Chapter 12:
Volcanoes!
Volcanoes and Earth's MovingPlates
A volcano is an opening inEarth that erupts gases, ash andlava.
Volcanic mountains formwhen layers of lava, ash andother material build uparound these openings.
Kilauea (kee low AY ah),located in Hawaii, is the world'smost active volcano.
The most recent series oferuptions began in 1983 andstill continues.
How do volcanoes form??
Magma is forced upwardbecause it is less dense than therock around it.
Ex: Air bubbles rise to surfacein shampoo/syrup bottles.
After many thousands or evenmillions of years, magma reachesEarth's surfaces and flows outthrough an opening called a vent.
As lava flows out, it coolsquickly and becomes solid,forming layers of igneous rockaround the vent.
The steep walled depressionaround a volcano's vent is calleda crater.
Where do volcanoes occur??
Divergent Plate Boundaries: themid-Atlantic ridge is an areawhere the plates are movingapart.
When plates separate, theyform long, deep cracks calledrifts.
Lava flows from these rifts andis cooled quickly by sea water.
Sometimes, the volcanoes andrift eruptions rise above sealevel, forming new islandssuch as Iceland.
In 1963, the new islandSurtsey was formed during avolcanic eruption.
Convergent Plate Boundaries: placeswhere the Earth's plates movetogether...especially form volcanoes wherean oceanic plate slides below a continentalplate.
Magma forms when the plate slidingbelow another plate gets deep enoughand hot enough to melt partially. Themagma is then forced upward to thesurface.
Volcanoes that form at convergent plateboundaries erupt more violently thanother volcanoes do.
Hot spots: form when areas between theEarth's mantle and core are unusually hotand rock at these areas is forced toward thecrust where it melts partially.
Magma breaks through the crust to formseveral volcanoes.
Not at a plate boundary.
Formed Hawaiian islands
What would happen to the big island of Hawaii if the Pacific plate stopped moving?
Types of Volcanoes
What controls eruptions?
Trapped gases like water vapor and carbondioxide build up in volcanoes.
If the gas escapes easily, it is a quiet eruption.
If gas and water vapor build up, eruptions canbe explosive.
At what plate boundary could water vaporbuild up?
Composition of Magma
Quiet eruptions: magma that is low in silica andare basaltic.
This type of lava pours from volcanic ventsand runs down the sides of a volcano (pg.338)
Explosive eruptions: magma that is high insilica and is andesitic.
This type of magma is thick and pressurebuilds up an explosion occurs...often carryingpieces of lava.
Forms of Volcanoes
Shield Volcano: broad, gently slopingvolcano formed by quiet eruptions ofbasaltic lava.
The basaltic lava flows over Earth'ssurface covering large areas withthis deposits of basaltic igneousrocks when it cools.
Basaltic lava can also flow ontoEarth's surface through large cracks(not volcanoes) called fissures andcreates flood basalts.
Accounts for the greatest volume oferupted volcanic material.
Much of the new seafloor thatoriginates at the mid-ocean ridgeforms as underwater flood basalts.
Cinder Cone Volcano: steep-sided, loosely packed volcanoformed when tephra falls to theground.
Tephra: bits of rock or solidifiedlava dropped from the air afteran explosive eruption.
Vary in sizes from volcanicash, cinders, large rocks andblocks.
Composite Volcano: volcano builtby alternating explosive and quieteruptions that produce layers oftephra and lava.
Found mostly where Earth'splates come together and oneplate sinks below the other.(What plate boundary is this?)
Igneous Rock Features
Intrusive Features
Most of the magma never reaches Earth's surface...as magma coolsunderground it produces underground rock bodies that could becomeexposed later at the surface by erosion.
Batholiths: large intrusive rock body that forms when magma beingforced upward toward Earth's crust cools slowly and solidifiesunderground.
Dikes: magma that is forced into a crack that cuts across rock layers andhardens.
Sills: magma that is forced into a crack parallel to rock layers andhardens.
Other features
Volcanic neck: solid igneouscore of a volcano left behindafter the softer cone hasbeen eroded.
When a volcano stopserupting, the magmahardens inside the vent.
Erosion, usually by waterand wind, begins to wearaway at the volcano.
Caldera: large circular-shaped opening formedwhen the top of a volcanocollapses.
Crater Lake in OR is acaldera that filled withwater and is now a lake.
Effects of Eruptions
When volcanoes erupt, they oftenhave direct, dramatic effects on thelives of people and their property.
Lava flows destroy everything intheir path.
Falling volcanic ash can collapsebuildings, block roads and cancause lung disease in people andanimals.
Sometimes, volcanic ash anddebris rush down the side of avolcano: pyroclastic flow.
Temperature inside a pyroclasticflow can be high enough to ignitewood.
When sulfurous gases fromvolcanoes mix with water vapor inthe atmosphere, acid rain forms.
