熔岩蒸發的溫度

　　　　Temp of melting　　Temp of boiling
SiO2　　~2000K　　　　　　2503K
Al2O3　　2345K　　　　　　3253K
CaO　　　2887K　　　　　　3123K
MgO　　　3125K　　　　　　3873K
FeO　　　1642K

Because of various composition for magma, heating over 2503K (2230 deg C) SiO2 will start to vapourize, further heating over 3123K (2850 deg C) CaO will start to vapourize, further heating over 3253K (2980 deg C) Al2O3 will start to vapourize, further heating over 3873K (3600 deg C) MgO will start to vapourize, until now almost all major composition have been vapourized. Usually naturally formed magma will not be so hot, and lava temperatures are usually less than 1200 deg C.

Volcanologists use the word "magma" for molten rock that is still under ground and hasn't yet erupted. Once it has erupted onto the surface, call it "lava".
In "Role of olivine cumulates in destabilizing the flanks of Hawaiian volcanoes: Bulletin of Volcanology, v. 56, p. 425-434." Clague and Denlinger report the following composition for average Kilauea primary magma: SiO2 (49%), TiO2 (1.9%), Al2O3 (10.7%), FeO (11.2%), MgO (16.5%), CaO (8.55%), Na2O (1.66%), K2O (0.30%), P2O5 (0.18%).

Comparing the Chemical Composition of Hawaiian Basalt and Mount St. Helens Dacite
Hawaiian Basalt
SiO2 (48.4%), Al2O3 (13.2%), FeO (11.2%), MgO (9.7%), CaO (10.3%), Na2O (2.4%), K2O (0.6%), TiO2 (2.8%), other (1.4%).

Mount St. Helens Dacite
SiO2 (63.5%), Al2O3 (17.6%), FeO (4.2%), MgO (2.0%), CaO (5.2%), Na2O (4.6%), K2O (1.3%), TiO2 (0.6%), other (1.0%).
so lava from Mount St. Helen is much cooler than from Hawaii.

Usually naturally formed magma will not be so hot, and lava temperatures are almost always less than 1200 degrees C. The highest temperature for lava probably about 1400 degrees Celsius in a basaltic lava. With higher silica content, the lower the melting temperature. Lava is molten rock that forms below the surface of the earth. It probably forms between 150 and 50 km down, and we call it magma before it reaches the surface. Once it reaches the surface to erupt and flow down the side of a volcano, we call it lava, even though it really is the same stuff. The heat that is required to form the magma in the first place comes from the radioactive decay of naturally-occurring radioactive elements within the earth, this heat is enough to partially melt rocks in the upper mantle, about 50-100 km below the surface. I say partially melt because the rocks don't completely melt. As you can guess, most rocks are made up of more than one mineral, and these different minerals have different melting temperatures. This means that when the rock starts to melt, some of the minerals get melted to a much greater degree than others. The main reason this is important is that the liquid (magma) that is generated is not just the molten equivalent of the starting rock, but instead something different.
Geochemists use several elements to track the history of a batch of magma. For our example we'll start with magnesium (and think of it as magnesium oxide, MgO, like the geochemists do). Beneath Hawaii, when the mantle melts it produces a magma with about 17 weight percent MgO. Volcanologists call magma with this composition a picrite. As the magma rises crystals begin to form. The first crystal to form is olivine, which contains the elements magnesium, silicon, and oxygen in the proportions 2 to 1 to 4. The crystal is more dense than the surrounding magma and it begins to settle. It is estimated that an olivine crystal 1 mm in diameter falls through the magma at a rate of 20 meters per hour. The key point is something, the crystals, is being removed and this causes the chemical composition of magma to change. As more and more olivine crystals settle, the magma has less and less magnesium oxide and more and more silica (silicon and oxygen). For most volcanoes, by the time the original magma from the mantle is erupted its composition has changed from a picrite to a basalt.