Differential Erosion, Diffraction, Discordant Coast, Dispersion, Diurnal Inequality: Word Origins, Descriptions of Coastal Features, Processes

In the current alphabetic series of word origins and physical descriptions of coastal features and processes, here are differential erosion, differential weathering, diffraction, discordant coast, dispersion, diurnal current, diurnal tide, and diurnal inequality.

Differential Erosion
The English word differential probably comes from French différentiel, from Middle French differentiel, from different plus -iel ("-ial").

In geology, the adjective differential means functioning or proceeding at a different rate.

Differential erosion, or differential weathering, refers to the process, or result, of the wearing away of some portions of a rock faster than other portions because of varying resistance to such erosion or weathering in different parts of the rock.

Diffraction
The English word diffraction comes from New Latin diffraction-, a combining form of diffractio, from Latin diffringere ("to break apart"), from dis- ("apart") plus frangere ("to break").

Diffraction is the spreading out of any type of wave, such as a sound wave or an electromagnetic wave, when it meets an obstacle.

Water waves, too, display this phenomenon when they meet any barrier that interrupts any part of the regular train of waves. The diffraction of water waves results in the multidirectional spreading of the waves.

Discordant Coast
The English word discordant goes back through Middle English and Middle French to Old French discordant, from Latin discordant-, a combining form of discordans, the present participle of discordare ("to discord"), from discord-, discors ("discordant, at odds"), from dis- ("apart") plus cord-, cor ("heart").

A discordant coastline is one where bands of rock at the shore area run perpendicular to the coast. In a concordant coastline, the rocks run parallel to the coast.

Dispersion
Modern English dispersion comes from Middle English dispersioun, from Latin dispersion-, a combining form of dispersio, from dispersus, the past participle of dispergere ("to scatter"), from dis- ("apart") plus -spergere, from spargere ("to scatter").

Dispersion, in general, is the act or process of breaking up or spreading widely. The term has important uses with respect to waves of all kinds. For example, when a beam of white light goes through a glass prism, the beam disperses into components that have different frequencies and wavelengths, producing a rainbowlike effect.

The separation of water waves according to their differing rates of speed is also called dispersion.

Diurnal Current, Diurnal Tide
The English word diurnal comes from Latin diurnalis, from diurnus ("of the day"), from dies ("day").

A diurnal current, or diurnal tide, is a tidal pattern of one high tide (flood) and one low tide (ebb) each day. Areas having such a pattern include Vietnam and the Caribbean.

Many places in the world have two high and two low tides, or semidiurnal tides, every day. Some places have diurnal tides on some days and semidiurnal tides on others.

Diurnal Inequality
The difference in height between the two high tides or between the two low tides at a particular location on a particular day is called the diurnal inequality.
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Encyclopaedia Britannica Ready Reference 2004. CD-ROM. Encyclopaedia Britannica, 2004.

Merriam-Webster's Collegiate Dictionary. 11th ed. Springfield, Mass.: Merriam-Webster, 2006.

Merriam-Webster's Geographical Dictionary. 3rd ed. Springfield, Mass.: Merriam-Webster, 2007.

The Oxford English Dictionary. 2nd ed. Oxford, England: Clarendon Press, 1989.