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Pitchstone : A black, opaque volcanic glass that may contain irregular, whitish clusters of minerals. Resembles pitch in appearance.

Piercing point : A feature that is cut by a fault and moved. Reconstruction of that object can show how much the fault has moved.

Phyllonite : A rock that macroscopically resembles phyllite but that is formed by mechanical degradation (mylonization) of initially coarser rocks (e.g., graywacke, granite, or gneiss).

Phyllite : Any of a set of argillaceous rocks in a condition of metamorphism between slate and mica-schist.

Phenocryst : A relatively large crystal in an igneous rock.

Phenoclast : A large, conspicuous fragment in sediment or sedimentary rock composed of various sizes of material.

Petrology : The branch of geology that studies the origin, composition, distribution, and structure of rocks.

Peridotite : An olive green when fresh, medium brown when weathered, saccharoidal intrusive igneous rock composed mainly of olivine, sometimes with pyroxene.

Pelite : A descriptive name for a clastic rock with a grain size of less than 1/16 mm (originally sand or silt).

Pegmatite : Exceptionally coarse-grained igneous rock.

Paralithic : A weathered layer of bedrock.

Palagonite : An alteration product from the interaction of water with volcanic glass of chemical composition similar to basalt or from the interaction between water and basalt melt.

Paleocurrent : An indication of the direction of fluid flow (at the time of deposition) visible in a rock.

Palaeozoic : Also spelled Paleozoic.

The earliest of the three geologic eras of the Phanerozoic Eon, spanning the time from roughly 541 to 252.2 million years ago. It is the longest of the Phanerozoic eras and is subdivided into six geologic periods: the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. The Paleozoic Era follows the Neoproterozoic Era of the Proterozoic Eon, and is followed by the Mesozoic Era.

Pahoehoe : A type of vesicular, basaltic lava often with a ropy surface texture.

Oxbow lake : A crescent-shaped lake found within a floodplain or fluvial terrace created by the cut-off and abandonment of an active meander within a river or stream channel.

Orthosilicate : An obsolete classification based on salt of hypothetical orthosilicic acid. Compare metasilicate.

Orthorhombic : Having a crystal structure with three perpendicular axes all of different lengths.

Orthogeosyncline : A geosyncline between continental and oceanic cratons containing both volcanic and non-volcanic belts.

Orthoclase : A silicate of potassium and aluminium which crystallises in the monoclinic system and occurs as an essential constituent in granitic and syenitic rocks and as an accessory in many other rock types.

Orogeny : Any set of forces and events leading to a large structural deformation of the Earth’s lithosphere due to the engagement of tectonic plates. It is the primary mechanism by which mountains are built on continents.

Orogenesis : The formation and growth of mountains related to tectonic activity.

Ordovician : A geologic period and system, the second of six of the Paleozoic Era, spanning the time between 485.4 ± 1.9 to 443.4 ± 1.5 million years ago. It follows the Cambrian and is followed by the Silurian.

Olivine : An orthosilicate mineral of iron and magnesium which crystallises in the orthorhombic system and occurs widely in the basic and ultramafic igneous rocks. It includes olivine-gabbro, olivine–dolerite, olivine-basalt, and peridotites, among others.

Oligoclase : One of the plagioclase feldspars consisting of the albite and anorthite molecules combined in the proportions 9:1 to 7:3. It is found especially in the more acid igneous rocks.

Normal fault : Also called an extensional fault.

Dip-slip faults can be sub-classified into the types “reverse” and “normal”. A normal fault occurs when the crust is extended such that the hanging wall moves downward relative to the footwall. Contrast reverse fault.

Non-clastic : Having the properties of any chemically and/or organically deposited rock, such as limestones, chalks, and evaporite deposits.

Neogene: A geologic period starting 23 million years ago and, depending on definition, either lasting until today or ending 2.6 million years ago with the beginning of the Quaternary.

Muscovite : The common or white mica; for the most part an orthosilicate of aluminium and potassium that crystallises in the monoclinic system.

