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Sedgwick Museum of Earth Sciences - Geological glossary Category: Earth and Environment > Geology Date & country: 15/12/2007, UK Words: 108

Weathering
the breakdown of rocks and minerals at and just below the Earth's surface caused by a number of physical and chemical processes. Physical or mechanical weathering is caused by atmospherically controlled processes such as temperature or humidity changes. Many of these processes are due to crystal growth - for example ice growth and expansion in freeze-thaw action, or the growth of salt crystals in salt weathering. Physical weathering does not involve chemical changes to rock. Chemical weathering is caused by decomposition of rocks making them more sensitive to erosion. This includes the effects of acid rain on limestones, or the decomposition of granite to clay. Physical and chemical weathering processes work together to disintegrate rocks

Vertebrates
animals that possess a backbone and an internal skeleton made of bone or cartilage. The individual bones which make up the spine or backbone are the vertebrae

Volcano
layers of lava or ash erupted on to the surface of the Earth forming a cone-shaped structure. Volcanoes are found in areas of the Earth which are geologically unstable, such as near plate boundaries, or 'hot-spots' where magma wells-up from deep inside the Earth

Valves
paired, hard shells, for example in bivalves

Vascular plant
A plant that has an internal water and food transport system (xylem and phloem) of specially modified cells that form tube or pipe-like structures

Tubercles
small knobbles on the surface of echinoid and trilobite bodies. On echinoids they are attachment sites for spines which are adapted for protection and movement. On trilobites they are thought to have had a sensory purpose

Triassic
the period of geological time between 248 and 213 million years ago. The name reflects the division of this time period into 3 parts. See Geological Timescale

Trilobite
member of an extinct group of marine arthropods characterised by three body sections and bearing a superficial resemblance to a woodlouse. First appeared in the Cambrian and became extinct in the Permian

Tetrapods
1. any four-legged animals, including mammals, birds, reptiles and amphibians. 2. descriptive name given to the first four-limbed creatures to emerge on to the land during the Devonian, for example Acanthostega

Terrestrial
1. associated with the Earth; 2. an animal or plant living on the land

Terrestrial Planets
The group of rocky planets in the Solar System closest in chemical composition and character to the Earth: Mercury, Venus, Earth and Mars

Tertiary
the period of geological time between 65 and 1.6 million years ago

Test
a hard shell or covering over the internal soft parts of an invertebrate which may then have attached flesh, for example the echinoids

Taxonomy
the science of the classification of living things, grouped by similarity: species are grouped into genera, genera into families, families into orders, orders into classes, classes into phyla, and phyla with similar characteristics at the top level of the classification system - the kingdom. Plants are classified in a similar way, although the phylum unit is called a division

Suture lines
the line marking the join between the septa and the inside wall of the shell of a cephalopod mollusc. Suture lines are visible when the shell has been preserved as a fossil mould. The lines can be very simple curves, or folded into complex crenulations. Ammonite evolution has been defined around the increasing complexity of the pattern of suture lines

Strata
distinctive beds or layers of rocks

Stratigraphy
the study of rock strata and the relationships between them

Species
the basic unit of taxonomy. A species is defined as a group of individuals that are genetically related and can interbreed to produce fertile young of the same kind

Shale
a thinly laminated sedimentary rock made of tiny clay-sized sedimentary particles.

Silurian
the period of geological time between 440 and 410 million years ago. The name comes from the Silures, a tribe of Celts. See Geological Timescale

Soft bodied
describing animals such as worms and slugs which do not have a shell or bones. They are found fossilised as body fossils in exceptional circumstances, for example the Burgess Shale

Solitary
an organism which lives alone

Sedimentary rocks
recycled rocks made of fragments most often of other rocks and minerals, or organic remains, such as shells or shell debris. The organic parts of any sedimentary rock are referred to as fossils

Sediment
the material produced by the effects of weathering rocks close to or at the surface of the Earth

Rock Record
The sequences of rocks from all over the world which give us snapshots of different parts of the geological history of the planet Earth

Rostrum
in belemnites, the rear part of the bullet-shaped, cylindrical internal shell, also called the guard. Its purpose was possibly to act as a counter-balance to the weight of the animal's head and tentacles

Rock Cycle
a collection of processes which shape the surface of the Earth. Rocks are constantly recycled by burial, melting, uplift and erosion. These processes are driven by energy from inside the Earth and from the Sun (which drives processes in the atmosphere and leads to the weathering of rock)

