10
3.
MANIFESTATIONS OF THE EARTH’S INTERNAL
ENERGY
Unlike other bodies in our Solar System, the Earth is geologically alive, with a lot of
geological activity. Volcanoes and earthquakes are the most evident manifestations
of this activity.
3.1.
Earthquakes
The
focus
or
hypocentre
is the point where the earthquake rupture starts. In
earthquakes that last a long time, this can be felt hundreds of kilometres from the
fault and can produce displacements of up to several metres.
Earthquakes are classified in three categories, depending on focal depth: superficial,
(< 70 km), intermediate (70-300 km) and deep (>300 km). The majority and most
destructive are superficial.
Earthquakes
are the violent shaking of the superficial layers of the Earth’s surface,
caused by the displacement of large masses of rock along a rupture or fault.
fault
S waves
P waves
super cial
waves
epicentre
speed of P waves
8 km/s
speed of S waves
4.5 km/s
depth
hypocentre
time (seconds)
displacement of the ground
(microns)
P
S
surface
waves
200
0
-200
5 10 15 20 25 30 35
Elements of an earthquake, P (primary) S (secondary)
Understand
18.
Look at the illustration, listen and say true or
false. Correct the false sentences.
Seismograms
Analyse
19.
This graph shows vibrations caused by an
earthquake. Explain which are more dangerous,
surface or internal waves.
volcanic
cone
crater
volcanic bombs
ash cloud
lava stream
volcanic
chimney
secondary
cone
magma chamber
Volcanic structure
Understand
20.
Look at the illustration and define
crater
,
chimney
and
volcanic cone
.
3.2.
Volcanoes
Magma originates in the mantle or in the lower part of the
Earth’s crust. It rises and accumulates in the
magma chamber
,
a few kilometres from the surface.
Magma rises to the surface due to its lower density, openings,
or the effect of dissolved gases which tend to rise and escape.
Volcanoes
are cracks or openings from which magma
emerges to the exterior.
Magma
is a mixture of melted
rocks, solid fragments and dissolved gases. When it
comes to the surface, it is called
lava
.
