Presentation on theme: "VOLCANOES Mt. Arenal – Costa Rica. It’s all in the magma… Mafic or Felsic ? Mafic – rich in ferromagnesian minerals, flows easily (low viscosity), gas."— Presentation transcript:
It’s all in the magma… Mafic or Felsic ? Mafic – rich in ferromagnesian minerals, flows easily (low viscosity), gas releases easier. (common in non-explosive) Felsic – high silica content, highly viscous, commonly plugs up creating explosive volcanism
A volcano that is composed of alternating layers of lava and pyroclastic material Steep sided slopes Silicic to intermediate magma = EXPLOSIVE Example: Cascade Mountains. Composite (Strato) Volcano
Mt. St. Helens – a typical composite volcano (prior to eruption)
Broad, low angle slopes Composed primarily of mafic lava Generally cover large areas Produced by non-explosive eruptions of large volumes of lava Mauna Loa on Hawaii is a good example Shield Volcanoes
Aa `A`a (pronounced "ah-ah") has a rough rubbly surface composed of broken lava blocks called clinkers. Typical aa lava is very viscous, either because of high silica or because the temperature is close to rocks melting point
Pahoehoe is basaltic lava that has a smooth, hummocky, or ropy surface that typically develops when the lava is very fluid. This is most likely if the lava has a basaltic composition. It is also most likely when the lava's temperature is well above the melting point
Pillow Lavas mounds of elongate lava "pillows" formed by repeated oozing and quenching of the hot basalt. First, a flexible glassy crust forms around the newly extruded lava, forming an expanded pillow. Next, pressure builds until the crust breaks and new basalt extrudes like toothpaste, forming another pillow. Formed from lava erupting underwater.
Columnar jointing/basalts The type of jointing that breaks rock, typically basalt, into columnar prisms. Usually the joints form a more or less distinct hexagonal pattern. Forms from contractional cooling. Aerial view of Devils Postpile showing columns' geometry Devils Postpile sheer wall face is 60 feet high
Pyroclastics When lavas are viscous and charged with gas, they are likely to erupt explosively rather than forming lava flows. The result is a cloud of pyroclastic debris that may shoot several kilometers into the air and then settle over a wide area. Fine-grained materials, known as ash ( 64 mm). Ash Cinders Bombs Lapilli
Nuee Ardente - pyroclastic flow A glowing cloud or Nuée Ardente is a gas generated eruptive phenomenon consisting of two parts. 1) glowing avalanche (lower denser part) 2) Lighter fraction of volcanic gases, ash, and dust which cauliflower upwards.
The city of St. Pierre, before the 1902 eruptions. The city of St. Pierre, after the 1902 eruptions, June 1904. The 1902 Eruptions of Mt. Pelée Nuee Ardente /Pyroclastic flow
Lahars Lahar - rapidly flowing mixture of rock debris and water that originates on the slopes of a volcano. Lahars are also referred to as volcanic mudflows or debris flows. They form in a variety of ways, chiefly by: the rapid melting of snow and ice by pyroclastic flows, intense rainfall on loose volcanic rock deposits breakout of a lake dammed by volcanic deposits.
Deadly Lahars from Nevado del Ruiz, Colombia November 13, 1985 Within four hours of the beginning of the eruption, lahars had traveled 100 km and left behind a wake of destruction: more than 23,000 people killed, about 5,000 injured, and more than 5,000 homes destroyed
Phreatic Eruptions Phreatic eruptions are steam-driven explosions that occur when water beneath the ground or on the surface is heated by magma, lava, hot rocks, or new volcanic deposits generating an explosion of steam, water, ash, blocks, and bombs. Mount St. Helens, Washington
Assignment Use Text – Chapter 4 to fill in and complete your notes package. Remember to include: Caldera, Phreatic eruptions, columnar basalt, lava tubes, pillow lava. Test your understanding by doing the chapter 4 fill in blank worksheet. Do it without the text first, and then look for the ones you don’t know.
Volcanism and Tectonic setting In general subduction zones will result in felsic magma (and lava) creating explosive volcanism Melting of basaltic slab, sediments, and water. The addition of water (gas) will increase melting of crust. Assimilation of silica rich crust turns magma into an intermediate compostion.