page_title: Peperite
hierarchical_section_title: Peperite
caption_attribution_description: English: Andesitic Peperite. This was removed from Ghyll Scaur quarry. It shows the reaction between hot andesitic magma (900C?) and groundwater rich volcanic derived ash. The andesite has explosively fragmented as mini steam explosions ripped through the ash. There is no baked margin in peperites as steam is an excellent insulator. The rock is part of the Borrowdale volcanic group. It represents Ordovician (450ma ish) rocks formed on an Avalonian island arc. The intermediate composition of the arc led to seriously violent ultra-Plinian explosions. The line of volcanoes preserves some of the most explosive eruptions known to man. Mapping of the Scafell caldera in the 1980s discovered more of these massive volcanoes. These eruptions would have blocked out the sun from most of the planet. Gas thrust of the rapidly degassing magma emplaced ash up into the stratosphere, with plumes reaching 50km. The ash matrix originated from pyroclastic density currents (PDCs) which sped down the volcano flank, possibly towards the end of the eruption with dome, caldera or plume collapse. (Battery at lower edge of rock, for scale.)
context_page_description: A peperite is a type of volcaniclastic rock consisting of sedimentary rock that contains fragments of younger igneous material and is formed when magma comes into contact with wet sediments. The term was originally used to describe rocks from the Limagne region of France, from the similarity in appearance of the granules of dark basalt in the light-coloured limestone to black pepper. Typically the igneous fragments are glassy and show chilled-margins to the sedimentary matrix, distinguishing them from clasts with a sedimentary origin.
context_section_description: A peperite is a type of volcaniclastic rock consisting of sedimentary rock that contains fragments of younger igneous material and is formed when magma comes into contact with wet sediments. The term was originally used to describe rocks from the Limagne region of France, from the similarity in appearance of the granules of dark basalt in the light-coloured limestone to black pepper. Typically the igneous fragments are glassy and show chilled-margins to the sedimentary matrix, distinguishing them from clasts with a sedimentary origin.
caption: Peperite of Cumbria, England. This example was formed during the Ordovician Period and it is of andesitic composition.
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