Land Ahoy!

You may have seen a few months ago that an earthquake created a new island off the coast of Pakistan. This island is around 200m long, 100m wide and 20m high, and formed as two plates pushed together, pushing the seabed up.
A second island, which is also around 200m long, has now been seen off the coast of Japan, a few thousand km's south of Tokyo. It formed after an underwater volcanic eruption occurred, but it currently isn't known if the island will stay, or if it will go.
The National Geographic has gone through other ways in which islands can form. Continental islands were originally connected to a larger landmass, but they broke off as the shifting continents moved apart. They can also form when there is an increase in sea level, covering low lying areas and creating smaller islands.
Tidal islands form when parts of a continent are eroded. The erosion is not complete, but when high tide occurs, it allows an island to form.
Barrier islands lie parallel to coastlines and are made of sediment or coral. Sometimes they form as currents pile up sand on sandbars near coastlines, allowing them to rise above the water. Another way they can form is after a glacier has retreated, leaving behind piles of rock. Floods cause these piles to be separated from the surrounding continent, allowing them to form islands.
Ocean/volcanic islands, such as the one off the coast of Japan, form during eruptions of volcanoes on the sea floor. As the volcanoes erupt, layers of lava are built up, allowing them to eventually reach above the water level.
Coral islands, as the name suggests, are made of colonies of corals that build up to form huge reefs, which can sometimes reach the sea level surface.
Finally, artificial islands are formed by people. They are created from material that is brought in from elsewhere and added to the seafloor, to eventually build up into an island.
I hope you've enjoyed this brief overview of how islands are formed!