10.2 The Nature of Volcanic Eruptions Earth Science - Schwartz

Review: melted mineral crystals dissolved gasses water What is the difference between Magma and lava? Magma that has reached the surface is called lava Lava cools and hardens to form rock What are the 3 things Magma is made up of? (see 10.1 notes for answer!) melted mineral crystals dissolved gasses water

Factors affecting Eruptions Primary factors that determine whether a volcano erupts explosively or quietly include characteristics of the magma- Viscosity specifically amount of dissolved gases in the magma

Viscosity Viscosity= Substances resistance to flow Viscosity is determined by Temperature Chemical composition More viscous = explosive eruption

Factors affecting Eruptions How does temperature change viscosity? When you heat maple syrup, it becomes easier to pour. Same thing with Magma, the hotter it is, the less viscous it is!

Factors affecting Eruptions How does chemical composition change viscosity? More silica= greater viscosity (thicker) Less silica= less viscosity (thinner) Silica = SiO2

Dissolved Gases Mainly water vapor and carbon dioxide More trapped gas = Greater Eruption A vent is an opening in the surface of Earth through which molten rock and gases are released. Trapped gases provide the force to propel lava

Which composition below would have the most explosive eruption? Which composition would have the quietest?

Volcanic Material Volcanic Material= Explosive eruptions eject lava as well as broken rock, lava bombs, fine ash and dust Eruptions also emit large amounts of gases 3 Types of Volcanic Material: 1. Lava Flows 2. Gases 3. Pyroclastic Materials

Volcanic Material Lava Flows Silica content and temperature affect lava flow Types of Lava Pahoehoe – hotter faster moving basaltic lava Aa – Cooler slower moving basaltic lava

Lava Flows The chunky brown lava is 'aa' while the ropy black lava is 'pahoehoe'

Gases Magma contains varied amount of gases As pressure is reduced the gases begin to escape (Think opening a soda can!)

Pyroclastic Materials Pyroclastic materials-fragments ejected during eruptions Includes dust, ash, and rock Range in size from very fine dust and ash to pieces that weigh several tons. 2 -64 mm called lapilli or cinders LARGER than 64 mm called blocks when hardened lava or bombs when ejected as glowing lava

Volcano vocab Reminder! Volcano – a mountain built by repeated eruptions of lava or pyroclastic material often separated by long inactive periods Crater – steep-walled depression located at the top of many volcanoes

Types of Volcanoes Volcanic activity often starts with a fissure, or crack, develops and magma is forced through it The form of the volcano is largely determined by the composition of the magma 3 Types : Shield Cinder Composite

Shield Volcano Shield Volcano – produced by the accumulation of fluid basaltic lavas Shape – broad, slightly domed structure that resembles a warrior’s shield Most have grown from the ocean-floor to form islands (Hawaii and Iceland)

Shield Volcano

Cinder Cone Volcano Cinder Cone – built by ejected lava fragments the size of cinders, which harden in the air Product of relatively gas-rich basaltic magma Shape – determined by the steep-sided slope that loose pyroclastic material maintains as it comes to rest Usually the product of a single eruption that lasts only a few weeks, rarely a few years The magma in the pipe solidifies after the eruption, and the volcano doesn’t erupt again Cinder cones are small, ~30-300 meters and less than 700 meters in height There are thousands of cinder cones around the world

Cinder cones

Composite Cone Volcano Most beautiful and potentially dangerous Also called stratovolcanoes Large, nearly symmetrical volcanic mountain composed of layers of both lava and pyroclastic deposits Gas – rich magma Most explosive eruptions Most located in the Ring of Fire, Mt. St Helens for example About 50 of these have erupted in the U.S. in the last 200 years

Composite cone

Other Volcanic Landforms Volcanic mountains are not the only landforms that result form volcanic activity 3 other landforms to know Calderas Volcanic Necks Lava Plateaus

Other Volcanic Landforms Calderas Depression in a volcanic mountain after end of activity Ex – Crater Lake

Formation of Crater Lake

Other Volcanic Landforms Volcanic Neck Landform made of magma that hardened in a volcano’s pipe and later was exposed by erosion

A volcanic neck in the Shiprock Volcanic Field, New Mexico.

Other Volcanic Landforms Lava Plateau Volcanic landform produced by repeated eruptions of very fluid, basaltic lava Erupts through long cracks called fissures Lava spreads out over a wide area

Volcanic Hazards Volcanic soil is good for farming Hazards include lava flows, volcanic ash, pyroclastic flows, and mudflows Mudflows called lahars occurs when water soaked volcanic rock and ash slide rapidly down hill.