320 Appendix
A6 Common Rock Types
Igneous Rocks. Mineral assemblages (dominated by silicates) which crystallized from hot, fluid
magma. Slow cooling produces coarse crystals, typical for intrusive rocks. Fast cooling produces
small crystals, typical for extrusive rocks (volcanic lava). Classification is by mineral types (from
high-silica to low-silica minerals) and by crystal size.
Intrusive rocks
(Coarse crystals)
Granite
Granodiorite
Diorite
Gabbro
Peridotite
Dunite
Extrusive rocks
(Fine-grained)
Rhyolite
Dacite
Andesite')
Basalt b )
Basalt b )
Basalt
Decreasing
proportion
of Si02, increase
in Mg and Fe
a) Typical volcanic rock in island arcs and landward of trenches (Andes! ). Partial melting of downgoing oceanic crust appears to be an important process in generating andesite (see Fig. A6.1)
b)Typical basement rock for deep sea floor, that is, ocean crust below sediments.
I. Macquane Island
2. Chatham Island
3. Kermadec Islands
4. Tonga Islands
5. Samoa
6: Fiji Islands
7. Santa Cruz
8. Solomon Islands
...
16
9. Bismarck Islands
10. Palau Islands
11. Yap
12. Mananas Islands
;'
13. Bonin Islands
14. Kurile Islands
15. Aleutian Islands
16. Galapagos Islands
Fig. A6.1 Andesite line . The " andesite line" separates andesitic volcanoes from basaltic ones. The
recognition of this boundary predates plate tectonics by many decades (see Fig. 1.2). Graph from
Kuenen Ph H (1950) Marine Geology. John Wiley and Sons, New York
A6 Common Rock Types
Igneous Rocks. Mineral assemblages (dominated by silicates) which crystallized from hot, fluid
magma. Slow cooling produces coarse crystals, typical for intrusive rocks. Fast cooling produces
small crystals, typical for extrusive rocks (volcanic lava). Classification is by mineral types (from
high-silica to low-silica minerals) and by crystal size.
Intrusive rocks
(Coarse crystals)
Granite
Granodiorite
Diorite
Gabbro
Peridotite
Dunite
Extrusive rocks
(Fine-grained)
Rhyolite
Dacite
Andesite')
Basalt b )
Basalt b )
Basalt
Decreasing
proportion
of Si02, increase
in Mg and Fe
a) Typical volcanic rock in island arcs and landward of trenches (Andes! ). Partial melting of downgoing oceanic crust appears to be an important process in generating andesite (see Fig. A6.1)
b)Typical basement rock for deep sea floor, that is, ocean crust below sediments.
I. Macquane Island
2. Chatham Island
3. Kermadec Islands
4. Tonga Islands
5. Samoa
6: Fiji Islands
7. Santa Cruz
8. Solomon Islands
...
16
9. Bismarck Islands
10. Palau Islands
11. Yap
12. Mananas Islands
;'
13. Bonin Islands
14. Kurile Islands
15. Aleutian Islands
16. Galapagos Islands
Fig. A6.1 Andesite line . The " andesite line" separates andesitic volcanoes from basaltic ones. The
recognition of this boundary predates plate tectonics by many decades (see Fig. 1.2). Graph from
Kuenen Ph H (1950) Marine Geology. John Wiley and Sons, New York