Such islands are formed of peaty masses of vegetation that rise up from the floors of shallow lakes due to the accumulation of gases as a result of decomposition of the vegetative matter. Such islands are ephemeral in nature and stay afloat till the gasses dissipate following which the vegetation sinks into the lake.
https://www.worldatlas.com/articles/how-are-lake-islands-formed.html
Reef islands may occur in isolation or in a chain along the length of the reef. Sometimes they take the form of long strips of land occupying most of the length of the central area of a reef top. There are two completely different kinds of origin of reef islands: uplift and accretion.
https://www.britannica.com/science/coral-island
Below is a list of new islands created since the beginning of the 20th century whether by means of volcanism, erosion, glacial retreat, or other mechanisms. One of the most famous new volcanic islands is the small island of Surtsey, located in the Atlantic Ocean south of Iceland. It first emerged from the ocean surface in 1963. In 1965, it was declared a nature reserve for the study of ecological succession; plants, insects, birds, seals, and other forms of life have since established themselves on the island.
https://en.wikipedia.org/wiki/New_islands
A landmass surrounded on all sides by the waters of a lake is a lake island. Such islands are formed in various ways. Accumulation of sedimentation over a period of time, separation of a landmass from the mainland surrounding the lake by erosion, influences of earthquakes, volcanic activities, meteor impacts, etc., also might lead to the formation of lakes and lake islands.
https://www.worldatlas.com/articles/how-are-lake-islands-formed.html
Sometimes, when a complex impact crater is formed, a central peak of the crater emerges from the crater’s floor. If the crater is filled with water and the central peak emerges above the surface of the water, then it forms an island. The René-Levasseur Island located in Canada’s Lake Manicouagan is an example of such an island.
https://www.worldatlas.com/articles/how-are-lake-islands-formed.html
The newly formed land is soon colonized by plants and animals, which also contribute their own remains to the island, helping soil to develop. Many of the reef islands in the central and southern Pacific and of the Maldive Islands in the Indian Ocean originated in this way. Reef islands, especially those close to sea level, are not very stable.
https://www.britannica.com/science/coral-island
Another noted new island is Anak Krakatau (the so-called child of Krakatoa, which formed in the flooded caldera of that notorious volcano in Indonesia), which emerged only in 1930. Ample rainforests have grown there, but they are often destroyed by frequent eruptions.
https://en.wikipedia.org/wiki/New_islands
If the volcano restarts its activity or continues to spew out magma then a resurgent dome might be formed by the upliftment of the center of the caldera. This dome that protrudes out from the surrounding waters of the crater lake is a crater island. An example of such an island is the Samosir Island in Lake Toba in Indonesia’s Sumatra island.
https://www.worldatlas.com/articles/how-are-lake-islands-formed.html
1 Coral reefs take four main forms. Fringing reefs consist of a flat reef area directly skirting a nonreef island, often volcanic, or a mainland mass. 2  Coral reef islands occur in association with all categories of reef but especially on reefs whose flat tops are well developed, perhaps a kilometre or more in width.
https://www.britannica.com/science/coral-island
List of recently created islands. This is a list new islands that formed during the 20th and the 21st centuries. Of those formed by submarine volcanoes, only Anak Krakatau, Surtsey, Nishinoshima, Sholan, and Jadid are currently islands, and Surtsey is the only one that is expected to survive. Ilha Nova is now connected to Faial Island.
https://en.wikipedia.org/wiki/New_islands
how was a islands formed
Islands are formed of peaty masses of vegetation that rise up from the floors of shallow lakes due to the accumulation of gases as a result of decomposition of the vegetative matter. Such islands are ephemeral in nature and stay afloat till the gasses dissipate following which the vegetation sinks into the lake.