Project Description

 

Andesite is an extrusive rock intermediate in composition between rhyolite and basalt with porphyritic texture. Andesite lava is of moderate viscosity and forms thick lava flows and domes.
Andesite is a common rock of the continental crust above subduction zones. It generally forms after an oceanic plate melts during its descent into the subduction zone to produce a source of magma. It is a fine-grained rock that formed when the magma erupted onto the surface and crystallized quickly.
Occasionally, andesites contain large, visible grains of plagioclase, amphibole, or pyroxene. These large crystals are known as «phenocrysts.» They begin forming when a magma, which is cooling at depth, approaches the crystallization temperature of some of its minerals. These high-crystallization-temperature minerals begin forming below the surface and grow to visible sizes before the magma erupts.

The word andesite was used initially by Leopold von Buch in 1835 to describe a group of rocks found in the Andes Mountains of Bolivia and Chile. Andesite is recorded as a type of rock associated with 422 of the 721 active volcanoes on Earth.

Chemical Composition of Andesite

Andesite is an intermediate sub-alkalic rock with SiO2 contents ranging between 57 and 63 wt. %, and Na2O + K2O contents around 5 wt. %. Intermediate rocks are also characterized by an increased CaO content compared to that in acidic rocks.

Igneous Rocks: “TAS” Classification

 

Igneous rock composition chart

 

The main minerals or those that dominate in the andesite are plagioclase (labradorite-andesine) and amphibole (hornblende). While the accessory minerals are pyroxenes (mainly augite), biotite and rarely quartz.

  1. Andesites with Pyroxenes: Rich in iron and magnesium. They make up the largest group of volcanic igneous rocks. They are dark and thick
  2. Andesites with Amphiboles: There is a remarkable presence of ferromagnetic minerals. Biotite, the mafic mineral of the group of trioctahedral micas, and the hornblende, dazzling amphibole.

Uses

  • It is used for making cobblestones because it is resistant to slipping – with cobblestones from Cabo de Gata, streets were paved in cities such as Madrid, Valencia, Almería or breakwaters in ports such as Motril and Almería. (Go to Disjunction Columnar)
  • It is used as filling material in construction and road construction
  • Used in landscaping and garden designs
  • Used in the construction of sculptures and monuments.

Cabo de Gata-Nijar GeoPark occurrences

Geological map of Cabo de Gata volcanic field (districts of Rodalquilar and San José). Lithologies: 1. Amphibolic Andesites. 2. Pyroclastic breccias and tuffs of amphibolic andesites. 3. Pyroxene Andesites. 4. Pyroxene Andesites with endogenous alteration. 5. Pyroclastic pyroxene andesite breccias. 6. Pyroclastic breccias of pyroxene andesite with endogenous alteration. 7. Lapilli or volcanic tuffs of pyroxene andesites. 8. Polygenic and tuff breccias of amphibole and pyroxene andesites. 9. Rhyolitic and dacitic breccias. 10. Rhyolitic and dacitic tuffs with endogenous alteration. 11. Andesite breccias. 12. Andesites breccias with strong alteration. 13. Dacites and amphibolic andesites. 14. Pycliclastic breccias of dacites and amphibole andesites. 15. Pyroclastic conglomerates and amphibolic dacite-andesite breccias with reddish matrix. 16. Pyroclastic and ignimbritic flows of the Cinto area with endogenous alteration. 17. Breccias of collapse of red-violet amphibole dacites. 18. Red-violaceous amphibole dacites with endogenous alteration. 19. Imitimbrite dacites with tuffs and basal ignimbrites with endogenous alteration (Las Lázaras unit). 20. Domes and flows of fine-grained quartz-amphibole dacites. 21. Dacitic and andesitic dams. 22. Shales, conglomerates, quartzites and limestones of the Maláguide complex. 23. Tertiary sediments. 24. Quaternary deposits. 25. Alluvial materials. 26. Dunes. (Taken from Rigol-Sánchez, 2000).

References

  • Bonewitz, R. (2012). Rocks and minerals. 2nd ed. London: DK Publishing.
  • Harangi, S. (2001). Neogene to Quaternary volcanism of the Carpathian-Pannonian region; a review. Acta Geologica Hungarica, 44(2), 223-258.
  • Atlas-hornin.sk. (2019). Atlas of magmatic rocks. [online] Available at: http://www.atlas-hornin.sk/en/home [Accessed 13 Mar. 2019].