Volcanic Eruption & Landform Creation

1. Volcanic Eruption & Landform Creation Dr. Prabha Joshi Department: B.E. Civil Engineering Subject: Engineering Geology Semester: III Teaching Aids Service by KRRC Information Section

2. Volcanic Eruption and landform Creation

3. Volcanic Landforms • What are volcanic landforms? • The term volcano refers not only to a volcanic vent, but also to the landform that develops as the materials from a volcanic eruption harden. • The shape and structure of a volcano are determined by the nature of the eruption and the materials that are ejected during the volcanic activity.

4. What is a volcano? • A volcano is a vent or 'chimney' that connects molten rock (magma) from within the Earth’s crust t or mantle to the Earth's surface. • The volcano includes the surrounding cone of erupted material. vent cone conduit magma chamber

5. How and why do volcanoes erupt? • Hot, molten rock (magma) is buoyant (has a lower density than the surrounding rocks) and will rise up through the crust to erupt on the surface. • Same principle as hot air rising, e.g. how a hot air balloon works • When magma reaches the surface it depends on how easily it flows (viscosity) and the amount of gas (H2O, CO2, S) it has in it as to how it erupts. • Large amounts of gas and a high viscosity (sticky) magma will form an explosive eruption! • Think about shaking a carbonated drink and then releasing the cap. • Small amounts of gas and (or) low viscosity (runny) magma will form an effusive eruption • Where the magma just trickles out of the volcano (lava flow).

6. How and why do volcanoes erupt? • As depth increases below the surface of the earth pressure also increases and this prevents actual melting of the very hot material. The material below the crust is therefore, is invariably at very high temperature and under high pressure likely to acquire a molten condition by a slight decrease in pressure. This creates the possibility of a Volcano. • As, when and where in the body of the earth there is some cause to release the pressure, a local pocket of molten material comes into existence.

7. Materials of Volcano • Material that falls following a volcanic eruption • Lava: • Solid material- thepyroclast • Tephra- is fragmental material produced by a volcanic eruption regardless of composition, fragment size or emplacement mechanism • “Ash” – less than 2 mm in diameter • Lapilli– between 2 mm and 64 mm in diameter • Volcanic bombs/blocks – greater than 64 mm in diameter • Gases: Gases impart mobility to the magma and supply pressure for its ascent Image credit: http://home.flash.net/~alanm52/Pinatubo_ASHFALL.JPG

8. Lava Flow Dominantly basalt (low viscosity and gas) Thin and laterally extensive sheets Pahoehoe flows: smooth, ropey flows Aa or block flows: rough and irregular flows Baked zones: oxidized zones due to contact with high temperature lava flow Types of Volcanic Products: Effusive

9. Lava Dome • Dacite or rhyolite (high viscosity, low gas content) • Thick, steep-sided flows

12. Bombs Tephra Ash Types of Volcanic Products: Explosive • Pyroclastic particles • Fragmental volcanic material (TEPHRA) • Vitric (glass shards) • Crystals • Lithic (volcanic rock fragments) • Broken during eruption of magma • Typically higher silica, high gas content • Categorized by size: • Ash (< 2.0 mm) • Lapilli (2-64 mm) • Blocks and bombs (>64 mm)

14. Types of eruptions Central eruption or Mountain like landforms • There are Several Kinds of Volcanoes: • Shield Volcanoes • Cinder Cones • Composite Volcanoes Lava Plateaus • Beneath continental plates hotspot generates large volume of basaltic lava and forms flood basalts.

15. Volcanic Landforms • Shield Volcanoes • Shield volcanoes are associated with mafic lava (basalt) • Lava thin, not viscous • Holds little gas • Usually quiet eruptions • Lava travels long distances, spreads out in thin layers • Shield volcanoes are rounded domes, with gentle slopes Shield Volcano: low, often large, dome-like accumulation of basalt lava flows emerging from long , radial fissures on flanks

16. Cinder Cones • A cinder cone is the simplest type of volcano; it forms when molten lava is thrown into the air from a vent. As it falls, the lava breaks into fragments that solidify in the air, before hitting the ground. These fragments accumulate in a cone shaped mound. • Cinder cones tend to be smaller than other types of volcanoes. • They form in groups, and on the sides of larger volcanoes.

17. Volcanic Landforms

18. Stratovolcanoes • The nature of an eruption depends on the type of magma involved. • Felsic lavas associated with stratovolcanoes • Rhyolite, andesite • Thick, resistant to flow • Builds steep slopes around volcanic vents • Tall, steep cone, with crater • Most of these volcanoes associated with subduction zones. • Large amount of gas under high pressure may emit glowing avalanche Glowing avalanche from a stratovolcano

19. Composite volcanoes Composite volcanoes are tall cone-shaped mountains that are typically steep-sided, symmetrical cones of large dimensions. The essential feature of a composite volcano is a conduit system through which magma from a reservoir deep in the earth's crust rises to the surface. The volcano is built up by the accumulation of material erupted through the conduit and increases in size as lava, cinders, ash etc. are added to its slopes. These volcanoes are built in layers by multiple eruptions, sometimes recurring over hundreds of thousands of years. Most composite volcanoes have a crater at the summit which contains a central vent or a clustered group of vents. St. Augustine volcano-Alaska is a composite volcano

20. Craters and Caldera A Crater is an oval opening in the middle of a volcanic cone and may be from a few meter to as much as a kilometer in diameter. A caldera is a very large (more than 1.5 km in dia.) crater shaped basin, which forms when the top of the volcano collapses inward. This happens when the eruption empties the underground magma chamber, and leaves an “empty” space underneath. Volcanic Necks: consists of pipe like masses of lava and frequently material cooled within the former conduits. Removal of material from the sides due to erosive processes leaves the neck standing as stamps, towers or exposed plugs. Chocosuela caldera-Costa Rica

23. Fissure eruptions Continental flood basalts, Columbia Plateau, U.S. Sometimes at plate tectonics boundaries, there are long, narrow cracks (fissures) in the crust. Basaltic lava pouring from the fissure spreads across the land, forming a lava plateau. Flood of lava continue for very long periods building up new landforms from the lava flows. The Deccan Trap in the peninsular India and the Panjal Traps in the Himalayas are best examples of fissure eruptions in the past. Fumaroles: These are cracks, openings in the crust of the earth through which only hot gases come out intermittently or regularly.