Rock
A consolidated or unconsolidated aggregate of minerals or organic materials. (Oxford Dictionary of Earth Sciences). Rocks can be made of a single type of mineral, or more than one mineral. For example, limestone is made (mostly) of calcite and quartzite is made just of quartz. Granite is made up of three main minerals: quartz, feldspar and mica

Prokaryote
an organism with cells which do not contain a distinct nucleus

Quartzite
a rock made entirely of the mineral quartz

Reptiles
a class of air-breathing vertebrates that include the alligators and crocodiles, lizards, snakes, turtles, and extinct forms such as dinosaurs and pterosaurs. Reptiles are characterized by a bony skeleton and a body usually covered with scales or bony plates

Polyp
soft-bodied adult form of a member of the Cnidaria (corals, jelly fish and sea anemones) which has a cylindrical body which is anchored down at one end. At the other end is a mouth, usually surrounded by grasping tentacles used for feeding. The young are often free-swimming medusoids. Jellyfish are free-swimming and remain in the medusoid form throughout their lives

Plate
1. One of about fifteen rigid sections of the Earth's surface; a section of the Earth's surface bordered by seismic activity (earthquakes and volcanoes). 2. Pieces of calcium carbonate forming the outer shell of some marine creatures, for example echinoids

Permian
A period of geological time between 286 and 248 million years ago. The name comes from Permia, an ancient kingdom in Central Europe. See Geological Timescale

Period
a unit of geological time, for example the Devonian Period

Palaeozoic
An era of geological time between 544 and 248 million years ago. Palaeozoic means 'ancient life'. See Geological Timescale

Partial Melting
incomplete melting of a 'parent rock' to produce a molten rock (like magma) with a different chemical composition. This happens because different minerals melt at different temperatures

Order
in taxonomy, a group of genera with shared characteristics

Ordovician
the period of geological time between 505 and 440 million years ago. The name comes from the Ordovices, a Celtic tribe which lived in Wales in the region where these rock units were first defined. See Geological Timescale

Opposing muscles
Muscles which work against one another to allow side to side or up and down movement, e.g. the biceps and triceps allow humans to bend or straighten their lower arms by relaxing and contracting muscles in opposition to one another. Fish have opposing blocks of muscles around their spines that allow their bodies to flex from side to side, propelling them through the water

Omnivorous
an animal which eats a mixed diet of meat and plants, for example, humans. Omnivores have a dentition adapted for their diet with a combination of grinding and tearing teeth

Moult
to shed a hard exoskeleton to allow body growth, or to lose thick body fur during warmer times of the year

Mosses
small, simple land plants which first appeared in the Devonian, although they are rarely found as fossils. Mosses rely on water for reproduction

Mollusc
a phylum of invertebrates which include modern creatures such as snails, slugs, cockles, and squids. Molluscs have a muscular 'foot' for digging, movement, or swimming. Many have a hard protective shell. Some forms such as most slugs and octopuses have lost their shells as they have developed other ways of protecting themselves. Molluscs are common fossils found in rocks from the Cambrian period onwards, and are especially common in the Mesozoic and Cenozoic. (See Geological Timescale)

Mineral
minerals are the building blocks of rocks. They are naturally occurring substances, which often have a crystalline form. They can be single elements (such as gold or diamond) or compounds (such as quartz or pyrite)

Metamorphism
The process by which rocks are changed by heating and / or squashing. As rocks are buried over millions of years, they are exposed to heat and pressure from the overlying rocks and from movements within the Earth's crust. This causes changes in their properties. The heat can make the minerals which make up the rock recrystallise, often in distinct layers or bands. Metamorphism can occur where two of the rigid crustal plates making the Earth's surface collide, or where a magma rising through the Earth's crust bakes the rocks surrounding it

Mesozoic
the geological era which spans the Triassic, Jurassic and Cretaceous periods, from 245 to 65 million years ago. The name means middle life. See Geological Timescale

Marine
1. associated with the sea. For example, a marine biologist studies the biology of the sea. 2. of or living in the sea. For example, a marine snail lives in the sea

Marine reptiles
a group of mostly large, carnivorous animals many of which lived in the oceans during the Mesozoic period. Marine crocodiles, turtles and sea-snakes are the only members of this group alive today, but during the Jurassic there were many more types, including ichthyosaurs and pliosaurs

Mantle
the internal layer of the Earth between the crust and the core. It is usually solid, but can also be molten or partially molten. The mantle is composed of iron- and magnesium-rich silica minerals

