A. Volcanic, or extrusive, igneous rock forms when molten material erupts onto the earth's surface and rapidly cools. B. Intrusive igneous rocks form when magma chambers remain buried below the surface, allowing very slow cooling and crystal growth. These cooled magma chambers, called plutons , are not exposed at the surface until the overburden of crustal material above them is eroded off. Landforms associated with volcanic activity A. Basaltic volcanism 1. Recall that basalts are extrusive igneous rocks of the oceanic crust, rich in iron and magnesium-bearing minerals.
Basaltic volcanism occurs in association with sea-floor spreading centers and oceanic hot spots.
Hawaiian islands formed as crust migrated over a hot spot. Character of basaltic volcanism. 2. forms a broad, gently sloping volcano 3. Shield volcano eruptions can produce prodigious quantities of lava, so that subterranean magma chambers are emptied and the mountain top collapses, forming a central depression called a caldera . Shield volcanoes may exhibit great relief.