how are islands made?

Answer 1

That depends on the type of island you are asking about.

Volcanic islands develop as the volcano builds itself up over a series of erruptions.

River islands are developed by the river flowing and cutting into the land on either side of the river. Sometimes the water cuts a wide chunk of land off and forms an island. Islands can also be formed by the slowing of the river causing a build up of silt being carried by the water (these rarely amount to more than sandbars or spits).

Other islands are formed by plate tectonics. Two plates rub against each other and one is thrust upwards Or two plates smash together and both get pushed upwards.

Answer 2

There are continental islands, oceanic islands, coral islands and barrier islands

Continental islands are parts of the continental shelf that rise above the surrounding water—Greenland and Newfoundland

Oceanic islands tend to be volcanic islands. They are usually in a pattern as they follow plate boundaries, along the mid-ocean ridges and are associated with sea-floor spreading. Some islands are just a build up of cooled magma on top of cooled magma. Some are actually the volcano tops
Peaks of underwater mountains (folding) may be above water forming islands.

Coral islands formed by the once living coral polyps leaving behind their limestone exoskeletons.

Barriers Islands - shallow-water, coastal areas tend to be more sand based.

Answer 3

Some are the result of continental drift - they are left behind as land masses drift apart.
Others are the tops of volcanoes. The tallest mountain in the world is the top of a volcano - Mauna Kea in Hawaii (or Hawai'i) - it's the tallest from bottom to top since most of the bottom is under water.

Answer 4

By the changes in sea pressure due to the motion geological plates of earth. Also other factors could be erosion, volvanic eruptions and earth quakes.