1. Global Significance of
Volcanic Activity
A2 Geography

2. Significance
• Volcanic eruptions are major events
broadcast worldwide.
• Recent statistics suggest that about an
average of 1000 people die each year
from volcanic disasters a year
• That‟s 1% of all geohazards
• Not specifically a major significance then

3. So?
• The average length of time between
events (Known as „The return period‟)
increases with the size (Magnitude) of the
event
• Historical records to date show a clear
magnitude-frequency distribution of
events.
• Large numbers of small eruptions
punctuated by rare but devastating „Great
Eruptions‟
• As yet known though over VEI 8 even
though it is clear these have occurred

4. Yeah but still?
• Ok well not all eruptions have adverse
affects on humans (Due to location or size)
• Yet Major eruptions tend to still surprise
human populations and have a massive
potential to cause damage (Vulnerable)
• Historically there are frequent small
volcanoes with small losses but as yet no
catastrophes.

5. Geography of Volcanoes
• There are around 450 classified active
volcanoes globally (They have erupted
before in historic times)
• Another 200+ sites which are capable of
eruption (Active or dormant)
• 1000‟s of dead or extinct posing no threat
• Huge numbers of submarines sites not yet
visible and presently pose no threat

6. Active and Dormant
• They show two clear features:
• Along the margins of tectonic plates (Inter-
plate) where crust is either being formed at
Constructive margins (Andean
Volcanoes of Chile and Peru)
• Above local hotspots within plates (Intra-
plate) e.g. Hawaii where the earth‟s crust
is thinner

7. Patterns of Behaviour
• The distribution of volcanic hazards is
fundamentally determined by magma
composition.
• 3 groups of eruptive behaviour can be
identified:

8. Type Mineral Viscosity Cooling Characterisitcs
Composition time
Basic or Iron rich, Silica Low Slow Lava domes
Basaltic poor
Intermediate Magma is rich viscous Quicker Vents get blocked,
(Andesitic) in Silica (Sticky) explosions occur,
pyroclastic material is
generated + Lava. Produce
Stratified volcanoes
Acid Magma is silica High Quickly Violent eruptions,
(Rhyolitic) rich and iron unpredictable, lots of
poor pyroclastic material
produced
Therefore it is the variations in magma composition that largely determines volcanic
activity and thereby the hazard. This in turn is determined by crustal structure and
composition

9. The Main types of Magma
Comparison of three different Magma types
Volcanic Rock Basalt Andesite Rhyolite
Type
Rock Description Black to Dark Grey Medium to dark Light coloured
grey
Volume of Earths 80% 10% 10%
surface
Silica Content 45-55% 55-65% 65-75%
Temp 1,000 – 1,200 800 – 1000 600 – 900
Viscosity Low (Heated Ice Intermediate High (Toothpaste)
cream)
Gas Escape from Easy Intermediate Difficult
magma
Eruptive Style Peaceful Intermediate Explosive

10. Basalt Andesite Rhyolite

11. Volcanic Types
• Oceanic crust is basic and produces
basaltic volcanoes
• “Active” Continental margins have magma
produced from both oceanic and
continental (Granitic) crust + subducted
water and sediment therefore contains
more silica and volatiles so becomes more
violent and unpredictable
• Continental Interiors have magma
produced predominantly from continental
crust and therefore produces acidic or
Rhyolitic volcanoes

12. So what does all this mean?
• Each volcano has its own hazard
geography determined by structure,
magma source, topographic form and
climatic factors such as prevailing wind
direction.
• Each eruption is unique and can be
different from near neighbours
• The greatest threat is posed from the
numerous „Active‟ “intermediate”
volcanoes associated with active
subduction zones, located where pop‟n
growth has resulted in high density e.g.
Indonesia and Parts of the Ring of Fire

13. References
• Geology.com
• Volcanoes by R.Tilling
• Prof. David Jones Dept. Geography and
the Environment LSE (Lecture)
Further Reading:
Web research for different lava flows as discussed in this
lecture can be found >>HERE<<