volcanoes (teach)

Volcanoes

Before beginning, let’s take a few minutes to review what we have learned about the inside of the Earth.

First, we learned that the Earth is made up of layers. What are the different layers?

What about the mantle?

What do you remember about Earth’s crust?

Yes, just under the Earth’s crust we find the mantle which is made up of melted rock called magma. Magma is very hot-- 1600 degrees Fahrenheit just below the crust to about

4000 degrees near the core!1600 degrees

Fahrenheit

4000 degrees Fahrenheit

Today we will learn something surprising about the Earth’s crust—it’s cracked. The layer of solid rock that surrounds the Earth 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 these cracks in the Earth’s crust along the edges of two of the plates.

So we have this red hot melted rock bubbling and churning just under the crust which, as you just learned, has cracks in it.

Where do you suppose the magma, needing to expand, finds its way through the crust ?

Sometimes, the magma which is very hot and lighter than the rock above tries to rise-- but its trapped beneath the crust. As more melted rock from below is added, greater pressure is created, squeezing the magma.As this pressure builds up, the magma tries to find a way to escape.

The easiest way is for the magma to ooze through the faults (cracks) between the plates. This magma called lava when it reaches the surface of the Earth cools and hardens into rock.

The easiest way-- is for the magma to ooze through the faults (cracks) between the plates. This magma, called lava when it reaches the surface of the Earth, cools and hardens into rock.

And when enough pressure builds up but there is no crack near by, the magma may push 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 definitions that are important to know in order to understand volcanoes.

Magma that flows onto the surface of the Earth or is blown out of a volcano is called lava.Lava is very hot and is red or orange colored. As it cools on the surface of the Earth or in the ocean water it hardens into solid rock.

Magma is a mixture of molten (melted) rock, solid pieces of rock and dissolved gases such as water vapor and carbon dioxide. It is found at the top of the mantle under the crust of the Earth.

Lava erupting from a volcano and flowing over the ground

Lava flowing into the ocean

Lava flowing into the ocean over lava rock

Volcanic ash is powder-size to sand-size particles of rock material (tephra) that have been blown into the air by an erupting volcano.

Tephra is volcanic rock of many sizes. These pieces of lava rock are blasted into the air out of a volcano by the explosions within the volcano. It can be as big as a house or a small as a piece of sand.

volcanic ash

tephra

When some volcanoes erupt, magma flows out in the form of lava (melted rock).

.

Others volcano eject mainly tephra, volcanic ash 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 might see inside a volcano if we were able to cut it open.

A volcano is a mountain topped with one main vent (hole or crack) and perhaps with many side vents (holes) where magma can escape.

Below the volcano “mountain” is a pool of magma called the magma chamber.

As more magma enters the magma chamber, pressure inside the chamber increases.

When the magma cannot find a path upwards it pools (gathers) in this magma chamber under the crust of the Earth.

And as magma collects and pressure increases, the magma dome inside the volcano rises.

rises

and rises.

see red block in next slide

When enough pressure develops inside the magma chamber, the volcano erupts.

Volcanoes (the mountain looking part) are built by the “stuff” that erupts from inside it—flowing lava and/or pieces of hardened lava.

As more lava accumulates, the volcano grows in size.

Accumulatinglayers of hardened lava

Although some volcanoes can take thousands of years to form, others can grow overnight.

Within a week it was 5 stories tall, and by the end of a year it had grown to more than 336 meters (1,100 ft) tall. It ended its growth in 1952, at a height of 424 meters(1,391 ft). By geology standards, that’s pretty quick.

During eruption After eruption

For example, the cinder cone volcano Paricutin appeared in a Mexican cornfield on February 20,1943.

Some volcanic eruptions are explosive and others are not. How violently a volcano explodes depends on: 1. how runny or sticky the magma is and 2. how much gas is dissolved in the magma.

If magma is thin and runny, the gases can escape easily from it. When this type of magma erupts, the lava flows out of the volcano 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 into the air, cools, turns to a solid and breaks apart into pieces of lava rock called tephra. Remember tephra can range in size from tiny particles of ash to house-size boulders. Lava may also be blasted from the main vent.

http://kids.discovery.com/games/build-play/volcano-explorer

In the following website, open “Build your own volcano and watch it erupt”. Here students may vary how thick the lava is (how viscous) and how much gas is dissolved in the lava and observe the volcanic eruption.

(bottom of menu left side of the page)

Volcanoes, fun to study…….

but would you want one in your backyard?