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Social sciences: geography

Grade 7

Natural hazards

Module 5


1. The occurrence and causes of volcanoes

Molten rock below the earth’s crust is called magma. When it flows to the surface it is called lava. Why lava flows to the surface is not clear enough for people to agree on the reasons for eruptions and no one has been able to investigate the heart of the earth to find out what happens there.

We do know, however, that the weight of the solid crust of the earth is so great that the fluid rock is forced upwards at weak places in the earth’s crust. This molten rock is extremely hot, with temperatures of between 800 and 1 500 C.

If you shake a can of cooldrink, the gas will propel the cooldrink out of the can with great force when you open the can. This is what happens in the case of volcanoes – the molten rock erupts through weak places in the earth’s crust and lava, rocks and ash are propelled into the air.

Then the lava cools down and solidifies to form new rocks known as igneous rock.

Some volcanoes produce very fluid lava that flows over a large area before solidifying. Thin plates of igneous rock are formed in this case. Other volcanoes build up domes because the lava is less fluid and does not flow far from the core.

A very forceful explosion can force out the whole of the dome and cause a large open hole called a caldera, or crater. There are roughly 450 active volcanoes worldwide. Fortunately, they do not erupt continuously and are therefore referred to as dormant volcanoes.

Where do most volcanoes occur?

Study figure 4.

There are about 5 000 volcanoes. They only occur in particular places. These places are situated above the margins of the large plates of the earth’s crust. The movement of these plates lead to volcanic eruptions and earthquakes.

Quake areasVolcanic areas

Figure 4

The distribution of the main volcanic and earthquake areas of the earth

Which volcanoes have caused the greatest destruction?

The most destructive volcano of all time was Krakatoa, in Indonesia. When it erupted in 1883, more than 36 000 people died. The bang of the explosion was heard 5 000 km away and the pollution that resulted from the eruption was visible in copper-coloured sunsets across the earth for years.

Another well-known example of the destructive force of a volcano comes from the eruption of Mount Vesuvius, which buried the city of Pompeii in the Bay of Naples under volcanic rocks and ashes in 79 AD.

2. The effect (consequences) of volcanoes on people’s lives and their socio-economic activities

Thousands of lights glitter in the dark dome of the night. From a distance the explosions that hurtle glowing rocks and streams of fire into the night look like an expansive display of fireworks. Rocks and ashes flung up high pollute the air.

A river of red, smoking lava slithers over the edge of the crater – the warm blood of the living earth, as someone has strikingly described it.

It may flow through fertile valleys for kilometres, devouring every blade of grass, even trees, along its way. Houses crumble and cities are buried under the glowing, driven mass. People who have survived, scramble down the mountainside; warm ash raining down on them. Suffocating sulphurous gases and a rain of ashes smother other people to death.

Questions & Answers

How we are making nano material?
what is a peer
What is meant by 'nano scale'?
What is STMs full form?
scanning tunneling microscope
what is Nano technology ?
Bob Reply
write examples of Nano molecule?
The nanotechnology is as new science, to scale nanometric
nanotechnology is the study, desing, synthesis, manipulation and application of materials and functional systems through control of matter at nanoscale
Is there any normative that regulates the use of silver nanoparticles?
Damian Reply
what king of growth are you checking .?
What fields keep nano created devices from performing or assimulating ? Magnetic fields ? Are do they assimilate ?
Stoney Reply
why we need to study biomolecules, molecular biology in nanotechnology?
Adin Reply
yes I'm doing my masters in nanotechnology, we are being studying all these domains as well..
what school?
biomolecules are e building blocks of every organics and inorganic materials.
anyone know any internet site where one can find nanotechnology papers?
Damian Reply
sciencedirect big data base
Introduction about quantum dots in nanotechnology
Praveena Reply
what does nano mean?
Anassong Reply
nano basically means 10^(-9). nanometer is a unit to measure length.
do you think it's worthwhile in the long term to study the effects and possibilities of nanotechnology on viral treatment?
Damian Reply
absolutely yes
how to know photocatalytic properties of tio2 nanoparticles...what to do now
Akash Reply
it is a goid question and i want to know the answer as well
characteristics of micro business
for teaching engĺish at school how nano technology help us
How can I make nanorobot?
Do somebody tell me a best nano engineering book for beginners?
s. Reply
there is no specific books for beginners but there is book called principle of nanotechnology
how can I make nanorobot?
what is fullerene does it is used to make bukky balls
Devang Reply
are you nano engineer ?
fullerene is a bucky ball aka Carbon 60 molecule. It was name by the architect Fuller. He design the geodesic dome. it resembles a soccer ball.
what is the actual application of fullerenes nowadays?
That is a great question Damian. best way to answer that question is to Google it. there are hundreds of applications for buck minister fullerenes, from medical to aerospace. you can also find plenty of research papers that will give you great detail on the potential applications of fullerenes.
what is the Synthesis, properties,and applications of carbon nano chemistry
Abhijith Reply
Mostly, they use nano carbon for electronics and for materials to be strengthened.
is Bucky paper clear?
carbon nanotubes has various application in fuel cells membrane, current research on cancer drug,and in electronics MEMS and NEMS etc
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Source:  OpenStax, Geography grade 7. OpenStax CNX. Sep 09, 2009 Download for free at http://cnx.org/content/col11021/1.1
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