eruption cloud . A cloud of volcanic gases, with ash and other pyroclastic fragments (tephra), that forms by volcanic explosion; the ash may fall from it the eruption cloud to deposit fallout tuff . Syn: explosion cloud; ash cloud; dust cloud [volc]; volcanic cloud. See also: eruption column.

coast . (a) A strip of land of indefinite width (may be many kilometers), bordering the ocean, that extends from the low-tide line inland to the first major change in landform features. Note that lakes and rivers have shores, while coasts are generally understood as bordering only oceans. (b) The part of a country regarded as near the coast, often including the whole of the coastal plain; a littoral district having some specific feature, such as the Gold Coast. Adj: coastal.

roadstead (road'-stead). An area of water near a shore, sheltered by a reef, sandbank, or island, or an open anchorage, usually a shallow indentation in the coast, where vessels may lie in relative safety from winds and heavy seas; it is often outside, and less sheltered than, a harbor. An "open roadstead" is unprotected from the weather. Syn: road [coast].

crust [interior Earth] . The outermost layer or shell of the Earth, defined according to various criteria, including seismic velocity, density and composition; that part of the Earth above the Mohorovicic discontinuity, made up of the sial and the sima. It represents less than 0.1% of the Earth's total volume. See also: continental crust; oceanic crust. Cf: tectonosphere.