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.
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.
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).
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.
the geological basement, the surface beneath which sedimentary rocks are not found; the igneous, metamorphic, granitised or highly deformed rock underlying sedimentary rocks.
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.
the splitting up of a continent or supercontinent (in this case the supercontinent Gondwana) into smaller continental blocks or fragments.
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.
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.
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.
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.