2.12 Summary

  • Volcanoes are erupting on earth all the time, but only in certain locations (see Figure 2.01).
  • Volcanoes occur principally at plate boundaries, both divergent plate boundaries and convergent plate boundaries.
  • Because seismic s-waves do not pass through liquids, but do pass through the crust and mantle of the earth, overall the crust and mantle cannot be liquid. Therefore, processes at plate boundaries must cause melting to occur.
  • Pressure-temperature conditions that can lead to melting of mantle peridotite have been determined by experiments in petrology laboratories. These experiments show that the temperatures needed for peridotite melting increase as pressure increases (see Figure 2.06).
  • Temperatures based on heat flow models for stable oceanic and continental lithosphere are too low to melt the crust or upper mantle, which is consistent with seismic s-wave data.
  • Decompression of the solid mantle by convection at divergent boundaries leads to peridotite partial melting beginning at a depth of about 40 km. The resulting liquid coalesces and rises, because its lower density, and erupts at the surface (see Figure 2.07).
  • Eruptions at ocean-ocean divergent boundaries (mid-ocean ridges) are principally tholeiitic basalt (MORB).
  • Eruptions at continent-continent divergent boundaries (rifts) are chemically more diverse and include basalt, rhyolite, and other volcanic rock compositions.
  • Geodynamic models of convergent plate boundaries (subduction zones) show that temperatures are lower in subduction zones than in the surrounding mantle because "cold" lithosphere is being carried down by convection into the otherwise "hot" mantle.
  • Based on laboratory melting experiments, model temperatures in subducting slabs are not high enough to melt the slab beneath the volcanic arcs along convergent boundaries.
  • Laboratory experiments of peridotite melting in the presence of water under pressure demonstrate that partial melting occurs at much lower temperatures than melting without water present (see Figure 2.09).
  • Water released by heating and metamorphism of hydrated ocean crust and the thin layer of subducted sediment, is believed to cause melting in the mantle above subduction zones (see Figure 2.10).
  • Andesite is the most common rock type found in volcanic arcs above subduction zones. Because partial melting of peridotite produces basaltic magma, processes must occur along convergent plate boundaries that start with basalt magma and lead to andesite and other rock types.
  • Volcanoes also occur with tectonic plates. Most within plate volcanism is best understood as a consequece of rising mantle plumes.
  • Plutonic rocks can form anywhere that there is a volcano. If the some of the magma that is feeding the volcano solidifies before reaching the surface, a plutonic rock is formed.
  • Continent-continent collisions thicken the crust and produce higher temperature condidtions that can cause metamorphism and crustal melting. Water released by metamorphic reactions involving hydrous minerals provides the "antifreeze" to lower the melting temperatures.
  • Reasons that magmas crystallize underground, rather than erupting, include the difficulty of rising magmas to break through the brittle rocks near the surface, and the solidification of water-bearing magmas because they cross solidus conditions as the pressure is lowered (see Figure 2.14).