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