In contrast, at convergent margins, while basaltic lavas are erupted, many of the erupted lavas have much higher silica contents. These more silicic lavas are thought to be partial melts of the crust. Their origins will be discussed later. The physical properties of these more silicic magmas differ greatly from those of basaltic magmas. The diagram at left illustrates the variation of the viscosities of magmas as functions of composition and temperature.
Viscosity is a measure of how fluid a magma is.
Magmas with very high viscosities are not very fluid and flow very slowly and magmas with low viscosities are much more fluid. The difference in fluidity of basalts and rhyolites is greater than the difference between water and maple syrup that has been srored in the freezer. For example, basaltic magma will flow down the side of a volcano at a velocity of a few kilometers per hour. In contrast, a rhyolite lava flow only flows at a few millimeters to centimeters per hour. In addition to magma composition, there are other factors that affect magma viscosity.