Kinds of Rocks

(Prof John J. Renton}
Outline Lecture Notes** (JA)

  • This lecture about igneous rocks, that formed from molten state
  • Mantle peridotite, crustal basalt and granites all igneous
  • Sedimentary rocks form from products of weathering
  • Metamorphic rocks form from other kinds of rocks subjected to pressure/temp. Original minerals may recrystallise.
  • Igneous rocks classified by texture and composition.
  • Texture: coarse-grained means visible minerals (> 0.5mm), otherwise fine-grained. If coarse, then cooled slowly (magma), allowing crystals to form and grow. If fine-grained, then cooled rapidly (lava), crystals all form at once, no time to grow.
  • Composition: made up of 9 silicate minerals covered before.
  • During cooling, minerals precipate out at defined temperatures, so in a fixed order.
  • Norman Bowen worked out order of crystallisation in 1920s, producing an imperfect order.
  • Not all magmas contain all 9 minerals. Basaltics made of olivine, anorthite,augite; andesitic of horneblende, biotite, albite; granitic of orthoclase, muscovite, quartz, with traces of biotite/albite.
  • Order complicated by freezing point only being constant when composition is fixed, whilst many minerals have elements in varying proportions (eg Iron, Magnesium Silicate - 2 metals can vary from 0-100%).
  • Taking this into account means a 2D diagram of mineral content. Quartz is anomaly, having different FP even though composition is constant - unexplained.
  • Coarse- and fine-grained minerals can both have same composition, depending on formation conditions. Eg fine-grained basalt and coarse-grained gabbro - former is erupted at oceanic ridge, cooling rapidly into oceanic crust, latter cools more slowly underneath, forming lower part of oceanic lithosphere.
  • Granitic magmas don't usually reach surface but, if they do, are thrown forcefully into the air, falling as as fine-grained rhyolite, as happened in Yellowstoen Park.
  • Peridotite found in mantle, basalt in oceanic crust, gabbros in lower oceanic lithosphere, andesites in arc volcanoes, granites in continental crust.

Further information

**more detailed lecture notes available to group members on request.