P.1
A2 : Plate tectonics
Which parts of the world have frequent earthquakes and volcanic eruptions ?
are these areas associated with building of fold mountain ?
To what extent
Explain your answer with reference
to specific examples.
I.
Des + Exp
12
Dis
8
Illustrations & Egs.
5
Introduction
Plate tectonics theory :
1.
The lithosphere is divided into broken segments called plates.
2.
The asthenosphere below is a soft layer where the rock is heated (by the decay of
radioactive elements in the mantle) close to its melting point.
3.
So, there are convection currents of magma in the asthenosphere.
4.
So, plates are drifting.
5.
They may converge at destructive plate margins, diverge at constructive plate
margins or slide past each other along conservative plate margins.
II. Content
A.
Des : Areas with frequent earthquakes + volcanic eruptions
1.
mostly lie along plate boundaries :
a.
Destructive plate margin
i)
Circum-Pacific Ring of Fire
SE Asia, Japan, Aleutians, W coast of N and S America
mainly convergent plate boundary, e.g. Kobe earthquake in 1995
Eruption of Mt. Pinatubo in 1991; Mt. Mayon in 1984, 1994, 2000
ii)
Mediterranean -Himalayan Belt
extrusive vulcanicity is rare in the Himalayas)
Eruption of Mt. St. Helens in 1980, 1983
b.
Constructive plate boundary
Mid-Atlantic Ridge
c.
Conservative plate boundary
Fault lines, e.g. San Andreas Fault in California
2.
Earthquakes and volcanic eruptions may also occur in plates’ interior
Hawaiian Islands : hot spot, Eruption of Mauna Loa in 1975
B.
1.
Exp : why both earthquakes + volcanic eruptions occur at these zones
Convergent/ Destructive plate boundary :
Sinking convection currents → plates converge
a.
subduction → melting of earth crust +
b.
collision → crushing + melting of crustal rocks
c.
subduction/ collision → stress builds up
When stress exceeds strength → pressure releases as seismic wave
→ shaking of the earth’s crust → earthquakes + volcanic eruption (Fig. 1)
(4)
P.2
2.
Divergent/ Constructive plate boundary :
Rising convection currents → plates diverge → magma ejects for pressure release to
produce earthquakes + volcanic eruption (Fig.2)
3.
Conservative plate boundaries,
e.g. along San Andreas Fault in California, although both N. American plate + Pacific plate
move in the same direction (NW), they move at different rates (Fig. 3).
past each others,
When plates slide
friction is created and so intense earthquakes results.
E.g. San Francisco earthquake :
1906 (8.2 on the Richter scale) & 1989 (6.9 on the Richer scale)
Since there is neither subduction nor divergence of plates, there’s no vulcanicity.
4.
Plates’ interior: e.g. Hawaii archipelago: hot spot uneven distribution of heat within the
earth → earthquakes + volcanic eruption
(12)
P.3
C.
Areas with fold-mountain building
1.
Def folding : bending of the earth crust (Fig. 4)
2.
Folding only occurs at convergent plate boundaries: Circum-Pacific belt & Alpine-Himalayan
Belt, there is no folding in Mid-Atlantic Ridge
3.
In case of convergence of two oceanic plates, island arc system with fold structures
formed, e.g. west Pacific in Japan and Indonesia
4.
In case of convergence of an oceanic plate and a continental plate, such as in east Pacific,
volcanic mountain chain systems with fold structures are formed, e.g. Rockies + Andes
5.
In case of continental collision, there is no subduction.
As two continental masses meet in
collision, the sedimentary rock strata will be folded up. e.g. the Himalayas (Fig. 5)
(8)
III. Conclusion
1.
All plate boundaries are zones of instability.
2.
Suggest measures to minimize the adverse impact.
e.g. prediction, warning system, landuse zoning, shock-proof constructions, earthquake drills
+ education , evacuation plans + well-prepared relief work
Illustration and examples
(5)
Max. 25