4.2 Plate Tectonics and Volcanism

The relationships between plate tectonics and volcanism are shown in Figure 4.4. As summarized in Chapter 3, magma is formed at three main plate-tectonic settings: divergent boundaries (decompression melting), convergent boundaries (flux melting), and mantle plumes (decompression melting).

The mantle and crustal processes that take place in areas of volcanism are illustrated in Figure 4.4. Hot mantle rock moves slowly upward at a spreading ridge by convection (centimetres/year), and within about 60 kilometres (km) of the surface, partial melting starts because of decompression. Over the triangular area shown in Figure 4.5a, about 10% of the ultramafic mantle rock melts, producing mafic magma that moves upward toward the spreading axis (where the two plates are moving away from each other). The magma fills vertical fractures produced by the spreading and spills out onto the seafloor to form basaltic pillows and lava flows. There is spreading-ridge volcanism taking place about 200 km offshore from the west coast of Vancouver Island.

Exercise 4.1 How thick is the oceanic crust?

Figure 4.5a shows a triangular zone about 60 km thick; approximately 10% of the mantle rock melts to form oceanic crust within this zone. Based on this information, approximately how thick do you think the resulting oceanic crust should be?

 

 

At an ocean-continent crust convergent boundary, the plate composed of the denser oceanic crust will subduct beneath part of another plate made up of continental crust (Figure 4.3b). Subduction zones also form where oceanic crust plunges beneath another oceanic crust plate at an ocean-ocean convergent boundary.  In both situations, the subducting slab is heated up, and while there isn’t enough heat to melt the subducting crust, there is enough heat to force the water out of some of its minerals. This released water rises up to the mantle above it, contributing to the mantle rock’s flux melting. Flux melting occurs when water or carbon dioxide added to rock lowers its melting temperature.  Melting of the solid mantle produces less dense magma than the surrounding solid rock, so it rises through the mantle to the base of the crust. It contributes to the partial melting of crustal rock, which is incorporated into the magma. This magma escapes from the upper mantle or crust base as a volcanic eruption or intrusion. Mount Garibaldi (Figure 4.1 ) is an example of subduction-related volcanism. An intrusion occurs when magma is forced into another geologic formation, like sedimentary rock (Figure 4.6).

A mantle plume is an ascending column of hot rock (not magma) that originates deep in the mantle, possibly just above the core-mantle boundary. Mantle plumes are thought to rise approximately 10 times faster than the rate of mantle convection. The ascending column may be on the order of kilometres to tens of kilometres across, but near the surface, it spreads out to create a mushroom-style head that is several tens to over 100 km across. Near the base of the lithosphere (the rigid part of the mantle), the mantle plume (and possibly some of the surrounding mantle material) partially melts to form magma that rises to feed volcanoes. Since most mantle plumes are beneath the oceans, the early stages of volcanism typically occur on the seafloor. Over time, islands may form like those in Hawaii.

Volcanism in Northern Cordilleran Volcanic Province (Figures 4.7 and 4.8) 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. A likely explanation is that 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.

Media Attributions

• Figure 4.4: Understanding Plate Motions by USGS. Public domain. Modified by Steven Earle.
• Figure 4.5: By USGS. Public domain. Modified by Steven Earle.
• Figure 4.6: By Andrew Shiva. Wikipedia, CC BY-SA 4.0
• Figure 4.7: “South-West Canada” by USGS. Public domain. Modified by Steven Earle. Volcanic locations from Edwards, B. & Russell, J. (2000). Distribution, nature, and origin of Neogene-Quaternary magmatism in the northern Cordilleran volcanic province, Canada. Geological Society of America Bulletin. pp. 1280-1293 [Steven Earle] Cordillera Volcanic Province, B.C.
• Figure 4.8: © Steven Earle. CC BY.