Glossary

This glossary contains simple explanations of a selection of the more important technical terms used in Chapter 1 and in the Introduction, Highlights and Conclusions sections of Chapters 2 to 7. These explanations do not pretend to be scientific definitions. Rock groups are usually explained in terms of their chemistry rather than by reference to their precise mineral content. Only major mineral groups are included. Stratigraphical terms are omitted as they are related to their contexts within the tables and figures. Bold face indicates a further glossary entry.

Throughout the glossary and the volume as a whole the following grain (i.e. crystal) sizes are assumed for the igneous rocks:

coarse-grained — grains over 3mm on average

medium-grained — grains between 1 and 3mm on average

fine-grained — grains under 1 mm on average (including non-crystalline glass)

Acid: coarse- to fine-grained igneous rocks relatively enriched in silica (SiO₂ nominally over 66%) which was originally thought to reflect the proportion of 'silicic acid'. An alternative term is 'silicic'.

Agglomerate: a volcaniclastic rock composed of large, often angular rock and mineral fragments (clasts).

Alkali-feldspar: see feldspar.

Allivalite: a coarse-grained ultrabasic igneous rock composed largely of the minerals plagioclase feldspar and olivine.

Amygdale: a vesicle infilled by minerals.

Aphyric: igneous rocks, especially those which are generally fine-grained, which contain no particularly large crystals; = non-porphyritic.

Arkose: sandstone containing abundant fragmental grains (clasts) of alkali-feldspar.

Aureole: the metamorphic rocks adjacent to an igneous intrusion.

Basalt: a fine-grained, basic igneous rock consisting largely of the minerals plagioclase feldspar, pyroxene(s) +/− Olivine. Usually a lava or a dyke.

Basic: coarse- to fine-grained igneous rocks relatively enriched in the 'bases' of early chemistry i.e. MgO, FeO, FeO₂, FeO₃CaO etc; silica (SiO₂) relatively low (nominally 45–53%.

Benmoreite: a fine-grained igneous rock, usually a lava, consisting essentially of soda-rich alkali-feldspar; see also hawaiite.

Bole: the iron-rich (sub-)soil produced by the surface weathering of basalts.

Breccia: a volcaniclastic, sedimentary, or fault-related rock composed of very large, usually angular rock fragments (clasts).

Caldera: a very large, approximately circular, fault-bounded basin formed by the collapse of a volcano.

Clast: a fragment.

Cone-sheet: a cone-like igneous intrusion which dips towards its centre, i.e. which closes downwards on projection. (cf. ring-dyke).

Crinanite: an alkali-(especially soda-)rich dolerite (or gabbro).

Diapir: a body, e.g. of igneous rock/magma which has risen through other rocks in consequence of its lower density and/or greater plasticity.

Diorite: a coarse-grained, lime-rich intermediate igneous rock containing plagioclase feldspar and various ferromagnesian silicate minerals.

Dolerite: a medium-grained, basic igneous rock containing plagioclase feldspar and pyroxene(s); usually a dyke or sill.

Dyke: a sheet-like body of igneous rock which cross-cuts the structure of the rocks it intrudes; often steeply inclined and composed of dolerite or basalt (cf. sill).

Eucrite: a coarse-grained, ultrabasic igneous rock containing plagioclase feldspar, pyroxene(s) and olivine.

Eutaxitic: the 'streaky' fabric (i.e. gross texture) exhibited by an ignimbrite.

Feldspars: a series of alumino-silicate minerals between lime-soda-rich (plagioclase) and potash-soda-rich (alkali-feldspar) end-members; the most abundant minerals in the earth's crust.

Felsite: medium- to fine-grained, equigranular, acid igneous rock consisting largely of alkali-feldspar and quartz; often a dyke or sill.

Gabbro: a coarse-grained, basic igneous rock consisting largely of plagioclase feldspar, pyroxene(s) +/− olivine; usually in large intrusions.

Gneiss: a coarse-grained, often banded metamorphic rock

Granite: a coarse-grained, acid igneous rock consisting largely of alkali-feldspar and quartz; usually in large intrusions.

Granophyre: a medium- to coarse-grained acid igneous rock which often displays a complicated, angular ( 'graphic') intergrowth between quartz and alkali-feldspar; usually in large intrusions.

Granulite: an even-grained granular metamorphic rock

Harrisite: a coarse-grained, ultrabasic igneous rock which displays branching crystals of olivine.

Hawaiite: a variety of trachybasalt rich in soda. Hawaiite, mugearite and benmoreite form a compositional series between alkali-basalt and trachyte; all usually occur as lava flows.

