The important point to understand is that the plume is stationary. The point where the hot material breaks out of the mantle and on to the surface stays in the same place. However, where it reaches the surface changes with time, because plates move , as the theory of plate tectonics tells us.
Above the hot spot a volcano forms and grows and grows until the plate carries it away. Once the volcano has been carried away and no longer sits over its hot spot, it loses its source of magma. A new volcanic cone begins to form on top of the hot spot.
A few active volcanoes do exist in locations well away from any plate margin. These are volcanic hot spots . They may be in the ocean, which is where the best known example, the Hawaiian islands , lies. Others are on land, for example the volcanic region around Rotorua in the North Island of New Zealand.
Iceland is unique in that it is a hot spot but is located on a mid ocean ridge. As their name suggests, these are points where molten material from the mantle breaks through the rocks of the Earth’s crust to reach surface. They are caused by convectional plumes of very hot magma in the mantle which burn through the crustal rocks to reach the surface.
The Hawaiian islands are a perfect illustration of this. All the Hawaiian islands are volcanic. The nearest land is over 3,000 km away. The islands lie on top of the Pacific plate which is moving north westwards by about l0cm a year.
Volcanic activity today only takes place in the south-east corner of the island chain on the big island, called Hawaii. Mauna Loa is a huge volcano It rises 9000 m from the floor of the Pacific Ocean to its summit, which itself is 4,170m above sea level. At its ocean base it is over 90 km wide
The magma comes from the mantle, it is very hot and as it is basalt it has a low viscosity. The activity now seems to be focused more and more on the south east part of the island where Kilauea has been active for over 150 years. Here we see the lava fountains and lava fountains. More recent eruptions have occurred on the coast.
Subsidence due to the pressure &/or erosion by waves the islands can transform into a flat toped sea mount called a guyot.
Thank you for taking the time to listen to my presentation which is arguably the best presentation in the class.