Rock types

Rock types

Igneous
1. Earth's crust 95% igneous 5% sedimentary and methamorphic
2. original rock type from which all others derive
3. volcanic origin, usually crystalline
4. Methods of classification; degree of acidity and location of cooling
5. Granite, basalt, gabbro
6. pseudo-bedding or sheet joints
  1. jointing (fractures) in any direction are common
    1. E.g. hexagonal jointing in Basalt Giant's Causeway NI
Sedimentary
1. 74% of exposed rock are sedimentary shales, sandstones and limestones
2. composed of particles of other rock derived from denudation
  1. held together by natural cement or chemical precipitation e.g sandstone, conglomerate, breccia
  2. sand, clay gravel
3. organic matter
  1. microscopic marine organisms E.g. foraminifera, coccoliths, sponge spicules
    1. E.g. Limestone, flint, chert
4. form from compressional, mechanical, chemical forces and precipitation of salts from solution.g. magnesium limestone composed of chemical compound magnesium carbonate
5. laid down in parallel layers called bedding planes
6. vertical jointing present
  1. potential zones of weakness for weathering so important in the development of the rock outcrop E.g. cave formation due to jointing Malham, Yorshire.
Methamorphic
1. means changed in shape or form (heat or pressure)
2. can be any igneous or sedimentary rock whose physical or chemical structure has been altered by heat or pressure
3. common in zones of tectonic activity i.e. margins
  1. where necessary folding, faulting, vulcanicity forces involved
4. usually crystalline
5. less easy to classify as parent rock may have changed into a variety of daughter forms
  1. sometimes the parent rock is indiscernable
  2. Sedimentary origin: Limestone = Marble or Igneous origin Gabbro = Biotite schist Epidorite
tectonic activity may produce joints of various orientations in all

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