Hornfels: a well-baked, hard metamorphic rock

Hyaloclastite: volcaniclastic rock composed of quenched, glassy fragments (clasts) formed when magma cools and shatters on coming into contact with water.

Hydrothermal: to do with hot water.

Igneous Rocks: rocks which have solidified (usually crystallized) from molten rock (magma).

Ignimbrite: a lava-like sheet of volcaniclastic rock formed by the compaction and welding of an ash-flow (see also tuff and eutaxitic).

Intermediate: coarse- to fine-grained igneous rocks intermediate in compositions between acid and basic.

Laterite: a red (sub-)soil rich in iron and alumina.

Lherzolite: a peridotite consisting largely of olivine and pyroxenes; commonly exhibits a metamorphic texture indicating (re-) crystallization deep in the earth.

Mafic: see basic.

Magma: molten rock; referred to as lava when on the earth's surface.

Metamorphic Rocks: rocks whose texture and mineralogy have been changed in the solid, i.e. without melting, by heat and/or pressure.

Metasomatism: a group of processes by which rocks change their chemical composition in the solid, i.e. without melting.

Meteoric Water: water derived directly from the atmosphere.

Mullite: a very high-temperature alumino-silicate mineral.

Mugearite: see hawaiite.

Olivine: a silicate mineral enriched in magnesium (and/or iron).

Pegmatite: applied to very coarse-grained varieties of igneous rocks; however, 'pegmatite' normally implies very coarse-grained granite.

Peridotite: a coarse-grained, ultrabasic igneous rock consisting largely of olivine and pyroxene(s).

Petrography: (the study of) the mineralogy and texture (fabric) of rocks.

Phyric: denotes the type(s) of the large crystals in a porphyritic igneous rock (usually a lava), e.g. feldspar-phyric.

Picrite: a lava of ultrabasic composition, particularly enriched in olivine.

Pitchstone: a rhyolite composed largely of volcanic glass.

Plagioclase: see feldspar.

Porphyrite: an igneous rock, often of intermediate composition and of medium grain-size, which displays porphyritic texture.

Porphyritic: the texture of those igneous rocks, often lavas, in which large crystals (megacrysts, phenocrysts) are surrounded by a matrix of smaller crystals and/or glass. Pyroclastic: see volcaniclastic.

Pyroxenes: the most abundant ferromagnesian silicate minerals in the earth's crust, e.g. augite and hypersthene.

Quartz: a mineral composed entirely of silica (SiO₂)

Rheomorphism: the processes by which a rock is (re-)melted.

Ring-Dyke: a near-cylindrical igneous intrusion which tends to close upwards on projection (cf. cone-sheet).

Skarn: a rock containing iron-rich and other minerals sometimes found at the contact between an igneous intrusion and its aureole, especially where this contains limestones; metasomatism is often invoked.

Sill: a sheet-like body of igneous rock which, in general, does not cross-cut the structure of the rocks which it intrudes; often gently inclined, medium-grained and composed of dolerite or basalt (cf. dyke).

Syenite: a coarse-grained, intermediate igneous rock consisting largely of alkali-feldspar and various ferromagnesian silicate minerals; usually in large intrusions.

Teschenite: an alkali-rich gabbro.

Tholeiite: basalt relatively enriched in silica (SiO)₂ and deficient in the alkalis (NaO₂ and K₂O).

Trachybasalt: basalt containing both plagioclase and alkali-feldspar.

Trachyte: a fine-grained, intermediate igneous rock consisting largely of alkali-feldspar and various ferromagnesian silicate minerals; usually a lava or dyke.

Transitional Basalt: basalt transitional between alkali-basalt and tholeiite.

Tridymite: a high-temperature equivalent (para-morph) of quartz.

Troctolite: a coarse-grained basic igneous rock consisting largely of olivine and plagioclase feldspar.

Tuff: consolidated volcanic ash (see also volcaniclastic rocks).

Tuffisite: a Tuff-like rock usually formed by the hydrothermal breakdown of volcanic rocks close to a rock fracture.

Ultrabasic Rocks: coarse- to fine-grained igneous rocks which are particularly enriched in 'bases' (see also basic) and relatively deficient in silica (SiO₂ nominally under 45%).

Vesicles: gas bubble cavities in consolidated lavas.

Volcaniclastic Rocks: rocks made up of volcanic fragments (clasts); also known as pyroclastic rocks.

Xenocrysts/-liths: (fragments of) crystals and rocks that are foreign to the igneous rock in which they are found.

Zeolites: a group of low-temperature, hydrous, elumino-silicate minerals.

References