ammonites or ammonoid
any of a group of Cephalopods of the subclass Ammonoidea, which appeared in the lower Devonian and were extinct by the end of the Cretaceous Period. They typically have an external chambered shell, planispirally coiled and often ornamented with ribs and knobs. Ammonoids are important as index fossils because of their rapid evolution and wide distribution in shallow marine waters.
basalt
dark coloured, fine-grained basic (low silica (45-52% and relatively high calcium, iron and magnesium) volcanic rock, composed mainly of the minerals calcium plagioclase and pyroxene, usually augite, with or without olivine.
basalt lava
is the most abundant lava type – it forms the upper layer of the oceanic crust, and is the chief constituent of intraplate oceanic islands. Varieties of basalt are found in island arcs and at active continental margins, and vast amounts of continental flood basalts have been erupted, associated with tension and rifting of the continents (eg Karoo flood basalts, southern Africa and Deccan flood basalts of India).
basaltic crust
the crust is the outermost layer of the Earth, the lower boundary of which is called the mohorovcic discontinuity or ‘moho’. Part of the crust is termed continental (an assemblage of igneous, sedimentary and metamorphic rocks rich in elements such as silicon and potassium), the other part, oceanic. Oceanic crust is much younger than the continental crust. The thickness of the oceanic crust (5 to 10km) is much less than that of the continental crust (25 to 90 km). Oceanic crust is constantly being generated at mid-ocean ridges, and destroyed as it moves to a subduction zone. Basalts are the main material of the upper oceanic crust.
basement
the geological basement, the surface beneath which sedimentary rocks are not found; the igneous, metamorphic, granitised or highly deformed rock underlying sedimentary rocks.
basins
a depression of large size, which may be of structural or erosional origin. The Karoo basin extended across much of southern Gondwana and records 120 millions of years of geological history, being the site of sediment deposition for that length of time.
breakup (Gondwana)
the splitting up of a continent or supercontinent (in this case the supercontinent Gondwana) into smaller continental blocks or fragments.
burrow
a pipe-like structure in sedimentary rock, made by an animal that lived in the soft sediment. Often filled with clay or sand, they may be along the bedding plane or may penetrate the rock.
Cambrian
The oldest Period of the Palaeozoic Era, having a duration of about 85 million years and beginning about 590 million years ago. Rocks of the Cambrian system are the oldest in which fossil remains are sufficiently abundant and distinct, because of hard parts, to provide reliable geological information.
clay
a smooth, earthy sediment or soft rock composed chiefly of clay-sized (less than 0.004mm in diameter) or colloidal particles and a significant content of clay minerals.
collision zone
a collision zone is the zone or belt where two pieces of continental crust have collided after closure of an intervening ocean.
conglomerate
coarse grained clastic sedimentary rock, composed of more or less rounded fragments or particles at least 2mm in diameter (granules, pebbles, cobbles, boulders), set in a fine-textured matrix of sand or silt, and commonly cemented by calcium carbonate, silica, iron oxide, or hardened clay.
continental blocks
separate or geologically unique blocks or rock masses that together form the continent – these would have formed independently of one another, having different geological histories and ages.
craton
a part of the continental crust that has been stable for at least 1000 million years. Cratons are typically formed of Lower to Middle Precambrian igneous and metamorphic rocks with a subdued surface relief which in places are overlain by largely undeformed Upper Precambrian or younger sedimentary rocks.
Cretaceous
the interval of geological time that began about 140 million years ago, and lasted about 75 million years. It is the final period of the Mesozoic era, and precedes the Tertiary Period.
crustaceans
any arthropod of the subphylum Crustacea, characterized chiefly by two pairs of antenna-like appendages in front of the mouth and three behind it. Present groups are represented by shrimp, crabs, lobsters, copepods and isopods; most forms are marine.
deformed
having undergone deformation – the alteration, such as faulting, folding, shearing, compression and extension of rock formations by tectonic forces.
desertification
is the slow expansion of the world’s deserts, either due to gradual climate change, and also nowadays due in part to rain falling in the desert margins, and the land being heavily farmed and overgrazed, allowing soil to dry out completely.
dinosaur
any of that particular group of reptiles that dominated or were prominent among Mesozoic life forms. They are divided into two forms on the basis of pelvic structure: the Saurischia with the normal reptile pelvis structure, and the Ornithischia, in which the pelvis resembles that of a bird. They ranged in size from 30cm to 26m. They were carnivorous, herbivorous, bipedal or quadrupedal. Most were terrestrial, but there were also aquatic and semi-aquatic representatives. Range, Triassic to Cretaceous.
