Fact: There is no a giant reservoir that molten rock simply hanging out beneath the crust, waiting to spurt come the surface at any obtainable weak spot. The mantle is hard rock. (Remember the lesson about the Earth"s interior earlier in this course: If the mantle were a liquid, earthquake S tide couldn"t travel with it.)

So wherein does the lava come from? exactly how do rocks melt? At surface pressures, all you need to do come melt a solid is to heat it approximately its melt point. That is true the the temperature rises as you walk deeper and also deeper right into the planet (15 - 20°C / km is the usual geothermal gradient). However, pressure is likewise rising as you descend into the Earth, and also increased push inhibits melting. There space three basic ways that rocks melt to type the lavas the erupt native volcanoes: decompression, addition of volatiles, and also conduction. Let"s discover each of these in turn.

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### Decompression

Decompression melting occurs in ~ mid-ocean ridges. As soon as two plates relocate apart, they produce a an are that can be fill by warm rock the rises buoyantly indigenous below. As lengthy as this warm rock rises quicker than the temperature have the right to cool off, the rock have the right to melt because the press is decreasing as the rock gets closer come the surface. Watch pencast lay out of decompression melt at a midocean ridge!

Let"s visualize what decompression melting looks choose as a plot in Pressure-Temperature space!

You deserve to construct Pressure-Temperature plots to display melting curve for every kinds of substances, not just lava in ~ a mid-ocean ridge. For example, the plot below shows data for table salt. Keep in mind that these scientists placed temperature top top the y axis, and pressure ~ above the x axis. Us did that the other way around (and had pressure increasing downwards ~ above the y axis) since we wanted pressure to be analogous to depth in the planet in ours plot.

### Volatiles

Rocks melt at a lower temperature in the existence of volatiles such together water and also carbon dioxide. Just how do you obtain water underneath a volcano? The many common means to do it is come send it under a subduction zone. As soon as a subducting plate sinks under the overriding plate, the water-saturated upper part of the lithosphere goes down, too. As the cold slab sinks, water is compelled out and also percolates upward into the overlaying hot, dry mantle rock. This sudden addition of water lowers the melting suggest of the mantle rock, and also it starts to melt. See my pencast that this process.

Volatile-driven melt happens at every subduction zones. The melt is generally formed at the allude when the slab gets to a particular depth (the depth in ~ which the pressure becomes high enough to pressure out the water). Pencast that a plot in Pressure-Temperature an are for volatile-driven melting! Therefore, you can number out the angle of subduction (i.e., exactly how steeply the subducting plate is sinking) based on how far the subduction zone volcanoes space inboard of the trench. If the volcanoes space close come the trench, it means the slab is subducting steeply, and also if the volcanoes are far away from the trench, it way that the slab is subducting in ~ a shallow angle.

Try this!

Below are two maps at the same scale showing two various subduction zones. Top top the peak is a subduction region in the main Andes in southern America. The trench is the dark virtually north-south line offshore. The slab is subducting native west come east. ~ above the bottom is a subduction zone south of Japan in the western Pacific Ocean. This slab is subducting from eastern to west. Keep in mind the positions of the trenches v respect to the volcano arcs connected with each one (the volcanoes are shown by the hokey little orange volcano symbols). Which plate is subducting at a steep angle and also which one is subducting at a shallow angle?

Central Andean subduction zone (top) and Western Pacific subduction region (bottom). Maps space at the exact same scale.

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### Conduction

The third means to melt a absent is through conduction. Conduction is the simplest method to transfer heat. At the atom scale, hotter corpuscle vibrate more. Once they come into call with other nearby particles, some of that vibrational heat energy is transferred to those nearby particles, heater them up. At the macroscopic scale, when rocks melt by one of two people decompression or by enhancement of volatiles, the more buoyant melt rises towards Earth"s surface. When this climbing melt come into contact with hard lithospheric rock on its path upward, it can transfer enough heat come the surrounding rock come melt it. This often happens in subduction zones as the early stage melt created at the slab/mantle boundary travels upward right into the absent of the overriding plate.