Volcanoes & Volcanic Landforms

Mini Quiz To Begin Volcanoes 1. What is the difference between magma and lava ? 2. What kind (type) of rocks are formed from the cooling of this magma or lava? 3. Why does magma force its way up to the surface of the earth ? (hint: what does it want to do?)

Volcano Background Volcano… a term borrowed from the Romans… Vulcan was the Roman God of fire that forged metal… the vent (or chimney) of his furnace was an island named Volcano… this was a simple attempt by people to try to understand the complex processes that cause volcanic activity

Volcano: A Definition Volcano is the modern term used to describe vents in the earth’s surface through which molten rock, heated rock debris, gases and water vapor are expelled from beneath the earth’s surface in igneous processes

Volcanic Materials n The stuff that results from volcanic activity can be in the form of solids, liquids and gases… some of these materials include n water vapor (the major gas expelled) n rock (lava or magma) n pyroclastic materials

Pyroclastic Material n Anything that is ejected from a volcano is termed Pyroclastic... n Pyro = fire & Clastic = particles n Pyroclastic materials that are ejected and go airborne are termed Tephra

Tephra n Classified according to size: –ASH : less than two millimeters in size –LAPILLI : 2 mm to 64 mm in size –BLOCKS : >64mm ejected as solid –BOMBS : >64mm ejected as liquid

Tephra n Although pyroclastic materials are born of fire (igneous), as they fall to the ground and settle they become sedimentary!

Igneous Rocks Igneous rocks fall into two categories: Intrusive (plutonic) that cool slowly and Extrusive (volcanic) that cool quicker

Magma Compositions n There are three general types of volcanic magma compositions : –Basaltic –Andesitic –Rhyolitic

Magma Compositions These magmatic compositions are classified according to the amount of silica present… n the less silica present, the more fluid the magma/lava will be n the more silica present, the more viscous the magma/lava will be

Magma Compositions n Magma compositions classified by the percent silica present turn out like this: –basaltic : 50 % silica –andesitic : 60 % silica –rhyolitic : 70 % silica

Explositivity of Magmas n The more silica that is present, the more viscous (stickier) that magma is… n Basaltic : 50% Si, fluid, not explosive n Andesitic : 60% Si, “regular” explosions n Rhyolitic : 70% Si, really big bangs !

Land Shapes via Magma Type n The magmatic composition not only controls the color and explositivity, but it also controls the external volcanic land features… those magmas that are fluid when they erupt tend to run like molasses while the really sticky magmas don’t really erupt at all… they just blow up.

Land Shapes via Magma Type n Basaltic : 50% Si, fluid. n Basaltic eruptions tend to flow out on the earth’s surface like molasses… initially it will mound up but will settle out under the influence of gravity forming a broad, low shield-like volcano called…. n You guessed it - a shield volcano

Land Shapes via Magma Type n Andesitic : 60% Si, kind of viscous n Andesitic eruptions are what most people typically associate with volcanic eruptions… there are lava flows, ash clouds, and nuee ardentes. Andesitic eruptions can form two volcanic landforms… n composite cones and cinder cones

Land Shapes via Magma Type n Composite volcanoes are volcanoes built of alternating flows of lava and pyroclastic material… composite cones are larger than cinder cones but aren’t as steep… the composition of these volcanoes is mainly andesitic… most large active volcanoes around the world are composite volcanoes…

Land Shapes via Magma Type n Cinder cones are volcanoes that have steep slopes (~30 degrees) and a large crater at the top… these volcanoes are composed entirely of fragments of volcanic rock and material… these fragments are generally basaltic or andesitic in composition…

Land Shapes via Magma Type n Rhyolitic : 70% Si, very viscous! n Rhyolitic eruptions generally don’t produce any distinctive landform at all… the stuff is so viscous (sticky) that it pressurizes until it just blows up creating a big hole in the ground… all that remains are usually basins that are hotbeds of volcanic activity

So, where are these landforms? n The low-slung broad landforms that characterize basaltic shield volcanoes are nicely demonstrated by some well- known island chains: n Hawaiian Islands n Galapagos Islands

So, where are these landforms? n The steep sloped sides of the cinder cone volcanoes can be found throughout Central and South America and many other places world-wide… just take, for example, Paracutin in Mexico...

So, where are these landforms? n The classic volcano shape that we all think of is demonstrated well by Mt Ranier in Washington State, or the most photographed volcano in the world… Mt Fuji in Japan

So, where are these landforms? n Rhyolitic landforms are noted by their absence… there is usually nothing left after a rhyolitic eruption but a hole in the ground that is active with geothermal energy…just like Iceland or Yellowstone National Park...