dolerite
a medium-grained intrusive igneous rock of basaltic composition, composed of the minerals pyroxene, plagioclase and iron-titanium oxides. It may be seen in the field as dykes and sills – feeding magma to form basalt lavas flows.
dyke
a tabular body of intrusive igneous rock that cuts across the layering or structural fabric of the host rock. It may be a composite or multiple intrusion.
erosion
the wearing away of any part of the Earth’s surface by natural agencies. These include mass wasting and the actions of waves, wind, streams and glaciers. Fundamental to the process of erosion is that material must be picked up and carried away by such agents (transportation).
estuarine
originating in an estuary – formed where a deeply cut river mouth is drowned following a land subsidence or a rise in sea level. Fresh water intermixes with seawater and tidal effects occur.
extinction
the dying-out of a plant or animal species. This may have arisen from a variety of causes, for example increased competition for certain niches, variation in the physical environment such as climatic changes or fluctuations in sea-level, which affect the range of habitats available.
faulting
the action or process of fracturing and displacement that produces a fault – a fracture in earth materials, along which the opposite sides have been relatively displace parallel to the plane of movement. The surface along which movement takes place is known as the fault plane, or fault surface. Faulting may mark the walls of the fault plane with slickensides – a striated and polished surface from the grinding and sliding of two rock masses against one another.
glacial pavement
bedrock surface with fine-cut parallel or near-parallel lines made by rock fragments carried in a glacier which travelled over the bedrock
Glossopteris
a late Palaeozoic genus of fossil plants (tongue-ferns) found throughout the glaciated regions of the Southern Hemisphere. Glossopteris flora bore clusters of simple spatulate leaves.
gneiss
a foliated metamorphic rock formed under conditions of high grade regional metamorphism. It is usually coarse grained and characterised by a layered appearance due to the segregation of ferromagnesian from quartzo-feldspathic minerals in discontinuous layers or lenticles.
Gondwana
the southern supercontinent taking its name from the Gondwana system of India, dating to the late Palaeozoic and early Mesozoic, and containing glacial tillites below coal measures. Similar rock sequences of the same age, containing identical fossil flora (Glossopteris) show connection of the southern continents of Antarctica, Africa, South America, Australia and India
granite
a light coloured medium to coarse-grained plutonic(of deep seated origin) igneous rock composed mainly of quartz and feldspar with biotite and/or hornblende as the commonest ferromagnesian minerals.
greenstone belt
embedded in the ancient granite and gneiss terrains of South Africa and Australia are greenstone belts. These are islands of ancient deformed rock, metamorphosed from basaltic lava and topped by sediments. They are between 2.5 and 3.5 billion years old. Because greenstone belts contain pillow lavas, geologists believe that they are ancient pieces of oceanic crust that formed under the sea in back-arc basins as the continents grew larger.
igneous rock
a rock that has solidified form molten rock material (magma) which was generated deep within the Earth. Igneous rocks are one of the three main groups of rocks that comprise the Earth’s crust ( the others being sedimentary and metamorphic).
ilmenite
a black or dark brown rhombohedral mineral FeTiO3, a major ore of titanium; it occurs in compact or granular aggregates.
intruded
intrusion, also called injection, is the emplacement of magma into pre-existing rock. It can take place either by deformation of the involved rock, or along some structural channels such as bedding planes, cleavages or joints.
kimberlite
a porphyritic, ultrabasic intrusive igneous rock containing abundant olivine and mica of the phlogopite variety in a serpentine and carbonate-rich matrix. Host rock of diamond.
lava
molten rock (or magma) that issues from openings at the Earth’s surface or on the ocean floor. Such openings may be located in craters or along flanks of volcanoes, or in fissures not associated with volcanic cones.
limestone
a sedimentary rock composed almost entirely of calcium carbonate mainly as the mineral calcite. Organic limestones consist of shell remnants or of calcite deposits precipitated by certain algae, e.g. coral limestone, crinoidal limestone, chalk. Chemically precipitated limestones form mainly in warm shallow seas.
Lystrosaurus
Lystrosaurus lived in Africa, India and China 200 million years ago. The discovery in the 1960’s of a fossil of Lystrosaurus in Antarctica was evidence to support the idea of continental drift.
magma
molten, mobile rock material that is a naturally occurring high temperature solution of silicates, water and gases. Suspended solids such as crystals and rock fragments may or may not be included. Magmas is generated deep within the Earth’s crust or upper mantle as a result of partial melting, and is the sources of igneous rocks.
