VOLCANOES
LEARNING OBJECTIVES
Related volcano links:
http://vulcan.wr.usgs.gov/home.html
http://volcano.und.nodak.edu/vw.html
http://www.volcanoes.com (this site has links to other volcano related web sites)
Molten rock at the Earth's surface is called Lava.
Lava is extruded onto the Earth's surface through cracks, fissures and volcano vent holes. As lava pours out onto the surface it either flows readily forming a Basalt Flow or it can build into a cone shape...Volcano. Sulfur rich gas is mixed in with the molten rock and is released as well through cracks, crevices and vents.
Parts of a Volcano:
Cone: cone shaped relief feature built by volcanic material. Varies in height from small to very large.
Vent: hole at the top of the volcano, lava is extruded from vent.
Crater: basin like depression over a vent.
Caldera: collapsed crater, forms when lava withdraws into magma chamber.
Magma Chamber: reservoir of magma below surface.
Volcanic neck: pathway that connects magma chamber to surface.
The silica content and the amount of gas in the lava determines how fluid the lava is and how easily it will flow.
High silica content produces a thick viscous lava that does not flow easily.
Low silica content produces a lava that is more fluid and flows more easily.
Lava is less dense than the surrounding rock and rises up to the surface through cracks and crevices under the Earth. The silica content determines how thick the lava is, how easily it flows and how easily dissolved gases can escape; therefore the silica content determines the characteristic of the volcanic eruption and the resulting volcanic cone.
Flood Basalts: if the lava is very low in silica it will flow out over the Earth's surface much like a flood. Columbia River flood basalts covered part of the surface of Washington, Idaho and Oregon to a depth of 9,000 ft. Map of Columbia River Flood Basalts
Most commonly however, the lava is thick enough to build up a volcanic cone.
Types of Volcanic Cones:
1) Shield Volcano: low silica content produces a fluid lava that flows out of a vent and builds up layer upon layer producing a "classic" volcanic cone shape. Shield cones are very tall with a broad base. Sometimes lava will erupt out of the sides of the cone in a fissure eruption.
Gas escapes the lava easily and often will rise out of cracks in the ground much like steam.
Characteristics of a shield type eruption: eruptions are "quiet". Lots of lava is produced but very little ash. These eruptions are not "dangerous" but can cause destruction of buildings and structures in the way of the lava. In 1992 an entire subdivision in Hawaii was destroyed by lava flows from Kilauea Volcano. Gas is released continually from the lava or through cracks in the ground.
Two types of lava flows that result are:
Pahoehoe: lava that is smoother with a "ropy" appearance.
Aa: lava that has more dissolved gas and has a "rough, sharp" appearance.
Tectonic Setting: shield type eruptions common along spreading centers and hot spots.
Examples: Hawaiian volcanoes
2) Cinder Volcano: produced from lava that has a high concentration of dissolved gases. Cinder cones are short with steep sides. The cone is composed of pyroclastics (material ejected from the volcano) usually a pumice material.
Eruptions from a cinder cone produce large amounts of ash and other pyroclastics. Cinder cones can be found in many tectonic settings (subduction zones, hot spots and spreading centers) because they are the result of gas concentration in the lava.
Example of a cinder cone: Mt. Paricutin, Mexico
3)
Composite of Stratovolcanoes: volcanic cone is composed of layers of pyroclastic material alternating with lava flows.
The cone is intermediate in size between a shield and a cinder cone. The silica content and the gas concentration varies with each eruptive sequence. Some composite cones are composed of mostly lava flows with little pyroclastics while others are composed predominantly of pyroclastics. Mt Rainier is composed of 90% lava flows and 10% pyroclastics. Mt St. Helens is almost completely composed of pyroclastics.
Eruptions of stratovolcanoes are unpredictable and can be highly dangerous. They are found along subduction zones.
Example of Stratovolcano: Mt St. Helens
Types of Volcanic Eruptions:
Lava with low silica content is very fluid and easily erupts onto the surface. This type eruption is described above under "shield volcano."
Lava with high silica content is thick and viscous that does not readily flow. Lava rises up toward the surface but is too thick to squeeze through the cracks and fissures in the Earth. As lava continues to rise upward pressure continues to build. Gas contained in the lava is not released because it is "trapped" in the viscous fluid. Pressure will continue to build under the volcano until it exceeds the strength of the overlying surface material. At this point the lava will explode outward much as champagne from a bottle when the cork is released. These type eruptions are very destructive and cause many deaths. Recent explosive eruptions include: Mt. St. Helens, Mt. Pinatubo and Monserat.
Material Extruded in Volcanic Eruptions:
LAVA:
Low silica lava flows easily across the surface producing pahoehoe and aa rocks. If there is a high concentration of dissolved gases the lava will be "frothy" producing more ash and pumice or scoria. If the lava is extruded underwater it produces rounded features known as pillow basalts.
High silica lava does not flow easily and becomes trapped in the volcanic cone. When the cone explodes the lava is expelled outward at great speeds. As the lava is expelled into the air it separates into many different sized pieces, cooling and becoming aerodynamically shaped, these pieces are called Bombs. Smaller bits of hardened lava are also expelled: ash, cinders, lapilli.
SURFACE MATERIAL:
When a high silica volcano explodes the surface material explodes outward in all directions, these pieces of surface material are called Blocks.
GAS:
In a volcano with low silica content gas escapes easily from the surface of the lava. It can also escape through cracks and crevices on the surface.
In volcanoes with high silica lava gas becomes trapped in the viscous lava and cannot escape. When the volcano explodes gas is expelled out of the cone at great speeds. Nuee Ardente or Pyroclastic flow is the term used for the gas expelled from volcanic cones at great speeds. This gas is mixed with ash and is very hot, it is often referred to as a "glowing gas avalanche" and looks like a cloud due to the ash. The pyroclastic flow is dense and moves at ground level flattening and scorching everything in its path .
LAHAR:
Mud flow produced when snow cap is melted from the top of a volcanic cone due to sudden heating. Water from the snow cap mixes with ash producing a thick mud with a consistency of wet concrete.
