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Islands
Land Surrounded by Water
How islands form and which ones people misplace.
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What is the largest island in the world?
How Islands Are Made
Almost every island on earth was made by one of a small number of processes. Work out which one and you can usually predict the shape of the island before you see it.
About this topic
There is no single way to make an island. A volcano can build one from the seabed up, a reef can keep growing while the volcano beneath it sinks, plates colliding can throw up a curved line of them, and rising seas can simply drown a landscape and leave the hilltops sticking out. This quiz works through those mechanisms with real examples: Hawaii for hotspots, the Aleutians for subduction, Norway for glaciation, and an atoll in the Marshall Islands where drilling finally settled an argument Darwin started in 1842.
Questions & answers
How Islands Are Made: all 10 questions and answers
Every question in this quiz is listed below with its correct answer and the reasoning behind it. Play first if you would rather not see the answers — or read through as a study sheet.
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1The Hawaiian island chain formed as the Pacific plate drifted over what?
- A A subduction zone
- B A rift valley
- C An ocean trench
- D A hotspot Correct
Answer: D. A hotspot
Why: The plume of hot mantle stays roughly in place while the plate moves north-west over it, so the islands get older in that direction. The oldest have eroded away entirely and survive as submerged seamounts.
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2Darwin's explanation for how atolls form says that the central volcanic island does what?
- A Erodes into sand that piles up in a ring
- B Splits apart along a fault to leave a circle
- C Sinks slowly while the reef around it keeps growing upward Correct
- D Rises as coral is pushed above the waves
Answer: C. Sinks slowly while the reef around it keeps growing upward
Why: He published the subsidence theory in 1842 after the Beagle voyage, with no way to test it. Drilling at Enewetak in 1952 hit volcanic rock beneath more than a kilometre of coral, exactly as he had predicted.
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3The Aleutian, Kuril and Mariana chains are all examples of what?
- A Barrier islands
- B Volcanic island arcs Correct
- C Coral atolls
- D Continental fragments
Answer: B. Volcanic island arcs
Why: Island arcs form where one oceanic plate dives beneath another and the melted rock rises to build a curved line of volcanoes. The deep trench always lies on the ocean side of the arc.
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4Surtsey, an island that rose out of the sea during an eruption in 1963, lies off which country?
- A Iceland Correct
- B Norway
- C Japan
- D New Zealand
Answer: A. Iceland
Why: The eruption continued until 1967 and built an island of fresh volcanic rock from nothing. Access is restricted to scientists so the arrival of plants, insects and birds can be recorded from the very beginning.
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5A barrier island is best described as what?
- A A ring of coral enclosing a shallow lagoon
- B A rocky outcrop left behind by a retreating glacier
- C A wedge of silt built up where a river meets the sea
- D A long sand ridge lying parallel to the coast behind a lagoon Correct
Answer: D. A long sand ridge lying parallel to the coast behind a lagoon
Why: Waves and longshore drift build them and keep reshaping them, so they migrate along and across the shore over time. The Outer Banks of North Carolina are the best-known example.
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6Great Britain is classified as which type of island?
- A A coral island
- B A barrier island
- C A continental island Correct
- D A volcanic island
Answer: C. A continental island
Why: It sits on the continental shelf and is separated from Europe only by shallow water. The land bridge of Doggerland connected the two until rising seas finally cut it off around eight thousand years ago.
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7Which Hawaiian island currently sits over the hotspot and remains volcanically active?
- A Kauai
- B Hawaii Correct
- C Maui
- D Oahu
Answer: B. Hawaii
Why: The Big Island holds Kilauea and Mauna Loa, both of which still erupt. Kauai at the far north-west end of the chain is several million years older and deeply eroded.
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8The thousands of rocky islands along the coasts of Norway, Sweden and Finland were mainly shaped by what?
- A Glacial erosion during the ice ages Correct
- B Volcanic eruptions along a rift
- C Coral growth in shallow water
- D Wind-blown sand building into dunes
Answer: A. Glacial erosion during the ice ages
Why: Ice sheets scoured out the valleys and hollows, and the sea flooded them when the ice melted, leaving the higher ground as islands. The same process produced the fjords cut into the mainland behind them.
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9Iceland owes its volcanic activity to sitting astride which feature?
- A The Mariana Trench
- B The East Pacific Rise
- C The Ninetyeast Ridge
- D The Mid-Atlantic Ridge Correct
Answer: D. The Mid-Atlantic Ridge
Why: Iceland is one of very few places where a mid-ocean ridge rises above sea level. The island is being pulled apart as the North American and Eurasian plates separate beneath it.
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10Krakatoa, which erupted catastrophically in 1883, sits between which two islands?
- A Bali and Lombok
- B Luzon and Mindanao
- C Java and Sumatra Correct
- D Borneo and Sulawesi
Answer: C. Java and Sumatra
Why: The volcano stands in the Sunda Strait, and the eruption destroyed most of the island it had built. A new cone named Anak Krakatau, meaning child of Krakatoa, has since grown in the same place.
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