Maestra

Lesson One


Where do igneous rocks come from?


The word igneous comes from the Latin ignis, which means fire. The term igneous describes rock formed from magma, melted rock that forms in the mantle and to a much lesser degree within the crust itself. There are two categories of igneous rock, intrusive rock formed within the Earth’s crust and extrusive rock formed at the surface ofthe Earth. People have always associated volcanic eruptions with fire and brimstone, death and destruction. This is still true when volcanoes erupt violently and without warning but igneous rock, which has formed within the Earth, provides us with a hard, resistant rock, which is used by all sectors of the construction industry.

Students should know that:

• The Earth is made upof different layers; the crust is the thin, cold and brittle solid layer that we live on. This sits on the mantle; the core of the Earth is divided in to two sections, the outer and the inner core. The outer core is liquid, the inner core solid.




Students will be introduced to the formation of intrusive and extrusive igneous rocks through a series of images downloaded from theInternet for use on an Interactive Whiteboard or for use as a Powerpoint display.







Extrusive igneous rocks



The 1980 eruption of Mt St Helens will be used as the main example of extrusive volcanic processes. Lava flows on Hawaii will be used to show the variations in viscosity and therefore flow and type of eruption. Images of granites both as hand specimens and asphysical features on Dartmoor will be used to illustrate intrusive processes.

Go to http://vulcan.wr.usgs.gov/volcanoes/MSH/Images/MSH80/framework.html


Select an image from the section: Prior to the 1980 eruption.


Then select the following 7 images from the section showing the impact of the eruption. There are brief notes alongside each thumbnail image.


oDevastation
o Debris avalanche helicopter
o Blast area spirit lake
o Debris avalanche looking downstream
o Volcanic ash with helicopter
o Reid Blackburn car
o Blowdown Smith creek

The 7 images show the eruption and consequence damage to the surrounding area, the image of the helicopter flying into the ash fall gives students someidea of the scale of the eruption and the image of the car virtually buried by ash continues this theme.


Teacher input


The origins of Mt St Helens can be found 100 to 330 kilometres below the Earth’s surface. Here in the Earth’s mantle it is so hot rock melts and a thick flowing magma is formed. Magma is more buoyant than the rock surrounding it and it begins to rise towards theEarth’s surface. Some of the magma will fill in the spaces between the rock in the Earth’s crust to form large reservoirs of magma. These are known as magma chambers. Gradually the rising magma will reach the Earth’s surface. The magma contains gases which until no have been held under pressure in the magma but now as the pressure from above is decreasing the gases begin to expand. Theexpanding gases push the magma upwards and it erupts through openings in the Earth’s surface - this is a volcanic eruption. Once the magma has erupted onto the Earth’s surface it is called lava. How explosive an eruption is depends on the viscosity of the magma. If it is thin and runny the gases escape easily, if thick like treacle the gases are trapped in the magma until the pressure of theexpanding gas becomes so great that there is a violent explosion. This explosion throws ash and lava high up into the air. The eruption of Mt St Helens in 1980 was just such a violent explosion.

If the magna reaching the surface is thin and runny then a different type of volcanic eruption will take place.

Go to
http://vulcan.wr.usgs.gov/Imgs/Jpg/Hawaii/puu_oo_lava_flow.jpg

and...