Reef

A reef ( Old Norse rif " ridge; Reef" ) is a more or less elongated elevation, which rises from the bottom of the water towards the water surface. A reef may lie deep below the surface, extend to approach this or stick out beyond. In the water it is usually around a sea.

There are essentially three different forms:

• Reefs: are made of solid rock of different origin; they protrude above the water surface, they are also called cliff
• Sand reefs: zusammengespültem consist of sand ( → shoal, sandbar → )
• Biogenic reefs: consist of fixed marine organisms with calcareous skeleton that grow on each other, thereby forming massive limestone body over time

The most common biogenic reefs are coral reefs. They are predominantly composed of colonies of hard corals. The Great Barrier Reef off the northeast coast of Australia is probably the most famous example of a coral reef complex. But smaller reefs can also be formed by other fixed -growing animals, eg Oyster reefs. In sea areas where reef-building organisms live, reefs can occur (→ Artificial Reef ) by human intervention.

In the course of the earth every era had their favorite reef builders. Even before 2-3 billion years in the Precambrian, created by the activity of blue-green algae, the so-called stromatolites could grow to be relatively large, riffartigen structures ( blue-green algae, however, are still subordinate to the Riffbau involved). In the Silurian and Devonian (ca. 440-360 mya ) prevailed Stromatoporenriffe, during the Cretaceous period, about 100 million years ago, were shells with coral- like appearance, the so-called rudists, significant Riffbildner.

Biogenic reefs are geologically very significant. Products resulting from the biogenic Riffbildung rocks are referred to as reef limestones. Open-minded fossil Riffkomplexe found as Massenkalke in almost all mountain ranges, including the Guadalupe Mountains in Texas, in the Rhenish Slate Mountains in West Germany and in the Dolomites in northern Italy.

Deep Massenkalke lying in the ground are important reservoir rocks for oil and gas.