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It depends on the type: volcanic ones arise from the accumulation of lava and continental ones can arise from soil erosion or the accumulation of sediments. There are also other less common types, such as atolls, which are formed when coral reefs settle over submerged rocks. These are the most common types, defined at school as “pieces of land surrounded by water on all sides”, although this concept is not exact – on continents, for example, there are pieces of land surrounded by rivers and lakes, but not are considered islands. :-0
THERE GO LAVA
Underground volcanic action can form islands in different ways
TRADITIONAL VOLCANIC
The floor of the deep sea is full of hot spots, places where magma from the Earth’s interior presses against the surface of the deep sea, forming volcanoes. These volcanoes on the sea floor erupt and, over millions of years, expel lava, which accumulates and, when crossing the water line, forms an island.
MOUNTAIN VOLCANIC
Magma from the Earth’s interior can also take advantage of gaps in tectonic plates to ask for passage. As the magma is expelled, it forms underwater mountain ranges that grow and give rise to islands. These undersea chains, called mid-ocean ridges, extend across the globe but form few islands. The most famous is Iceland.
SHOCK VOLCANIC
At the bottom of the sea, there are areas where plates collide and one slides under the other. These are called subduction zones. As this happens, pressure from the sinking slab causes lava to be forced out onto the remaining slab. This forms volcanoes, which form islands. As the movement of plates in subduction zones is constant, the areas where they occur are highly subject to earthquakes and even tsunamis. Japan was formed through this process, which continues to this day.
ON FIRM LAND
See how islands are formed within continents
SEA
A piece of land on the coast of the continent begins to suffer erosion, caused by the action of sea currents in the place. The soil wears away and, over time, the hole is so big that one of the pieces of land separates from the other on the surface, even though they remain united at the bottom of the sea.
FRESH WATER
In rivers and lakes, the accumulation of waste such as earth and sand generates a mound of earth, which rises and exceeds the waterline, as in Ilha do Bananal (TO)