Composition

1. V o s Part I l c e a o n Composition Types of deposits Types of volcanoes Distribution

2. Volcano: A mound of material that is extruded to the Earth’s surface from a vent that is connected to a magma chamber via a feeder conduit.

3. Volcanoes are classified according to their form. The form of a volcanoes depends on the type of material that it is made up of. The nature of the extruded material (and the volcano itself) depends on the properties of the magma. Magma: Molten rock within the Earth. Magma is called lava when it reaches the surface.

4. The composition of magma determines the type of rock that forms when it cools and its behavior during an eruption. Main controls on behavior: chemical composition (largely silica dioxide - SiO2 - content) and gas content (largely water vapor and CO2). SiO2 content controls the viscosity of a magma. Viscosity: a measure of how easily a fluid flows. Water has a low viscosity, molasses has a much higher viscosity.

5. Viscosity, in turn, controls the amount of gas that can be trapped in the magma. The greater the viscosity the more gas in the magma. There are three basic types of magma: Basaltic Magma Andesitic Magma Rhyolitic Magma The names are based on the rock type that forms when the magma crystallizes.

7. Overall, the behaviour of the magma determines the type of volcano that develops: Low SiO2 magmas, with little gas and low viscosity, flows readily through their vents and across the land surface when the lava escapes the vents. High SiO2 magmas, gaseous and with high viscosity, tend to plug their vents until the force of escaping magma blows the vent clear; such magmas cause explosive volcanoes.

8. Types of volcanic deposits (photos from USGS) Volcanoes also vary in terms of the types of deposits that they produce. Lava: Hot (up to 1200 degrees C), fluid,molten rock that flows along the land surface.

9. Lava can flow like viscous water, including forming lava falls.

10. Pahoehoe: Lava with a ropelike surface texture due to partial cooling as the lava flowed. Relatively hot, low viscosity lava.

11. Pahoehoe

12. A thick deposit of pahoehoe lava

13. Aa: Blocky, rough lava flow. Due to high viscosity lava that flowed pushing chunks of solid and semi-solid blocks. www.volcanovideo.com/Movies/p8vdclp.htm

14. Lava tube: A tube formed by cooling and solidifying of the lava walls while fluid lava continued to flow inside.

15. Pillows: A form of closed lava tube (with a bulbous end) that forms when a lava flows into water (e.g., a lake or ocean) and cools very rapidly. http://oceanexplorer.noaa.gov/explorations/04fire/background/volcanism/media/pillow_lava_video.html Pyroclastic material: Debris formed by a volcanic explosion. Results when magma is very viscous. Tephra: The general term for all pyroclastic material that is ejected from a volcano. Different terms apply according to the size of the tephra. (syn. Ejecta)

16. Ash: tephra that is finer than 2 mm in diameter.

17. Lapilli: from 2 mm to 64 mm in diameter. Blocks: hard fragments greater than 64 mm in diameter.

18. Bombs: soft, partially melted fragments greater than 64 mm in diameter.

19. Tuff: A deposit made up of ash. Volcanic ash fall during mid-day with the eruption of Mount Pinatubo in the Philippines. Welded tuff: A deposit of pyroclastic material that was laid down while still very hot and particles become fused together. Ash fall: Fallout of very fine ash from the air.

20. Ash flow: Pyroclastic debris that flows downslope. Lahar: A water saturated slurry of ash and other volcanic debris that flows downslope.

21. Nuée Ardente (glowing cloud): A hot, gaseous cloud of ash that flows down slope. http://volcano.und.nodak.edu/vwdocs/volc_images/img_mt_pelee.html Flow speeds can reach 160 km/hr and temperatures can exceed 600 degrees C.

22. Classification of volcanoes Volcanoes are classified according to their morphology. The processes and deposits dictate the morphology of volcanoes. Three types of volcano:

23. Muana Loa Volcano – the world’s largest volcano. http://hvo.wr.usgs.gov/maunaloa/ Photograph by J.D. Griggs on January 10, 1985 Shield volcanoes: dominated by lava flows.

24. Forms an isolated conical mound of tephra. Photograph by J.P. Lockwood on 1 December 1975 http://volcanoes.usgs.gov/Products/Pglossary/CinderCone.html Cinder cones: dominated by pyroclastics.