Lithify
to turn to rock

Magma
hot molten rock full of dissolved gases. Magma can form as rocks melt deep in the Earth's crust, or can sometimes come directly from the mantle

Lava
the term for magma which has reached the Earth's surface and lost its dissolved gas content

Limestone
a sedimentary rock which composed mostly of calcium carbonate (calcite)

Kingdom
the top unit in taxonomy. Originally there were just two kingdoms; Animals and Plants. Now there are thought to be at least five kingdoms: animals, plants, fungi (mushrooms and lichen), protists (true algae) and monera (bacteria)

Jurassic
the period of geological time running from 213 to 145 million years ago. The name comes from the Jura Mountains on the French-Swiss border, where the most complete section of these rocks is found. See Geological Timescale

Invertebrate
an animal without a backbone, for example a starfish

Igneous
igneous rocks are new rocks, formed when hot magma rises up from inside the Earth and solidifies. Intrusive igneous rocks do not reach the surface, but form bodies of rock within the crust. Granite is an intrusive igneous rock. Extrusive igneous rocks are formed when the magma breaks through the crust and erupts on to the surface of the Earth. Once magma has reached the surface of the Earth it becomes known as lava - these rocks, such as basalt, can form volcanoes

Ichthyosaur
a Mesozoic marine reptile which bears a superficial resemblance to a modern dolphin

Horsetail
a plant which first appeared in the Devonian. Horsetails have jointed stems with a ring of long, pointed leaves and branches at each joint. Equisetum is the only type of horsetail alive today. Fossil horsetails grew up to 30 m in height, for example Calamites, which had a very thin stem but grew extremely long and probably crept like bind weed

Hemichordate
a group of animals first found in the Burgess Shale which have bodies very similar in arrangement to the chordates, but without the central stiffening rod (notochord)

Grasses
a group of flowering plants which now dominate the world's vegetation. They first appeared during the Tertiary

Graptolite
a stick-like group of colonial, marine animals which lived from the middle Cambrian to the Lower Carboniferous. They are very important for defining the stratigraphy of Ordovician and Silurian rocks in the U.K

Granite
a light-coloured intrusive igneous rock made up of the minerals quartz and feldspar with biotite and / or muscovite mica. The crystals are big enough to be visible without magnification. Granite is formed from the melting of the continental crust rocks

Fossil Record
the name given to the history of life on Earth as shown by fossils preserved in the rocks

Gastropod
a class of mollusc which typically have a single, often coiled shell, and a head and unsegmented soft body. They are found as fossils in rocks from the early Cambrian, and now live successfully on the land, in the sea, and in fresh water. The name means “stomach foot�. Examples are snails, whelks and slugs

Fossil
Fossils are the remains of plants and animals which lived on the Earth during the geological past and are now preserved in the rocks

Foodchain
the transfer of energy through an ecosystem starting with primary producers (plants) which use the energy of the sun to produce sugars, through a series of organisms that eat and are in turn eaten. An example might be: lettuce > slug > blackbird > hawk. A food web is a more realistic model of energy flow, which demonstrates how food chains combine

Fauna
the animal life of a region or period of geological time

Fern
a type of plant with large, divided leaves. First found in the Devonian. During the Palaeozoic era tree-ferns, some up to several metres in height, dominated the forest vegetation. Today ferns grow mostly as smaller plants and tree ferns are rare

Family
in taxonomy a group of genera which show very similar characteristics. For example, the cat family (Felidae) is made up of different genera which all have whiskers and sharp claws

Eukaryote
an organism with cells with a true nucleus. This includes fungi, plants and animals. Compare with prokaryote

Exoskeleton
a hard body covering found on the outside of many invertebrates. The term is most often applied to the body covering on arthropods. This is made of a protein compound which is secreted by a layer of cells underneath it. Because it is not stretchy, the creature has to moult regularly to accommodate growth

Erosion
the part of the process of denudation which includes the physical breakdown, chemical solution and transportation of material from the surface of the Earth. This includes the removal of the surface layer of rock or soil by agents of erosion such as ice, wind and water

Element
any of more than 100 fundamental substances that consist of atoms of only one kind. Single types of element or elements in combination constitute all matter in the known universe

Echinoid
Invertebrate marine organism with five-fold symmetry. Includes starfish and sea urchins

Ecosystem
a term used to describe a natural unit that consists of living and non-living parts which interact to form a stable system. The ecosystem idea can be applied at different scales in the same way e.g. a pond or an ocean

Division
in plant taxonomy, the second highest level of classification beneath kingdom. The equivalent to phylum in animal classification

Devonian
the period of geological time between 410 and 360 million years ago. The name comes from the strata in Devon, S.W. England, which define this part of geological history. The Devonian is also known as the Old Red Sandstone (often referred to as O.R.S.) because the characteristic colour of the rocks is red. The red colour is because of a high iron content in the rock, and tells us that the climate was hot and dry when they were deposited. See Geological Timescale

Denudation
the removal of rock material from the surface of the Earth though the processes of weathering and erosion.

