount of material erupted from a supervolcano! Supervolcano eruptions from the Yellowstone hotspot can be traced back about sixteen million years and were larger than the well-known Krakatoa eruption in Indonesia. But don't worry, Earth scientists are monitoring it closely and they do not believe that it is likely to erupt any time soon!

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  1. Underwater hotspot volcanoes are:
    They can grow to be so large that they form islands above the surface of the sea
  2. Mauna Kea is a volcano on the largest of the Hawaiian islands which lie in the middle of the Pacific tectonic plate. What type of volcano is it likely to be?
    Even though you can't see most of it (unless you have a submarine!!), counting its height from its base on the sea bed to its summit, it is the tallest mountain on Earth, taller even than mount Everest
  3. How can you describe the lava from a shield volcano?
    The lava that forms a shield volcano comes directly from the mantle
  4. The Deccan Traps in India is an area of lava flows created when India was over the same hotspot that powers the large volcano on the Isle of Reunion. Some scientists believe that it could have been responsible for the extinction of the dinosaurs. Which of the following effects would be the most likely to have killed off the dinosaurs worldwide?
    Hotspot eruptions don't produce large earthquakes, the lava could not have flowed all over the Earth but the gases could have spread round the whole of the atmosphere creating climate change
  5. About 250 million years ago, almost all of the life on Earth became extinct. There was a massive volcanic eruption that lead to the formation of the Siberian Traps at the same time. Some geographers believe that the eruptions that created this geographical feature came from a hotspot volcano. Which of the following pieces of evidence might help to support that idea?
    The word trap comes from the Swedish word for stars and descibes the appearance of the region. The lava flows form one on top of the other in a series of steps
  6. The hotspot volcano Piton de la Fournaise is a volcano on the island of Reunion in the Indian Ocean. This is very active and most years, lava from the volcano destroys the road that leads up towards the summit. This is an example of a:
    A primary effect is one that is caused directly by the eruption of a volcano
  7. Which of the following is a secondary effect of the eruption of a volcano?
    Even hotspot volcanoes can produce a lot of ash during an eruption though most of them produce only lava and gases
  8. The volcanic event that created the Siberian Traps appears to have created a major global warming event. Which of the following gases could have been released?
    Out of the four gases, only carbon dioxide is associated with global warming. Sulphur dioxide leads to the formation of sulphuric acid in the atmosphere which has the opposite effect, causing global cooling
  9. Yellowstone National Park in the USA is famous for its hot springs and geysers. It is nowhere near either a destructive or constructive plate boundary so which of the following is the most likely to be the source of heat?
    The hotspot under Yellowstone National Park has been active for about 16 million years and the line of volcanoes it has created show the American plate has moved west and south-west during that time
  10. Mauna Loa, an active volcano of the Hawaiian islands, erupted in 1984. The nearest town is Hilo which is 50 km away. Hilo was not evacuated during the eruption. Why not?
    Even though the lava from a hotspot volcano is regarded as being a runny lava, by the time it has travelled 50 km, it has cooled a lot and is only moving slowly (takes several weeks to cover a kilometre)

Author: Kev Woodward

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