Melanorosaurus
large quadrupedal, herbivorous sauropod dinosaur from Triassic rocks, specifically the Elliot Formation of South Africa.
metamorphosed
having undergone metamorphism – the processes which produce structural and mineralogical changes in any type of rock in response to physical and chemical conditions differing from those under which the rocks originally formed. Changes brought about by near-surface processes such as diagenesis and weathering are excluded.
mudstones
commonly used synonym for mudrock, a fine-grained sedimentary rock composed chiefly of particles in the silt-clay size range. Mudrock is a general term that can be used to distinguish the finer-grained sedimentary rocks from sandstones or limestones.
pillow lavas
hot, fluid magma rapidly cooled into pillow structures when coming into the sea or water saturated sediments.
pillow structures
spherical or ellipsoidal structures usually composed of basaltic lava, generally about 1m in diameter. These are the result of the rapid cooling of hot, fluid magma that comes into the sea or into water saturated sediments.
rhyolite
one of a group of extrusive rocks (those extruded at the Earth’s surface) commonly showing flow texture, and typically porphyritic, with phenocrysts of quartz and potassium feldspar in a glassy to microcrystalline groundmass. Rhyolite is the extrusive equivalent of granite.
rift
a zone where two tectonic plates are pulling apart, often forming a mid-ocean ridge with the associated upwelling of hot magma.
rutile
a yellow, red, brown or black tetragonal mineral, TiO2, an important ore of titanium. Rutile occurs as a very common accessory mineral in intrusive igneous rocks, or dispersed through quartz veins.
sandstone
a sedimentary rock composed of sand-sized grains in a matrix of clay or silt, and bound together by a cement that may be carbonate. Quartz forms about 65% of the detrital fraction of the average sandstone, and feldspars about 10 to 15%. Most sandstones originate as underwater deposits, usually marine; many sandstones were originally beach deposits.
sea level
the level corresponding to the surface of the sea half way between mean low and high tide.
sedimentary rocks
rocks formed by the consolidation of sediment settled out of water, ice or air and accumulated on the Earth’s surface, either on dry land or under water. Sediments are consolidated into a rock mass by lithification. Sedimentary rock is typically stratified or bedded, beds can vary greatly in thickness.
shales
fine-grained sedimentary rock formed by the compaction of silt, clay, or sand that accumulates in deltas and on lake and ocean bottoms. It is the most abundant of all sedimentary rocks. Shales may be black, red, grey or brown.
sill
a tabular igneous intrusion with boundaries conformable with the planar structure of the surrounding rock.
silt
a detrital particle, finer than very fine sand and coarser than clay, in the range of 0.004 to 0.062mm.
Striation directions
the direction (compass) of one of many thin lines or scratches, generally parallel, incised on a rock by some geological agent such as a glacier or stream. See glacial pavement.
stromatolites
laminated calcareous sedimentary formations produced by lime-secreting cyanobacteria. Living stromatolites are in the shape of stony cushions or massive columns. Fossilized stromatolites (silicified) dating well back to the Precambrian are found in the Gunflint Chert of Lake Superior, in cherts of Africa and Australia, and in calcareous sediments.
subduction
the movement of one crustal plate under another so that the descending plate is “consumed”.
supercontinent
a collection of many smaller continents to form one giant landmass.
tillite
a sedimentary rock formed by the compaction and cementation of till. Till is generally not layered and deposited directly by glacial ice. It is poorly sorted with a wide range of grain sizes from clay to boulders. Clasts are usually angular because they have undergone little or no water transport.
transport
the carrying away (transportation) of sediment by the agents of waves, wind, streams and glaciers. See erosion.
unconsolidated sediments
sediments which have not yet been compacted and cemented to form rock.
uplift
process by which regions of the Earth’s crust is raised above sea-level. This process is most commonly associated with rifting and collision of continental crust.
weathering
destructive natural processes by which rocks are altered with little or no transport of the fragmented or altered material. Mechanical weathering occurs with the freezing of confined water and the alternate expansion and contraction due to temperature changes. Chemical weathering produces new minerals. The main chemical reactions are oxidation, hydration and solution.
Witwatersrand rocks
rocks belonging to the 3000 million year old Witwatersrand Supergroup which outcrop around Johannesburg and south into the Free State. The Witwatersrand rocks are significant in that more than half of all the gold produced has been extracted from these rocks.
zircon
silicate mineral, ZrSiO4, an important ore for zirconium, hafnium and thorium; some varieties are used as gemstones. Zircon is a typical accessory mineral of acidic igneous rocks and their metamorphic derivatives. It also occurs as a detrital mineral.
Sources Consulted:
Farndon, J. (1998). Concise Encyclopedia Earth, Dorling Kindersley, Great Britain, 192.
Lapidus, D.F. (1987) Collins Dictionary of Geology, Winstanley, I. (Ed.), HarperCollins, Great Britain, 565pp.