Final answer:
It is true that mid-ocean ridges have more volcanoes than island arcs. The process of sea-floor spreading at divergent plate boundaries leads to the formation of many underwater volcanoes along mid-ocean ridges, which are more numerous than those found in island arcs formed at convergent boundaries.
Step-by-step explanation:
The student asks whether it is true that mid-ocean ridges have more volcanoes than island arcs. The answer is true. Mid-ocean ridges, such as the Mid-Atlantic Ridge, are formed by divergent plate boundaries where the Earth's tectonic plates pull apart, causing magma to rise and create new oceanic crust. This process results in a large number of underwater volcanoes along these ridges, which are more abundant than the volcanoes found in island arcs associated with subduction zones.
In comparison, island arcs are formed at convergent plate boundaries, where one tectonic plate is forced beneath another into the mantle. This causes partial melting of the asthenosphere above the subducting slab, and the resulting magma rises to form a chain of volcanic islands. While these are also sites of significant volcanic activity, the linear nature and scale of mid-ocean ridges contribute to a higher concentration of volcanoes.
Another form of volcanic activity is the 'hot spot' volcanism, exemplified by the Hawaiian Islands, where mantle plumes result in volcanic activity away from plate boundaries. However, the question specifically contrasts mid-ocean ridges with island arcs, and in this comparison, mid-ocean ridges do indeed have more volcanoes.