science world 2 – chapter 11
dynamic earth
FOLDING & FAULTING
Huge forces caused by movements of the mantle cause changes
in the Earth’s crust. When rocks bend slowly without breaking,
folds are formed. The folds that bend downwards to form a Ushape are called synclines. Those that bend upwards like a
dome are called anticlines.
If the pressure in the Earth’s crust is very intense the rocks may
break and cracks may appear. A crack in the Earth’s crust along
which rocks move is called a fault. This movement can be upand-down or sideways, and can be hundreds of metres or a few
millimetres.
Folding and faulting are the reason that many mountains and
valleys form.
EARTHQUAKES
Earthquakes occur when intense pressure in the Earth’s crust
causes the rocks to break and move along a fault. The place
inside the Earth at which an earthquake starts is called the focus.
The point on the Earth’s surface directly above the focus is
called the epicentre – this is usually where the strongest shaking
is felt.
Earthquakes are recorded on instruments called seismographs.
They measure the vertical and horizontal movements of the
Earth. A record of the waves caused by an earthquake is called
a seismogram. It shows the three different types of waves that
are produced – P (primary or compression) waves, S (secondary
or shear) waves and L (surface) waves. The difference in the
time taken for the waves to travel certain distances enables
scientists to determine the exact location of the epicentre.
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THE RICHTER SCALE
Scientists use seismograms to work out the strength or
magnitude of an earthquake. The magnitude of an earthquake
is how much energy it has, and is measured on a scale from 0 to
10. This scale is called the Richter scale, after its inventor,
Charles Richter.
On the Richter scale, the intensity or energy of an earthquake
increases tenfold for a single increase in magnitude. For
example, an earthquake of magnitude 6 causes 10 times more
ground motion than one of magnitude 5, and 100 times more
than one of magnitude 4. (Look at “The Richter scale” on p
246)
CONTINENTAL DRIFT & PLATE TECTONICS
Back in 1915, Alfred Wegener suggested that continents such as
Africa, South America, Australia, Antarctica and India were
joined about 200 million years ago. He proposed that this
‘supercontinent’, called Gondwana, gradually broke apart and
the continents separated. Wegener’s hypothesis was called
continental drift. (Look at Figure 34 on p 248)
The continental drift hypothesis has since been modified.
Scientists now believe that giant sections or plates of crustal
rock move over the Earth’s surface. This movement of the
Earth’s plates is called the plate tectonic theory.
WHAT HAPPENS WHERE PLATES MEET?
The Earth is made of two different types of plates – oceanic and
continental plates. When an oceanic plate collides with a
continental plate, earthquakes and volcanoes are produced along
this line (often called a fault line). Colliding continental plates
explain the formation of mountain ranges such as the
Himalayas. (Look at all Figures on p 250 and 251)
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SPELLING WORDS
No.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
EASY
birth
death
interior
fault
ranges
buckled
brittle
valley
sideways
pressure
focus
primary
shear
surface
strength
effects
plate
collapse
drift
boundary
HARD
dynamic
explosion
eruption
composition
mountain
syncline
anticline
horizontal
earthquake
devastation
epicentre
seismograph
seismogram
secondary
compression
intensity
magnitude
Richter
continental
tectonic
3