Wave-formed ripples
Encyclopedia
In sedimentology
Sedimentology
Sedimentology encompasses the study of modern sediments such as sand, mud , and clay, and the processes that result in their deposition. Sedimentologists apply their understanding of modern processes to interpret geologic history through observations of sedimentary rocks and sedimentary...

, wave-formed ripples or wave-formed ripple marks are a feature of sediment
Sediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....

s (sandstone
Sandstone
Sandstone is a sedimentary rock composed mainly of sand-sized minerals or rock grains.Most sandstone is composed of quartz and/or feldspar because these are the most common minerals in the Earth's crust. Like sand, sandstone may be any colour, but the most common colours are tan, brown, yellow,...

s, limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

s, siltstone
Siltstone
Siltstone is a sedimentary rock which has a grain size in the silt range, finer than sandstone and coarser than claystones.- Description :As its name implies, it is primarily composed of silt sized particles, defined as grains 1/16 - 1/256 mm or 4 to 8 on the Krumbein phi scale...

s) and dune
Dune
In physical geography, a dune is a hill of sand built by wind. Dunes occur in different forms and sizes, formed by interaction with the wind. Most kinds of dunes are longer on the windward side where the sand is pushed up the dune and have a shorter "slip face" in the lee of the wind...

s. These ripple marks are often characterised (and thus distinguished from current ripples) by symmetric cross section
Cross section (geometry)
In geometry, a cross-section is the intersection of a figure in 2-dimensional space with a line, or of a body in 3-dimensional space with a plane, etc...

s and long relatively straight crests, which may commonly bifurcate. Commonly, these crests can be truncated by subsequent flows. Their wavelength (periodicity) depends on the sediment grain size, water depth and water-particle orbits in the waves. On tidal flats the pattern of wave-formed ripples may be complicated, as a product of changing depth and wind and tidal runoff directions. Symmetrical ripples are commonly found in shallow waters. Beaches are a good environment in which to locate these ripples.

While wave-formed ripples are traditionally described as symmetrical, asymmetric wave ripples are common in shallow waters along sandy shores. They are produced by bottom oscillations generated by passing breaker waves, which have unequal intensity in opposite directions.

Wave-formed ripples indicate an environment with weak currents where water motion is dominated by wave oscillations.

Although symmetrical ripples are also called bi-directional ripples there is a difference between them. Bi-directional ripples are rarely symmetrical due to the difference in force of the two directions, where as the wave formed or oscillation ripples are formed from the circular water movement pattern of water molecules. These ripples are formed parallel to shore line. They usually display rounded troughs and rounded crests.

Ripples

Ripples are relatively small, elongated ridges that form on bed surfaces perpendicular to current flow. With continuous current flow in one direction asymmetrical ripples are formed. Asymmetrical ripples contain a steeper slope downstream. With an alternation in current flow from one direction to the opposite symmetrical ripples are formed. Symmetrical ripples tend to have the same slope on both sides of the crest.

Formation

Symmetrical ripples are formed as water molecules oscillate in small circles. A particle of water within a wave does not move with the wave but rather it moves in a small circle between the wave crest and wave trough. This movement of water molecules is the same for all water molecules effected by the wave. The water molecules continue to do this to a depth equal to 1/2 the wave length. The water molecule traveling in a circular patern interacts with the sediment on the floor and moves the sediment into symmetrical ripples. These ripples can be either straight crested or sinuous crested ripples.
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