Volcanic Island

An island or isle is any piece of sub-continental land that is surrounded by water. Very small islands such as emergent land features on atolls can be called islets, skerries, cays or keys.
A volcanic island is not formed at a plate boundary. It forms as a result of narrow mantle plumes bringing material from deep within the lower mantle. The magma of both volcanic islands and mid-ocean ridges forms as hot, but solid rock moves closer to Earth’s surface. As the hot rock rises, the pressure drops, and the rock melts, forming magma. The melted lower mantle material forms runny basalt magma that is less dense than the surrounding rock. The basalt magma melts its way through the lithospheric plate above it. An underwater volcano forms when the magma breaks through the surface of the plate. If the eruption is strong enough, the magma will reach the surface of the sea, forming a volcanic island.

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The plate on which a volcanic island sits is moving, but the mantle plumes stay in one place. The top of an established mantle plume is called a hot spot. Hot spots are places within the mantle where rocks melt to generate magma. The presence of a hot spot is inferred by anomalous volcanism (i.e. not at a plate boundary), such as the Hawaiian volcanoes within the Pacific Plate. As the plate moves, it carries the volcanic island away from the hot spot that formed it. Without the hot spot to supply magma, the volcano becomes extinct. At the same time, the hot spot begins to form a new volcano beside the old one. In this way, a volcanic island chain is formed. Mantle plumes are areas of hot, upwelling mantle. A hot spot develops above the plume. Magma generated by the hot spot rises through the rigid plates of the lithosphere and produces active volcanoes at the Earth's surface. As oceanic volcanoes move away from the hot spot, they cool and subside, producing older islands, atolls, and seamounts. As continental volcanoes move away from the hot spot, they cool, subside, and become extinct.
The Hawaiian volcanoes were produced by the Hawaiian hot spot. Presently, it resides under the Big Island of Hawaii. In general, when you move along the island chain from southeast (Hawaii) to northwest, (Kauai), the volcanoes become older and older with ages from 400,000 to more than 5 million years. Further out along the archipelago, they are significantly older. For example, the island of Midway is more than 27 million years old and others still further out along the chain date back almost 70 million years from when they passed over the Hawaiian hotspot.
Currently there are 3 Hawaiian volcanoes that are classified as active: Kilauea, actively erupting since 1983; Mauna Loa, which last erupted in 1984 and is building for a new eruption in the next few years; and Loihi, which erupted in 1996. All three of these active Hawaiian volcanoes share the Hawaiian hot spot, but retain unique volcanic histories and compositions.

Reference
Wikipedia
http://volcano.oregonstate.edu/what-is-a-hot-spot
https://www.soest.hawaii.edu/GG/HCV/haw_formation.html
https://www.soest.hawaii.edu/GG/ASK/hawaii_volcano_age.html