This is a computer model of the thermal structure of a subduction zone (convergence at 6 cm/year) showing how cool rocks of the slab pass to great depths, only warming gradually. If this type of model is correct (and it is consistent with seismic velocity information) it suggests that the ocean crust of the subducted slab will only melt at depths greater than 600 km. In fact the volcanic line of island arcs lies above the slab which has reached depths of just 120-140 km. This modeling shows the temperature there to be just 200-300C - far too cool for the ocean crust too melt - but warm enough to drive off fluid into the overlying mantle wedge.

Click for Subduction zone relationships

Coseismic period (A few minutes)

Interseismic period (100’s of years)

Click for Generation of magma in subduction zone

The generation of magmatism above subduction zones can be rather complicated - as illustrated in this simplified (honestly!) diagram. The down-going slab hydrates the overlying mantle. This can then melt, emitting basaltic (with added silica that was dissolved in the water), leaving a residue (called harzbergite) in the mantle. Alternatively the hydrated mantle might rise buoyantly and melt at shallower depths. The complexity of magma is increased by fractional crystallisation as it rises or is ponded in magma chambers.

Andean magmatism can show all the complexity of island arcs but with the added complication of contamination. Adding hot molten basalt to the base of continental crust can melt this crust - because granitic rocks have much lower freezing points than basalts (c. 6-700C rather than >1000C). The basalt and granitic melts can mix to make hybrids or drive new granitic melt too.

A seismic reflection profile through the front of the accretionary prism at the Japan trench. Sediments are being scraped off the down-going slab.

As rocks are pressurized by burial or heated up their chemical constituents can recombine into new compounds (minerals). The association of minerals (called facies) for specific bulk chemistry are diagnostic of particular conditions of pressure and temperature. This diagram illustrates how different facies reflect different depths for the temperature-depth relationship typical of subduction zones.

The effect of adding water to mantle is to lower the freezing point, makingit easier to melt. Here a part of the mantle (green circle) is too cool andtoo pressured to melt when dry but if water is added it lies above the wetsolidus and so will melt.

Click for different ways to melt rock

Oceanic crust:As the name already suggests, this crust is below the oceans. There, the crust is 4-7 miles (6-11 km) thick. The rocks of the oceanic crust are very young compared with the rocks of the continental crust. The rocks of the oceanic crust are not older than 200 million years. The material of which the oceanic crust consists is for the greater part tholeiitic basalt (this is basalt without olivine). Basalt has a dark, fine and gritty volcanic structure. It is formed out of very liquid lava, which cools off quickly. The grains are so small that they are only visible under a microscope. The average density of the oceanic crust is 3g/cm³.

Continental crust:When you look at the globe, you see that the surface of the earth consists of a lot of water (71%). The other 29% consists of land. You can divide this land into six big pieces, which are called continents. The different continents - arranged in decreasing order of size - are: Eurasia (Europe and Asia together), Africa, North-America, South-America, Antarctica and Australia. In the past the division of the continents was different (see plate tectonic). The earth's crust is the thickest below the continents, with an average of about 20 to 25 miles (30 to 40 km) and with a maximum of 45 miles (70 km). The continental crust is older than the oceanic crust, some rocks are 3.8 billion years old. The continental crust mainly consists of igneous rocks and is divided into two layers. The upper part mainly consists of granite rocks, while the lower part consists of basalt and diorite. Granite is lightly-colored, coarse-grain, magma. Diorite has the same composition, but it's scarcer than granite and is probably formed by impurities in the granite-magma. The average density of the continental crust is 2.7g/cm³.

Continental CrustAll of the Earth's continents have a core foundation that is made of mixtures of very old granite, gneiss, schist, sedimentary, and volcanic rocks. This core foundation is often referred to as a shield or basement rock. Rocks found in the shields were formed during the Precambrian and are some of the oldest rocks found on the Earth. In Canada, some of the metamorphic rocks have been dated to an age of 3.96 billion years. Geologists believe that the major continental cores were formed by the early solidification of the lighter components of magma between 3.9 and 3.8 billion years ago. The continental shields are generally covered by younger sedimentary deposits. These sedimentary rocks constitute the interior platforms of the continents. The oldest platform rocks were laid down in shallow seas about 600 million years ago. In central North America, the platform sedimentary deposits are between 1000 to 2000 meters thick. Together the shield and platform form what geologists call a craton. Most of the Earth's continental cratons have been tectonically stable and have experienced little deformation for hundreds of millions of years.

Continental collision animation