The relationships between plate tectonics and volcanism are shown on Figure 11.3. As summarized in chapters on plate tectonics and igneous rocks, magma is formed at three main plate-tectonic settings:
- divergent boundaries where melting happens by decompression
- convergent boundaries where flux melting occurs as a result of water being released from slabs of subducting ocean crust
- mantle plumes where hot mantle material is decompressed as it rises up from deep within the mantle, and can cause eruptions away from plate boundaries both on land and in the ocean
Mantle and crustal volcanic processes are illustrated in more detail in Figure 11.4.
Decompression Causes Volcanism Along Spreading Centres
At a ocean spreading ridge (Figure 11.4a), hot mantle rock moves slowly upward by convection at rates of cm per year. At approximately 60 km below the surface, the mantle rocks have decompressed is enough to permit partial melting. Over the triangular area shown in Figure 11.4a, about 10% of the ultramafic mantle rock melts, producing mafic magma that moves upward toward the axis of spreading (where the two plates are moving away from each other). The magma fills vertical fractures produced by the spreading and spills out onto the sea floor to form basaltic pillows (more on that later) and lava flows. There is spreading-ridge volcanism taking place about 200 km offshore from the west coast of Vancouver Island.
Volcanism in northwestern B.C. (Figures 11.5 and 11.6) is related to continental rifting. This area is not at a divergent or convergent boundary, and there is no evidence of an underlying mantle plume. The crust of northwestern B.C. is being stressed by the northward movement of the Pacific Plate against the North America Plate, and the resulting crustal fracturing provides a conduit for the flow of magma from the mantle. This may be an early stage of continental rifting, such as that found in eastern Africa.
Water Causes Partial Melting Along Subduction Zones
At an ocean-continent or ocean-ocean convergent boundary, oceanic crust is pushed far down into the mantle (Figure 11.4b). It is heated up, and while there isn’t enough heat to melt the subducting crust, there is enough to force the water out of some of its minerals. This water rises into the overlying mantle where it contributes to flux melting of the mantle rock. The mafic magma produced rises through the mantle to the base of the crust. There it contributes to partial melting of crustal rock, and thus it assimilates much more felsic material. That magma, now intermediate in composition, continues to rise and assimilate crustal material; in the upper part of the crust, it accumulates into plutons. From time to time, the magma from the plutons rises toward surface, leading to volcanic eruptions. Mt. Garibaldi (Figures 11.1 and 11.2) is an example of subduction-related volcanism.
Decompression of Mantle Plumes Can Cause Volcanism Away from Plate Boundaries
Hot spot volcanoes (Figure 11.4c) can occur within plates, far from where plate boundary processes are taking place. They occur above mantle plumes, which are rising columns of hot solid rock. The rising column may be kilometres to 10s of kilometres across, but near the surface it spreads out to create a mushroom-like head that is 10s to over 100 kilometres across. Mantle plumes are different from the convection that normally occurs beneath ocean spreading centres: plumes rise approximately 10 times as fast as regular mantle convection occurs, and may originate deep in the mantle, possibly just above the core-mantle boundary.
Near the base of the lithosphere (the rigid part of the mantle), pressure on the mantle plume is low enough to permit partial melting of the plume material, producing mafic magma. Heat carried by the mantle plume may also melt rock adjacent to the plume. The magma rises and feeds volcanoes. Most mantle plumes are beneath the oceans, so the early stages of volcanism typically take place on the sea floor. Over time, islands may form like those in Hawaii.
Exercise 11.1 How Thick Is Ocean Crust?
Figure 11.4a shows a triangular zone about 60 km thick. Within this zone approximately 10% of the mantle rock melts to form oceanic crust. Based on this information, approximately how thick do you think the resulting oceanic crust should be?