Measuring the temperature of lava. Photograph by R.L. Christiansen, U.S. Geological Survey, January 9, 1973.
The temperatures of lavas vary depending on their chemical composition. Hawaiian lava (basalt) is usually around 1100 C. Volcanoes such as Mt. St. Helens erupt lava that are around 800 C.
Source of information:
Francis, P., 1993, Volcanoes: a planetary perspective: Clarendon Press, 443 p.
Here is a list of temperatures for the common types of lava: