glossary

Glossary Of Terms

Pluton; the higher protruding cusps of a batholith punched into the upper regions of the earth's crust.
Silicic; denotes an igneous rock high in silicon dioxide (quartz).
Batholith; a deep seated igneous intrusion into the earth's crust.
Igneous; this term descibes a rock that has crystallised from a melt. A rock is made up of constituent minerals.
Laccolith; a form of igneous intrusion where the form is mushroom shaped.
Orthoclase; a mineral, a potasium aluminium silicate. Part of the feldspar group of minerals. These are potasium, and sodium to calcium aluminium silicates.
Plagioclase; the sodium to calcium rich sub group of feldspars. Albite and Oligoclace are part of this group.
Biotite; a platy iron-rich, magnesium aluminium silicate, part of the mica group.
Quartz; a mineral, silicon dioxide.
Metasomatic; A change within a rock involving the introduction of material from elsewhere.
Variscan; a time period describing the montain building or continental collision activity towards the end of the carboniferous geological period. Synonomous with Hercynian and Amorican.
Tor; the tops of dartmoor hills are crowned with rocky granite masses called tors.
Chilled Margin; a cooling igneous rock will show a chilled rim, the crystals in this rim will be smaller (fine grained) because they have cooled faster. Crystals well within the main body will have cooled more slowly allowing them to develop a coarser grain size (larger crystals). The intermediate term is medium grained.
Magma; rock melt.
Diorite; a coarse grained plutonic igneous rock consisting of plagioclase feldspar and one or more ferromagnesian minerals such as biotite, hornblende or augite. Quartz may be present up to 10%, pottasium and sodium rich feldspars may also make up one third of the total feldspar.
Dykes & Sills; not really a term mormally applied to the intrusion of igneous rock, but in this case describes further igneous activity either cutting the existing joint (dyke) or filling the existing system (sill).
Tourmaline; a mineral, black Boron silicate.
Greisening; an alteration process.
Basic Segregations; a high percentage of dark coloured minerals with high calcium and magnesium concentrations.
Groundmass; the finer grained mass of crystals seen in an igneous rock, commonly called a matrix through strictly it shouldn't.
Phenocryst; large crystal in this case of feldspar formed from a melt. Also called megacrysts however these might not have formed froma melt.
Joints; these are the partings seen within the granite at outcrop.
Outcrop; a rock exposure.
Porphyry; a rock with large crystals possibly indicating slow cooling.
Lode; a metal ore rich pipe of igneous material.
Cornubian; name given to the earth's crustal block that contains the SW England or Conubian Batholith or vice versa.
Isostacy; the effect of crustal blocks moving vertically on ontop of the lower crust. Common recent and more well known isostatic movement has been the uplift caused by the melting of the ice sheets at the end of the last ice age.
Ma; million annum eg. 303 Million years ago. Isotopic dating can commonly be accurate using methods of isotope decay and half lives.
Stephanian; a sub group of the Carboniferous geological period.
Palingenetic; origin from the melting or partial melting of a pre-existing rock.
Differentiate; minerals that crystalise at high temperatures may be removed from a melt leaving a melt composition different from the original.
Carboniferous; geological time period during which time Britain was sub tropical. Famous for the period when most of our coal was formed.
Devonian; geological time period before the carboniferous. Britain was largely a desert area during this time.
Plate Configurations; the earth's crust is a complicated assebly of plates, those under the oceans are termed oceanic and those with continents on top are known as continental plates. where the plates move apart, new crust is formed by volacnism at ocean ridges. Where the plates move together and the ocean plate is able to slip beneath the continental plate, we have subduction. Where continents collide we have periods of mountain building. This happened with the Himalayas when the Indian plate collided with the Eurasian plate. Today where we see plate collision such as along the america's west coast, where the pacific plates are subducted beneath the continental plates, plutons have been punched into the crust notably at Yosemite and in the Sierra Nevadas. There is always associated volcanism (where magma reaches the surface). This includes andesite-rhyolite-dacite lava flows. These are rock types manifested as lava flows.
A rhyolite is virtually the same composition as a granite but because the rock melt reached the surface, it cooled faster thus forming a different rock type of the same composition.
Mantle; high pressure and temperature melt-rock beneath the earth's crust.
Xenolith; a block of roof-rock adjoining an emplacement of palingenetic origin may become partly consumed and altered by the melt.