Chemical
If a rock is aphanitic, it can be very difficult to classify.
Instead of using the mineral content of a rock, geologists have developed
classification schemes that are based on the chemistry of the rock. Most
rocks are made of different amounts of the elements listed in the table
at the top of this page (plus the elements titanium and phosphorous).
Geologists keep track of the elements by making them oxides, a
combination of the element with one or more oxygen atoms. The oxides
that show the greatest variation in rocks are silica (SiO2), iron (FeO or
Fe2O3), magnesium (MgO), sodium (Na2O), and potassium (K2O).
Table 3. Classification of volcanic rocks by silica content.
________________________________________________________________________
Basalt Andesite Dacite Rhyolite
________________________________________________________________________
SiO2 48-52 52-63 63-68 >68
FeO 10-12 7 5 2.5
MgO 10- 6 3 2 ~1
Na2O 3 3.5 3.8 3.5
K2O 1 1.5 2.2 4.3
________________________________________________________________________
All numbers are in weight percent.
When geologists analyzed numerous types of volcanic rocks from around the
world, they found that the silica (SiO2) content varied by as much as 40
weight percent. Because of this large variation, silica content is used
to classify rocks. A simple classification scheme based on silica
content is shown above. This table also shows the abundance of other
elements. Notice that the weight percent of the other elements do not
vary as much as silica does.
1. Basalt 6. Basaltic andesite 11. Basanites and Tephrites
2. Hawaiites 7. Andesites 12. Phonolitic tephrites
3. Mugearites 8. Dacites 13. Phonolites
4. Benmoreites 9. Rhyolites 14. Nephelinites
5. Trachytes 10. Trachyandesites 15. Phonolitic nephelinites
16. Picrite
Classification scheme for volcanic rocks based on alkali element and
silica content. Classification scheme of Cox, Bell, and Pankhurst
(1980). Data for Hawaiian volcanoes is from Peterson and Moore
(1987).
In addition to silica content, volcanic rocks are also classified by
their alkali element (Na2O+K2O) content. This provides a more detailed
classification compared to the scheme based on silica content alone. At
the same silica content, alkali basalts have greater abundances of alkali
elements relative to tholeiitic basalts.
Table 4. Composition of Hawaiian eruptive products.
________________________________________________________________________
Eruptive Stage Rock Types Eruption Rate Volume (percent)
________________________________________________________________________
Rejuvenated stage Alkali basalt Very low <1
Basanite
Nephelinite
Nepheline melilitite
Postshield stage Alkali basalt Low ~1
Transitional basalt
Ankaramite
Hawaiite
Mugearite
Benmoreite
Trachyte
Shield stage Tholeiitic basalt High 95-98
Picritic tholeiitic basalt
Preshield stage Basanite Very low <1
Alkali basalt
Transitional basalt
________________________________________________________________________
From Clague and others (1989). Transitional basalt is transitional in
composition between alkali and tholeiitic. Picrite refers to
olivine-rich basalts.
To VolcanoWorld