Mullion : A particular type of reworked boudin. The term is likely derived from an architectural structure with the same name.

Moraine : A glacially formed accumulation of unconsolidated debris which can occur in currently glaciated and formerly glaciated regions, such as those areas acted upon by a past ice age.

Monoclinic : Having three crystal axes of unequal lengths, with one intersection oblique and the other two perpendicular.

Monocline : A fold with a single limb which produces a sudden steepening of the dip; the rocks, however, soon approximate to horizontal on either side of this flexure.

Molding : The process of manufacturing by shaping pliable raw material using a rigid frame or model called a mold.

Mohs scale of mineral hardness : Also simply called the Mohs scale.

Miogeosyncline : A geosyncline in which volcanism is not associated with sedimentation, or the non-volcanic part of the orthogeosyncline located near the craton.

Mineralization : The hydrothermal deposition of economically important metals in the formation of ore bodies or “lodes”.

Mineralization : The hydrothermal deposition of economically important metals in the formation of ore bodies or “lodes”.

Migmatite: A composite rock composed of igneous or igneous-looking and/or metamorphic materials which are generally distinguishable megascopically.

Mid-oceanic ridge : An underwater mountain range typically having a valley known as a rift running along its axis, formed by plate tectonics.

Micropaleontology : A branch of paleontology which studies microfossils.

Mica : Any of a set of minerals which crystallise in the monoclinic system. They have similar chemical compositions and highly perfect basal cleavage.

Metatexis : Low-grade anatexis: partial or differential melting of rock components with low melting point.

Metasilicate : A salt of metasilicic acid H₂SiO₃.

Metapelite : A metamorphosed pelite rock.

Metamorphism : The solid state recrystallisation of pre-existing rocks due to changes in heat and/or pressure and/or the introduction of fluids, i.e. without melting.

Mesozoic : The era of geological time including the Triassic, Jurassic, and Cretaceous ages.

Mesocratic : A term applied to igneous rocks which in respect of their content of dark silicates are intermediate between those of leucocratic and melanocratic type, and contain 30–60% of dark heavy minerals.

Melanocratic : A term applied to rocks which are abnormally rich in dark and heavy ferro-magnesium minerals.

Mélange : Large-scale breccia formed in the accretionary wedge above a subduction zone.

Massive : A description applied to a homogeneous rock which lacks internal structure or layers.

Marl : A calcium carbonate or lime-rich mud or mudstone which contains variable amounts of clays and aragonite.

Marine terrace : A narrow, flat area often seen at the base of a sea cliff caused by the action of the waves.

Marble : A fine to coarse-grained granoblastic calcium carbonate that effervesces in dilute hydrochloric acid. Often banded with various colours and sometimes veined.

Mantle : The highly viscous layer of molten rock situated directly beneath the Earth’s crust and above the outer core.

Malmesbury group : An 830 to 980 Ma basal group of the Western Cape comprising at least eight distinct formations, including the Tygerberg, Piketberg, Porterville, Berg river, Klipplaat, Moorreesburg, Franschhoek, and Bridgetown formations.

Magnetite : An oxide of iron which crystallises in the cubic system. It is attracted by a magnet but does not attract iron itself.

Magma : Molten rock that sometimes forms beneath the surface of the Earth (or other terrestrial planets) and often collects in a magma chamber.

Mafic : A silicate mineral or rock that is rich in magnesium and iron. A mnemonic term for the ferromagnesian and other non-felsic minerals actually present in an igneous rock rich in dark (ferromagnesian) minerals (greater than 60% by volume). Basic [alkali gabbro (alkali basalt), syeno-gabbro (trachybasalt), gabbro (basalt and dolerite)].

Ma : A non-standard but widely used abbreviation for one million (1,000,000) years, using the metric prefix M (for “Mega”) to indicate a quantity of one million. When not otherwise qualified, it usually indicates 1,000,000 years Before Present (or 1,000,000 years ago).

Lustre: Also spelled luster.