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VOLCANOES Mt. Arenal – Costa Rica. It’s all in the magma… Mafic or Felsic ? Mafic – rich in ferromagnesian minerals, flows easily (low viscosity), gas.

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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:

1 VOLCANOES Mt. Arenal – Costa Rica

2 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

3 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

4 Mt. St. Helens – a typical composite volcano (prior to eruption)

5 After Eruption…

6 Sequence of events prior to Mt. St. Helens eruption in May of 1980.

7 Volcanic Dome Extremely viscous lava Very steep sided Often forms within crater of erupted volcano Dome inside Mt. St. Helens has recently been very active.

8 St Helens Lava Dome

9 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


11 Fluid Mafic Magma Forms Lava Tubes


13 Olympus Mons Caldera 650 km 24 km high

14 Built from ejected lava (mainly cinder- sized) fragments Steep slope angle Rather small size Frequently occur in groups Cinder Cone

15 Near Mt. Edziza, Northern B.C. Manu loa, Hawaii Cinder Cones tend to be semi – explosive.

16 Fissure Eruptions Fluid basaltic lava extruded from crustal fractures called fissures


18 The Gorge Amphitheater – George, Washington Flood Basalts Ben Harper

19 Caldera Calderas (form by collapse of evacuated magma chamber) Steep-walled depressions at the summit Size generally exceeds 1 km in diameter

20 Caldera of Mt Mazama now filled by Crater Lake

21 Ash Plume

22 Lava Flows

23 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

24 A typical aa flow A typical Aa lava flow

25 Pahoehoe

26 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

27 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.

28 Pillow lava movie

29 Pillow Lava Deposit

30 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

31 Columnar jointing/ basalts

32 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


34 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.

35 30/61 Nuee Ardente off of Mt. St. Helens, 1980



38 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

39 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.

40 Lahars from Mt. St. Helens

41 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

42 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

43 A Final Summary




47 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.

48 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.

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