Ignimbrite EC (East Auckland) EarthCache
Ignimbrite EC (East Auckland)
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An ignimbrite is the deposit of a pyroclastic density current, or pyroclastic flow, a hot suspension of particles and gases that flows rapidly from a volcano, driven by a greater density than the surrounding atmosphere.
Ignimbrites are made of a very poorly sorted mixture of volcanic ash (or tuff when lithified) and pumice lapilli, commonly with scattered lithic fragments. The ash is composed of glass shards and crystal fragments. Ignimbrites may be loose and unconsolidated, or lithified (solidified) rock called lapilli-tuff. Near source, ignimbrites commonly contain thick accumulations of lithic blocks, and distally, many show m-thick accumulations of rounded blocks (or cobbles) of pumice. The term 'ignimbrite' derives from 'fiery rock dust cloud' (from the Latin igni- (fire) and imbri- (rain)), and forms as the result of immense explosions of pyroclastic ash, lapilli and blocks flowing down the sides of volcanoes.
Ignimbrites may be white, grey, pink, beige, brown or black depending on their composition and density. Many pale ignimbrites are dacitic or rhyolitic. Darker coloured ones may be densely welded vitrophyre (i.e. glassy) or, less commonly, mafic in composition.
Ignimbrite also occurs in the Coromandel region of New Zealand, where the striking, orange-brown ignimbrite cliffs form a distinctive feature of the landscape. The nearby Taupo Volcanic Zone is covered in extensive, flat sheets of ignimbrite erupted from caldera volcanoes during the Pleistocene and Holocene.
The white volcanic pumice deposits in the cliffs at GZ are rhyolitic ash. The ignimbrite here was deposited from central North Island eruptions. Near GZ you'll find an information board.
To claim the earth cache you'll need to answer the following questions:
- How old is the ignimbrite?
- How thick is the ignimbrite over here?
- How thick is the ignimbrite higher up the Tamaki river?
- From volcanic eruptions in which areas are these ignimbrites?
Send your answers by email to the cache owner. On line logs with answers will be deleted. You also need to take a picture of you and/or your GPS'r near GZ. Post the picture with your log.
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Treasures
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