Volcanoes are common occurrences along the boundaries of Earth’s tectonic plates. These boundaries allow super-heated molten rock called magma, along with ash and gases, to rise through Earth’s crust and emerge on the surface, often dramatically. Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge.
It was volcanically active just over 100 million years ago. The youngest is Nashville Seamount, which formed about 80 million years ago. The volcanoes rise from 500 m to several kilometers above the surrounding seafloor, and they are conical in shape, often with flat tops. Submarine landslides down the sides of many of these volcanoes have changed their shapes.  Map of the entire track of the hotspot that formed the New England Seamounts and other associated volcanoes, from the Monteregian Hills (red dots) in the west to Great Meteor Seamount in the east. The bottom graph shows in red the height of the land and depth of the seafloor along the hotspot track.  When magma reaches the level of the seafloor, it meets cold ocean water and quickly cools to form basaltic rock, often termed pillow lava due to its rounded shape. This pillow lava, along with slower-cooling magma beneath it, forms the vast majority of oceanic crust. Volcanoes are common occurrences along the boundaries of Earth’s tectonic plates. These boundaries allow super-heated molten rock called magma, along with ash and gases, to rise through Earth’s crust and emerge on the surface, often dramatically. A seamount is a mountain rising from the ocean seafloor that does not reach to the water's surface (sea level), and thus is not an island. Seamounts are typically formed from extinct volcanoes that rise abruptly and are usually found rising from the seafloor to 1,000–4,000 metres (3,300–13,100 ft) in height. Seamounts can be found in every ocean basin in the world, distributed extremely widely both in space and in age. A seamount is technically defined as an isolated rise in elevation of 1,000 m (3,281 ft) or more from the surrounding seafloor, and with a limited summit area, of conical form. Most seamounts are volcanic in origin, and thus tend to be found on oceanic crust near mid-ocean ridges, mantle plumes, and island arcs. Overall, seamount and guyot coverage is greatest as a proportion of seafloor area in the North Pacific Ocean, equal to 4.39% of that ocean region. Seamounts can be found in every ocean basin in the world, distributed extremely widely both in space and in age. A seamount is technically defined as an isolated rise in elevation of 1,000 m (3,281 ft) or more from the surrounding seafloor, and with a limited summit area, of conical form. Age of oceanic crust; youngest (red) is along spreading centers. Seafloor spreading is a process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge. Seafloor spreading helps explain continental drift in the theory of plate tectonics. The idea that the seafloor itself moves (and carries the continents with it) as it expands from a central axis was proposed by Harry Hess from Princeton University in the 1960s. Frequent eruptions along divergent plate boundaries such as the Mid-Atlantic Ridge form new ocean bottom in a process known as seafloor spreading. This process occurs slowly (1-15 centimeters per year) yet relentlessly, moving the plates—and the ocean floor and continents above them—further apart. Volcanoes are common occurrences along the boundaries of Earth’s tectonic plates. These boundaries allow super-heated molten rock called magma, along with ash and gases, to rise through Earth’s crust and emerge on the surface, often dramatically. As the plate moves over a fixed spot deeper in the Earth where magma (molten lava) forms, a new volcano can punch through this plate and create an island. The Hawaiian Islands are believed to be formed from one such 'hot spot'. As the plate moves away, the volcano stops erupting and a new one is formed in its place. With time, the volcanoes keep drifting westward and getting older relative to the one active volcano that is over the hot spot. 
http://www.divediscover.whoi.edu/hottopics/seamounts.html http://oceanexplorer.noaa.gov/explorations/05stepstones/background/geologic_history/geologic_history.html http://oceanexplorer.noaa.gov/facts/volcanoes.html https://en.wikipedia.org/wiki/Seamount https://en.wikipedia.org/wiki/Seamount https://en.wikipedia.org/wiki/Seafloor_spreading http://oceanexplorer.noaa.gov/facts/volcanoes.html http://www.soest.hawaii.edu/GG/HCV/haw_formation.html
how are seafloor volcanoes formed