Agglomerates (from the Latin 'agglomerare' meaning 'to form into a ball') are coarse accumulations of large blocks of volcanic material that contain at least 75% bombs. Volcanic bombs differ from volcanic blocks in that their shape records fluidal surfaces: they may, for example, have ropy, cauliform, scoriaceous, or folded, chilled margins and spindle, spatter, ribbon, ragged, or amoeboid shapes. Globular masses of lava may have been shot from the crater at a time when partly molten lava was exposed, and was frequently shattered by sudden outbursts of steam. These bombs were viscous at the moment of ejection and by rotation in the air acquired their shape. They are commonly one or two feet in diameter, but specimens as large as twelve feet have been observed. There is less variety in their composition at any one volcanic centre than in the case of the lithic blocks, and their composition indicates the type of magma being erupted.

Agglomerates are typically found near volcanic vents and within volcanic conduits, where they may be associated with pyroclastic or intrusive volcanic breccias. Older (pre 1970) publications, particularly in Scotland, referred to any coarse-grained volcaniclastic rock as 'agglomerate', which led to debris flow deposits, talus deposits and other types of breccia being mistaken for vents. Agglomerates are typically poorly sorted, may contain a fine ash or tuff matrix and vary from matrix to clast support. They may by monolithologic or hetrolithic, and may contain some blocks of various igneous rocks. There are various differences between agglomerates and ordinary ash beds or tuffs. Agglomerates are coarser and less frequently well-bedded. Agglomerates can be non-welded or welded, such as coarse basaltic 'spatter'. They typically form proximally during Strombolian eruptions, and are common at strongly peralkaline volcanoes. Some large agglomerate deposits are deposited from pyroclastic density currents during explosive caldera-forming eruptions, such as at Santorini, Taal, and Campi Flegrei. They may be massive to crudely bedded, and can attain great thicknesses.

Crystalline masses of a different kind occur in some numbers in certain agglomerates. They consist of volcanic minerals very much the same as those formed in the lava, but exhibiting certain peculiarities which indicate that they have formed slowly under pressure at considerable depths. They bear a resemblance to plutonic igneous rocks, but are more correctly to be regarded as agglomerations of crystals formed within the liquid lava as it slowly rose towards the surface, and at a subsequent period cast out by violent steam explosions. The sanadinites of the Eifel belong to this group. At Vesuvius, Ascension, St Vincent and many other volcanoes, they form a considerable part of the coarser ash-beds. Their commonest minerals are olivine, anorthite, hornblende, augite, biotite and leucite.

Wikimedia Foundation. 2010.

Игры ⚽ Поможем написать курсовую

Look at other dictionaries:

  • Agglomerate — Ag*glom er*ate, n. 1. A collection or mass. [1913 Webster] 2. (Geol.) A mass of angular volcanic fragments united by heat; distinguished from conglomerate. [1913 Webster] …   The Collaborative International Dictionary of English

  • Agglomerate — Ag*glom er*ate, v. t. [imp. & p. p. {Agglomerated}; p. pr. & vb. n. {Agglomerating}.] [L. agglomeratus, p. p. of agglomerare; ad + glomerare to form into a ball. See {Glomerate}.] To wind or collect into a ball; hence, to gather into a mass or… …   The Collaborative International Dictionary of English

  • Agglomerate — Ag*glom er*ate, v. i. To collect in a mass. [1913 Webster] …   The Collaborative International Dictionary of English

  • Agglomerate — Ag*glom er*ate, Agglomerated Ag*glom er*a ted, a. 1. Collected into a ball, heap, or mass. [1913 Webster] 2. (Bot.) Collected into a rounded head of flowers. [1913 Webster] …   The Collaborative International Dictionary of English

  • agglomerate — index accumulate (amass), agglomeration, aggregate (noun), aggregate (verb), cement, coadunate, cohere ( …   Law dictionary

  • agglomerate — (v.) 1680s, from L. agglomeratus, pp. of agglomerare to wind or add onto a ball, from ad to (see AD (Cf. ad )) + glomerare wind up in a ball, from glomus (gen. glomeris) ball of yarn, from PIE root *glem . Related: Agglomerated …   Etymology dictionary

  • agglomerate — agglomerate, agglomeration conglomerate, conglomeration, *aggregate, aggregation Analogous words: combination, association (see corresponding verbs at JOIN): accumulation (see corresponding verb ACCUMULATE): heap, pile, mass (see under HEAP vb) …   New Dictionary of Synonyms

  • agglomerate — ► VERB ▪ collect or form into a mass. ► NOUN ▪ a mass or collection of things. DERIVATIVES agglomeration noun. ORIGIN Latin agglomerare add to …   English terms dictionary

  • agglomerate — [ə gläm′ər āt΄; ] for adj. & n. [, ə gläm′ər it] vt., vi. agglomerated, agglomerating [< L agglomeratus, pp. of agglomerare < ad , to + glomerare, to form into a ball < IE base * glem , var. of * gel , form a ball, sphere, hence akin to… …   English World dictionary

  • agglomerate — I. transitive verb ( ated; ating) Etymology: Latin agglomeratus, past participle of agglomerare to heap up, join, from ad + glomer , glomus ball more at clam Date: 1684 to gather into a ball, mass, or cluster II. adjective Date: 1828 gathered… …   New Collegiate Dictionary

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”