25. Stratovolcanoes: mixture of lavas and pyroclastics. Syn. Composite volcanoes Mount Mageik volcano, Alaska Photograph by R. McGimsey on 15 July 1990 http://volcanoes.usgs.gov/Products/Pglossary/stratovolcano.html

26. Shield Volcanoes Low, dome-shaped profile, like an inverted shield. http://geoimages.berkeley.edu/GeoImages/Johnson/Landforms/Volcanism/ShieldVolcano.html Dominated by fluid, high temperature, low viscosity basaltic magma.

27. Typical slopes approximately 15 degrees. Many shield volcanoes have a central caldera: USGS Lava flows downslope, away from a central vent or a series of vents. Calderas form after an eruption when the surface collapses. Each caldera is located at the site of a former eruption.

28. Hawaiian Isands and Iceland are built from shield volcanoes. Mauna Loa is the largest volcano on Earth. It makes up most of the island of Hawaii. The volcano rises 4,170 m above sea level. It covers an area of 5,271 km2. Total volume of rock: 80,000 km3

30. Began to form 700,000 to 1,000,000 years ago when lava began to flow to the sea floor. Its great weight depresses the underlying crust by 8 km. Eruptions reached the surface 400,000 years ago.

31. Low viscosity lava forms fountains of lava flowing from vents near the volcano summit.

32. The lava flows easily down the gentle slopes….reaching the ocean during some eruptions.

33. Where the lava is relatively cool eruptions form small cinder cones on the volcanoes surface.

34. Cinder Cones Mount Edziza, British Columbia Dominated by viscous, gaseous magmas Relatively cool basaltic magmas or andesitic magmas predominate.

35. Internally constructed entirely of layers of pyroclastic deposits (blocks, bombs, lapilli). Angle of repose: the natural maximum angle that a pile of loose, unconsolidated material will form. Slopes are steep, at angle of repose.

36. Typical angles: 30 to 40 degrees. Range from several metres to over 300 m in height. Commonly associated with old shield volcanoes with a relatively cool, basaltic magma.

37. Paricutinvolcano began to erupt in a corn field in Mexico in 1943 and continued until 1952. The farmer had noticed a fissure (vent) had opened in the field one morning and from it was pouring black ash. In the first year the volcano grew to 336 m (almost 1 metre per day). Rate of growth decreased steadily; by 1952 the volcano was 424 m in height.

38. Stratovolcanoes © Noemi Emmelheinz 2001 Volcanoes that alternate between periods of lava flows (constructive phase) and periods of explosive eruptions (destructive phase). Commonly called “composite volcanoes” because they are made up of both lava and pyroclastic deposits. Steep slopes, at angle of repose or greater.

39. May lay dormant for thousands of years. On average, andesitic magmas with a high gas content. Actually, a mix of basaltic and rhyolitic magmas in many cases. Gases add great pressure when the feeder conduit becomes plugged, contributing to the explosive power. Can grow to thousands of metres high during constructive lava flow phases. The constructive phase often ends with a destructive phase – an explosive eruption.

40. Mt. St. Helens Before Mt. St. Helens After

41. Extensive ash falls and ash flows are commonly produced during explosive phases.

42. After an eruption a large caldera remains. Crater Lake is a caldera that remains following an explosive eruption 7,700 years ago. The eruption was 42 times more powerful than Mt. St. Helens.

43. USGS Styles of Volcanic Eruption Eruption style is determined by the explosiveness and the height of the column of tephra.

44. Hawiian Eruptions Eruptions dominated by lava flows. No significant ash column. Any explosions are of a small scale.

45. Strombolian eruptions Intermittent explosion or fountaining of basaltic lava from a single vent or crater. Caused by the release of volcanic gases, and they normally occur every few minutes. Tephra ranges from ash to bombs.

46. Vulcanian Eruptions Periodic eruptions (decades apart) that are moderate explosions. Ejecta are hard (not melted). Andesitic, gaseous magmas.

47. Pelean Eruptions Similar to Vulcanian Eruptions but include hot gas clouds (Nuees Ardentes)

48. Plinian Eruptions Very explosive volcanic eruptions; long periods between eruptions. Involve a very viscous, rhyolitic magma. Form a plume of tephra that extends upwards to the stratosphere (>11 km high). Dust can remain in the stratosphere for years and significantly cool the Earth.

49. Ultraplinian Eruptions Very explosive eruptions that eject approximately 100 km3 or more of tephra and produce ash columns exceeding 25 km in height. Phreatic Eruptions Eruptions of tephra and large volumes of steam produced when water makes contact with the magma. Water flashes to steam and causes a very violent eruption.

50. The Distribution of volcanoes