Dictionary of Geotourism

2020 Edition
| Editors: Anze Chen, Young Ng, Erkuang Zhang, Mingzhong Tian

Classification of Igneous Rocks

Reference work entry
DOI: https://doi.org/10.1007/978-981-13-2538-0_333
Igneous rocks can be categorised as plutonic rocks when they are formed by intrusions, hypabyssal rocks when formed near the surface, and volcanic rocks when formed from eruptions. The types of igneous rocks are complex, and their classification is based on their chemical composition, mineral content, structure and formation. The visual identification of igneous rocks can be performed as follows (Table 1):
Table 1

Visual identification of igneous rocks

Item

Feldspar

No feldspar

Mainly plagioclase

Approximately equal amounts of alkali feldspar and plagioclase

Mainly alkali feldspar

pH indicating mineral

With olivine

No quartz, sometimes with olivine

No olivine, sometimes with quartz

With quartz

No quartz

With quartz <20%

With quartz

No quartz

With feldspathoid

<20%

>20%

Colour %

>75

75–35

35–20

35–20

35–20

35–20

35–20

<20

35–20

<30

Formation and structure

Rock name

I. Intrusive rock,

lava flow, rock cover, cupola porphyritic or non-porphyritic cryptocrystalline structure

Meimechite

Basalt...

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