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Geology Group Diary (23)

Started by johnd, September 14, 2017, 07:49:33 pm

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The Geology Group met at Merlin's Bridge Village Hall at 10'30am on Wednesday 13 September 2017. The topic was 'Igneous Rocks and their formation'.
As an introduction we looked at a collection of pebbles from the storm beach at Newgale. The pebbles were graded according to size starting with the fine sand and gravel on the beach and moving up to pebbles and cobbles on the crest of the storm beach. These were moved during powerful winter storms when spring tides raised the water level above mean high water. A variety of rock types could be identified in the pebbles including porphyrytic rhyolite from Ramsey Island, dark dolerite pebbles, white quartz pebbles and a banded gneiss specimen.
Rocks that originate as molten magma within the earth's crust are referred to as Igneous rocks. When magma starts to cool several kilometres below the surface it slowly begins to crystallise forming coarse grained igneous rocks like granite in which you can see the interlocking crystals with the naked eye. When the magma reaches the surface it cools more rapidly producing volcanic lavas that have a fine grained microcrystalline texture like basalt.
There are several types of volcanic eruptions:
Dome volcanoes are formed of viscous felsic magma with a high silica content such as rhyolite and dacite lavas. The Puys of Auvergne are classic dome volcanoes.
Composite volcanoes (or strato volcanoes) are composed of alternate layers of ash and lava such as andesite & trachyte.   Vesuvius (Italy) and Cotopaxi (Ecuador) are typical composite volcanoes. Minor intrusions such as dykes (vertical intrusions) and sills (horizontal intrusions) are often associated with composite volcanoes.
Shield volcanoes have a low profile and cover a wide area. They are formed of fluid basaltic lava with a low silica content.  Mauna Loa in Hawaii is a good example of a shield volcano.
Volcanic eruptions. Dome volcanoes erupt explosively throwing blocks of solid lava and ash out of the crater. Composite volcanoes often produce a Plinian eruption where hot ash and gas rise in a column to form a dense cloud above the cone. Mt.Redoubt in Alaska produced air fall deposits in 2009; the ash fell to the SW of the volcano due to the influence of the prevailing winds. When the eruption column collapses a pyroclastic surge occurs; this is a fast flowing river of pumice and rock fragments in a matrix of ash that rushes down the  slopes of the volcano. When Vesuvius erupted  in AD 79 the town of Pompeii was engulfed in a pyroclastic surge. On Mt Teide in Tenerife beds of white pumice and dark scoria represent the products of alternate pyroclastic flows. When pyroclastic material cools and solidifies it forms a rock called ignimbrite. Composite volcanoes are so called because they consist of alternate layers of ash and lava produced by alternate phases of explosive eruption and quiet lava effusion.
When shield volcanoes erupt they produce a steady flow of basaltic lava which bubbles in the crater as gases are emitted and incandescent debris creates fire fountains. If the lava flow reaches the sea then it cools rapidly on contact with the water forming a skin (reaction rim) around the still molten lava. The term pillow lavas refers to the numerous rounded masses of lava that accumulate on the sea floor.
Classification of Igneous Rocks
Igneous rocks can be classified according to their mineralogical composition.
Felsic Rocks contain > 66% by weight of SiO2. They include Granite and Rhyolite which contain quartz, orthoclase feldspar and mica.
Intermediate Rocks such as Diorite and Andesite contain plagioclase feldspar and amphibole
Mafic Rocks contain < 52% by weight of SiO2. They include Gabbro and Basalt which contain plagioclase feldspar and ferromagnesian minerals (pyroxene and olivine). Ultra mafic rocks like peridotite are rich in olivine.
Igneous rocks which contain large crystals set in a finer grained ground mass are said to have a porphyritic texture. The large crystals are called phenocrysts and these are often used to describe the rock such as quartz porphyry or rhomb porphyry. 'Porphyry' was a highly prized ornamental stone in Ancient Egypt and Imperial Rome, used for statues, vases, columns, etc. The red/purple colour of porphyry was associated with royalty. Another type of igneous rock that is characterised by large crystals is pegmatite which forms from residual silica rich fluids during the later stages of crystallization and is usually found in thick veins fed directly from the magma chamber.


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