What is lava made of?

Photograph by Steve Mattox.

Lava is made up of crystals, volcanic glass, and bubbles(volcanic gases). As magma gets closer to the surface and cools, it begins to crystallize minerals like olivine and form bubbles of volcanic gases. When lava erupts it is made up of a slush of crystals, liquid, and bubbles. The liquid "freezes" to form volcanic glass.

Chemically lava is made of the elements silicon, oxygen, aluminum, iron, magnesium, calcium, sodium, potassium, phosphorus, and titanium (plus other elements in very small concentrations. Have a look at the background information in Minerals, Magma, and Volcanic Rocks.

Chemistry:

Most rocks are made of the following elements:

Element (Symbol)	Weight percent
Oxygen (O)	46.6
Silicon (Si)	27.7
Aluminum (Al)	8.1
Iron (Fe)	5.0
Calcium (Ca)	3.6
Sodium (Na)	2.8
Potassium (K)	2.6
Magnesium (Mg)	2.1
Total:	98.5

The amount of these different elements in a rock can be determined. Geologists report the values as oxides (the element combined with oxygen). For example, a rock might have 6 weight percent FeO (iron oxide).

Different types of lava have different chemical compositions. Here are a few examples:

	Basalt	Andesite	Dacite	Rhyolite	Trachyte
SiO2	49.20	57.94	65.01	72.82	61.21
TiO2	1.84	0.87	0.58	0.28	0.70
Al2O3	15.74	17.02	15.91	13.27	16.96
Fe2O3	3.79	3.27	2.43	1.48	2.99
FeO	7.13	4.04	2.30	1.11	2.29
MnO	0.20	0.14	0.09	0.06	0.15
MgO	6.73	3.33	1.78	0.39	0.93
CaO	9.47	6.79	4.32	1.14	2.34
Na2O	2.91	3.48	3.79	3.55	5.47
K2O	1.10	1.62	2.17	4.30	4.98
P2O5	0.35	0.21	0.15	0.07	0.21

Source of Information:
LeMaitre, R.W., 1976, The chemical variability of some common igneous rocks: Journal of Petrology, v. 17.