Before beginning, let’s take a few minutesto review what we have learned about theinside of the Earth.
First, we learned that the Earth is madeup of layers. What are the differentlayers?
What do you remember about Earth’s crust? What about the mantle?
Yes, just under the Earth’s crust we find themantle which is made up of melted rockcalledmagma. Magma is very hot-- 1600 degreesFahrenheit just below the crust to about 4000 degrees near the core! 1600 degrees Fahrenheit 4000 degrees Fahrenheit
Today we will learn something surprising aboutthe Earth’s crust—it’s cracked.The layer of solid rock that surrounds theEarth is made up of several huge plates ,like the cracked shell of a hard boiled egg. These cracks called faults go right down to the mantle, but we don’t usually see them because they are covered by oceans or soil.
Here we can see thesecracks in the Earth’scrust along the edgesof two of the plates.
So we have this red hot melted rockbubbling and churning just under the crustwhich, as you just learned, has cracks in it.Sometimes, the magma which is very hot andlighter than the rock above tries to rise-- butits trapped beneath the crust. As more meltedrock from below is added, greater pressure iscreated, squeezing the magma.As this pressure builds up, the magma triesto find a way to escape.Where do you suppose the magma, needing toexpand, finds its way through the crust ?
The easiest way--is The easiest way for the magma tois for the magma ooze through theto ooze through faults (cracks)the faults (cracks) between thebetween the plates. Thisplates. This magma calledmagma, calledlava when itit lava when reaches thereaches the surface of thesurface of the Earth cools andEarth, cools and hardens into rock.hardens into rock.
And when enough pressure builds up butthere is no crack near by, the magma maypush right through a weak part of the crust.A volcanic eruption occurs! Molten rock may pour out or a cloud of lava rock and ash may violently shoot up into the air.
Now, let’s look at some definitionsthat are important to know in order tounderstand volcanoes.
Magma is a mixture of molten (melted) rock,solid pieces of rock and dissolved gases suchas water vapor and carbon dioxide. It is foundat the top of the mantle under the crust ofthe Earth.Magma that flows onto the surface of theEarth or is blown out of a volcano is calledlava.Lava is very hot and is red or orange colored.As it cools on the surface of the Earth or inthe ocean water it hardens into solid rock.
Lava eruptingfrom a volcanoand flowing overthe ground
Tephra is volcanic rock of many sizes. Thesepieces of lava rock are blasted into the air outof a volcano by the explosions within thevolcano. It can be as big as a house or asmall as a piece of sand.Volcanic ash is powder-size to sand-sizeparticles of rock material (tephra) thathave been blown into the air by anerupting volcano.
When some volcanoes erupt, magma flowsout in the form of lava (melted rock).Others volcano eject mainly tephra, volcanicash and gases. .Others throw out a mixture of both lava,tephra, volcanic ash and gases.
This diagram shows a volcano ejecting all three---lava, tephra and gases.http://www.computerclipart.com/computer_clipart_images/erupting_volcano_cartoon_0515
Now let’s look at what we mightsee inside a volcano if we were ableto cut it open.
A volcano is a mountain topped with one mainvent (hole or crack) and perhaps with manyside vents (holes) where magma can escape. Below the volcano “mountain” is a pool of magma called the magma chamber.
When the magmacannot find a pathupwards it pools(gathers) in thismagma chamberunder the crust ofthe Earth.As more magma enters the magma chamber,pressure inside the chamber increases.
And as magma collects and pressure increases,the magma dome inside the volcano rises.
Although some volcanoes can takethousands of years to form, others cangrow overnight.
During eruption After eruptionFor example, the cinder cone volcano Paricutinappeared in a Mexican cornfield on February 20,1943.Within a week it was 5 stories tall, and by the endof a year it had grown to more than 336 meters(1,100 ft) tall. It ended its growth in 1952, at aheight of 424 meters(1,391 ft). By geologystandards, that’s pretty quick.
Some volcanic eruptions are explosive andothers are not. How violently a volcanoexplodes depends on:1. how runny or sticky the magma is and2. how much gas is dissolved in the magma. If magma is thin and runny, the gases canescape easily from it. When this type ofmagma erupts, the lava flows out of thevolcano and down its sides.
When magma is thick or there is a lot of gas dissolved in the magma, the eruption is more explosive.In this type of eruption, the lava blasts intothe air, cools, turns to a solid and breaksapart into pieces of lava rock calledtephra. Remember tephra can range in sizefrom tiny particles of ash to house-sizeboulders. Lava may also be blasted from themain vent.
In the following website, open “Buildyour own volcano and watch it erupt”.Here students may vary how thick thelava is (how viscous) and how much gasis dissolved in the lava and observe thevolcanic eruption.http://kids.discovery.com/games/build-play/volcano-explorer (bottom of menu left side of the page)
Volcanoes, fun to study…….but would you want one in your backyard?