Igneous Rocks

  • Igneous rocks (Granites). Igneous rocks are formed by the crystallisation of a magma. The difference between granites and basalts is in silica content and their rates of cooling. A basalt is about 53% SiO2, whereas granite is 73%.
    • Intrusive, slowly cooled inside the crust. (Plutonic rock = formed in the earth). Large crystals.
      • Granite. (Continental crust) Density 2.7-2.8. High silica content (acidic). = quartz + mica + K-feldspar in solid solution. 60% orthoclase and plagioclase fledspars + 25% quartz + 5% darker minerals (biotite, hornblende). Color from flesh to black. Crystals intermingled. Hard, rigid, tough. Granitic rock is much less common on the other terrestrial planets, a fact having to do with the fractionation (where early crystallizing minerals separate fromt he rest of a magma), a process that takes place uniquely on earth, due to the prevalence of plate tectonics.
      • Granodiorite. An intermediate form between granite and diorite.
      • Diorite. High silica content (acidic)
      • Gabbro. Density? Medium silica content. (intermediate). Similar to granite = quartz + feldspar + pyroxene + amphibole + mica + olivene. A layer of gabbro is found in the ocean crust, unerneath the basalt layer (0.5-2.5km), from 2.5 to 6.3 km deep. The lunar highlands have many gabbros (made largely of potassium feldspar - also known as plagioclase)
      • Peridotite.
    • Extrusive. cooled rapidly at the surface. Small crystals.
      • Rhyolite. Medium silica content (intermediate). A fine-grained volcanic rock of granitic composition.
      • Dacite.
      • Andesite. (Volcanic arcs) Density >2.8. Low silica content (basic) = sodium feldspar + amphibole. Dark, dense.
      • Basalt. (Ocean crust) Density 2.9. Low silica content. (basic). Dark, dense. = olivene + pyroxene + Ca-Feldspar in solid solution. Basaltic rocks (gabbro & basalt) are made up of feldspars and other minerals common in planetary crusts. They have been identified as major surface rocks on the dark lunar planes and much of Mars, Venus and the asteroid Vesta.
  • Pyroclastic rocks: debris ejected by volcanoes
    • Tuff is made of compacted debris from old volcanic ash showers.
    • Volcanic breccia is composed of angular mineral fragments embedded in a matrix, the product of explosive eruptions.
    • Ignimbrites are sheets of coalesced fine particles which once flowed at high speed, extremely hot, fluid avalanches.
Notes: Density in kg/litre or g/cm3
Classification of igneous rocks
Classification of igneous rocksThis diagram shows the makeup of igneous rocks from the various minerals inside a magma chamber. Density increases from bottom right to top left. 
Intrusive rocks are coarse-grained in texture and crystallise slowly from magma deep in the earth's crust. Extrusive rocks are fine-grained in texture and crystallise quickly from lava on or near the earth's surface. The mineralogy determines the type of rock. Granites and rhyolites consist predominantly of quartz and potash feldspar; gabbros and basalts, predominantly of pyroxene and plagioclase feldspar. Other rock types have intermediate mineral compositions. Note that amphibole = horneblende. Note that the density of the minerals increases from top left (2.6) to bottom right (3.4). Top left: high silica content (acidic); bottom right: low silica content (ultrabasic). The temperature range at which magma solidifies is 1100-700ºC.

(Paul R Pinet in Oceanography, an introduction to Planet Oceanus. 1992.)

Excerpted and edited from: http://www.seafriends.org.nz/enviro/soil/rocktbl.htm