2.5 Why? Divergent plate boundary geotherm
The variation of temperature with depth in the earth is called a geotherm. Figure 2.07 (above) shows a model geotherm for an oceanic setting calculated by McKenzie and Bickle (1988). A model is needed because it is not possible to measure the temperature directly. The low pressure part of the geotherm is based on conduction modeling of heat flow measurements in stable oceanic crust. The more vertical geotherm at higher pressures is based on convection modeling. Results of laboratory melting experiments by Takahashi and others (1993) on rock samples like the upper mantle (they used a peridotite xenolith) are also shown in Figure 2.07. Percent liquid contours from their experiments over the melting interval between the solidus and liquidus for peridotite are also shown. The geotherm for stable oceanic lithosphere does not intersect the solidus for peridotite. But at divergent plate boundaries, mantle convection will bring hot mantle close to the surface without much cooling. Mouseover the figure to add the predicted geotherm at a divergent boundary calculated by McKenzie and Bickle (1988).
The temperature path for convectively rising mantle shown by the dashed line in Figure 2.07 indicates that mantle peridotite should start melting at a depth of about 40 km below a divergent plate boundary. It is the decreasing pressure, not rising temperature, that leads to melting. When the melt fraction is large enough (~5%), the partial melt liquid will begin to coalesce and rise to the surface because of its lower density. If the divergent plate boundary is a mid-ocean ridge, the lava that erupts is mostly tholeiitic basalt (MORB). If the divergent plate boundary is a continental rift, other lava types erupt in addition to tholeiitic basalt. Look at Figure 2.01 and chose the "Holocene Rift Setting" dataset to see volcanic rock types at divergent settings.
Decompression causes melting at divergent plate boundaries. Why are there volcanoes at convergent plate boundaries?
Figure 2.07. Geotherm with peridotite melting conditions. The variation of temperature with depth in the upper mantle is shown, with depth increasing downward. The contours between the solidus and liquidus are percent melt from experiments by Takahashi and others (1993). When you mouseover this image, the P-T path followed by convecting mantle beneath rifts is shown by a dashed line. On this path, convecting mantle crosses the peridotite liquidus boundary at a depth of about 40 km and begins to melt.