Deposition
the process by which sediment is laid down and builds up on the Earth's surface - the direct opposite of erosion.

Dendroid
a branching type of graptolite. Most types lived attached to the sea-bed, and were upright and bushy in appearance

Cycad
a seed-bearing plant similar in appearance to modern palm trees. They first appeared in the Permian, and were common in the Mesozoic, but are rare today, having been displaced by flowering, fruit-bearing plants which appeared in the Cretaceous

Crust
the thin, outermost, solid layer of the Earth. It varies in thickness from 5 km beneath the oceans (oceanic crust) to 60 km beneath mountain chains (Continental crust). It is broken into a number of plates

Cretaceous
the period of geological time running from 145 â€` 65 million years ago. The name comes from the chalk which was the main rock type deposited during this period. See Geological Timescale

Correlation
the method by which rocks units or strata are compared and time-relationships between them are established. This can be done by examining the rock type and succession, the fossil content, or by chemical analysis

Core
1. The iron-rich central part of the Earth. The core is divided into two zones (the inner core and outer core) because of the different way in which they transmit seismic waves. The core is responsible for the Earth`s magnetic field, and accounts for 32% of the mass of the planet. 2. a long sample of sediment or rock extracted by drilling down into the Earth. This is used to help to analyse strata which are not exposed at the surface

Coral
important reef-building organisms known from the late Palaeozoic onwards. Fossil corals are good indicators of warm shallow marine environments. Corals can be solitary or part of a colony. A corallite is the skeleton formed by an individual coral polyp

Conifer
a plant belonging to a group of seed-bearing trees and shrubs, which have needle or scale-like leaves and resinous sap. Fertile parts of the plant are cones. The group first appeared in the Carboniferous

Compound
a substance formed by the chemical combination of two or more elements. Minerals such as calcite (made of calcium, carbon and oxygen) and quartz (made of silicon and oxygen) are compounds

Coal
a carbon-rich mineral deposit made of the remains of fossil plant life. It is first deposited as peat, but over time is buried, compressed and heated which changes it physically and chemically. There are different grades of coal depending on the amount of water and gases left inside the deposit, and the percentage of carbon present

Class
in taxonomy, a group of orders with shared characteristics. For example, Mammals are a class of vertebrates which are warm-blooded and give birth to live young which they breast-feed. This includes humans, cows, whales and cats

Chordate
an animal with an internal 'rod' of flexible tissue which supports its body. This rod can be bone or cartilage. Chordates with bony notochords are called vertebrates - for example mammals (including humans), fish, birds and reptiles. Invertebrate chordates include worms

Chalk
a variety of white limestone made from tiny mud-sized particles. The Chalk forms many of the low rounded hills in the south-east of England and was formed during the Cretaceous period

Cephalopod
a group of marine molluscs which have a soft body, grasping tentacles and a well developed nervous system. Fossil examples include ammonoids and belemnoids. Living examples include octopus, nautilus and squid.

Cartilage
flexible skeletal tissue found in vertebrates and chordates, made of fibres of a rubbery protein. In most animals the embryo has a skeleton made entirely of cartilage, most of which is replaced by bone as it develops. Some fish, such as sharks and rays, retain a cartilage skeleton throughout life

Carboniferous
the period of geological time between 360 and 286 million years ago, named for the thick deposits of coal found in rocks of this age. Forests covered much of the land during this time, the decomposition of which gave rise to the coal. See Geological Timescale

Cambrian Explosion
the term used to describe the very sudden appearance of a huge variety of fossil organisms with hard skeletons in the sedimentary rocks of the middle Cambrian.

Cambrian
the period of geological time between 544 and 505 million years ago. The name comes from the Cambrian Mountains in Wales, where the strata of the Cambrian were first defined. See Geological Timescale

Calcite
the mineral form of calcium carbonate. Limestones are made mostly of calcite