Table of Contents
LITHOSPHERE
3
STRUCTURE OF THE EARTH
3
CORE
GUTENBURG DISCONTINUITY
MANTLE
MOHOVORIC DISCONTINUITY
CRUST
3
3
3
4
4
THE THEORY OF PLATE TECTONICS
4
SEA-FLOOR SPREADING
SUPPORTING EVIDENCE
SUBDUCTION
EVIDENCE
MANTLE CONVECTION CURRENTS
SOURCE OF ENERGY
CONVECTION CURRENT
4
4
5
6
6
6
6
GLOBAL STRUCTURAL LANDFORMS
7
DIVERGENT/CONSTRUCTIVE PLATE BOUNDARIES
RIFT VALLEYS
MID-OCEAN RIDGES
CONVERGENT/DESTRUCTIVE PLATE BOUNDARIES
TRENCHES
CONTINENTAL VOLCANIC ARCS
ISLAND ARCS
FOLD MOUNTAINS
TRANSFORM PLATE BOUNDARIES
THE SAN ANDREAS FAULT
HOT SPOTS
THE HAWAIIAN CHAIN OF ISLANDS
OTHER HOT SPOT ACTIVITIES
SUMMARY
7
7
9
10
10
10
11
12
12
12
12
13
13
13
EXTRUSIVE VOLCANISM
13
COMPONENTS
LAVA FLOWS
PYROCLASTS
LAHARS (MUDFLOWS)
GASES
TYPE OF VOLCANIC ERUPTIONS
VISCOUSITY
14
14
15
15
16
16
16
NATURE OF ERUPTION
TYPES OF ERUPTIONS
FEATURES OF EXTRUSIVE VOLCANISM
SHIELD VOCANOES & COMPOSITE VOLCANOES (STRATOVALCANOES)
CINDER CONES
BASALT PLATEAU
CALDERAS
VOLCANIC HAZARDS
VOLCANIC HAZARD MANAGEMENT
PREDICTIONS
RESPONSE
16
17
18
19
20
20
20
20
21
21
22
Lithosphere
Structure of the Earth
Earth in an oblate spheroid
made up of concentric layers.
 Inner core
 Outer core
 Gutenburg dicontinuity
 Lower mantle
 Upper mantle
 Mohovoric dicontinuity
 Crust
o Oceanic
o Continental
Other ‘layers’ include:
 Mesosphere
 Asthenosphere
 Lithosphere
Core
Density
Composition
State
Thickness
Inner
Outer
Density of 12
32% of Earth’s mass although it’s only 16% of the volume
Made of nickel-iron alloy called Nife
Solid
Liquid
1255 km
3500 km
Gutenburg Discontinuity



Separates the core from the mantle
Depth from surface is 2900 km
Seismic waves slow down in this region
Mantle
Thickness
Type of Rock
Density

Upper
2895 km of solid rock
Peridotite
~3
Lower
Olivine
~5.7
Divided to 3 layers:
o Lithosphere
 Rigid layer above asthenosphere
 Made up of uppermost part of mantle and crust.
o Asthenosphere

 Semi-molten layer near melting point
 80 km beneath continental crust; ~40km beneath oceanic crust
 P and S waves slow down here
 300 km thick
o Mesosphere
 Rigid layer beaneath asthenesphere
 Consists of rest of mantle
Rigid lithosphere is broke into large units called lithosphere plates and has the capacity of
moving bodily over the molten asthenosphere.
Mohovoric Discontinuity




Separates crust from mantle
Marks lower limit of kinds of rocks which make the outer-most earth-shell we live on
Depth from surface varies (~5-70 km)
Meaning that crust thickness varies a lot as well
Crust
Thickness
Age
Type of
Rock
Oceanic (sima)
~ 5 km thick
~ 200 million years old
Basalt: dark, compact, heavy igneous rock,
3g/cm3, made of silicates of Mg, Al, Ca and
Fe
Continental (sial)
~ 25 – 70 km
~ 3800 million years old
Granite: Pale, coarsely crystalline, light
igneous rock, 2.7g/cm3, silicates of Al, Na
and K
The Theory of Plate Tectonics
Sea-floor spreading




MOR are locations for generation of new crust
When continents split apart, magma wells up along line of the rift.
It cools and forms new crust along the rift.
Process is continuous with constant replenishment of fresh upwelling of manga
Supporting evidence
Rock magnetism


‘Conditions’:
o Preservation of record of
magnetic field: Rocks rich in
iron are weakly magnetised
with respect to earth’s
magnetic field when they
solidify
o Earth’s frequent polarity
reversals: At least 9 in 4.5
million years
As magma wells up along rift and
solidify, they become magnetised to




earth’s magnetic field then.
Since there are continuous reversals of polarity, both flanks of the ridge acquire a fossil
magnetism where there are symmetrical alternating bands of rock with normal and reversed
polarity.
Example: 1966 Magnetic Survey of Reykjanes Ridge (south of Iceland)
Since magnetic bands can be dated, rates of SFS can be calculated.
Examples:
o Iceland: 1cm/year
o Part of East Pacific Rise: up to 9cm/year
Geothermal Heat Flow
 Heat generated by the interior of earth
 Measured by thermistor probe
 Temperature several times the norm due to injection of mantle material
Seismic Activity Distribution
 MOR are centers of activity and are important seismic zones
 Shows crustal movement
Dating of Volcanic Activity
 Example: Iceland on Mid Atlantic Ridge
o Most recent activity occurs down the center of island and become progressively
older when moving away from center
 Example: Volcanic Islands in North Atlantic
o Islands further from ridge show that they are older
Pattern of Sedimentation




Newly formed oceanic crust closest to the ridge has little and younger sendiment compared
to thickening and older sediment further away on older crust
Since SFS is constantly occuring, sediment on crust is only slightly younger and thinner
because it can only accumulate after crust formation
Hence younger crust will ensure thinner layer and younger sediment
Example: Deep Sea Drilling Project of 1968
Subduction


Trenches are zones of crust destruction
Crust is thrust back into mantle, re-melted and re-absorbed
Evidence
Geothermal Heat Flow
 Heat generated by the interior of earth
 Measured by thermistor probe
 Temperature is lower over trenches due cold crust descending into and hence cooling mantle
Seismic Activity Distribution
 Trenches are centers of activity and are one of the most intense seismic zones
 Shows crustal movement
Distribution of Earthquake Foci



Earthquakes near trenches generally
have shallow foci and occur at
progressively greater depths toward
the island arcs
Earquake foci lie on the Benioff Zone
which is ~45
Marks the line of distrurbance of plate
during subduction into mantle
Mantle Convection Currents





Carried by convection current
o New crust generated at MOR: plates
move away along
o Old crust destroyed at trench: plates
collide
Lithosphere: rigid layer above asthenosphere
consisting of crust and upper most part of
mantle that is broken into 7 large plates and
many smaller ones
Mohovoric Discontinuity: layer which marks base of crust and velocity of seismic waves
increases due to denser mantle material
Asthenonsphere: Low Velocity Zone at 70-80 km depth where wave velocity falls
Crust attached to upper part of mantle (lithosphere) moves on top of semi-solid
asthenosphere
Source of Energy
 Residual heat from earth’s formation 4.55 billion years ago
 Continuing decay of radioactive elements
Convection Current
 Earth releases internal heat causes rocks near the Gutenburg discontinuity to increase in
temperature and hence expand, leading in decrease in density
o 300-400C leads to 1% expansion and hence 1% decrease in density
 Hence they are more buoyant than surrounding rocks and thus rises to the base of the
lithosphere, moves laterally and releases heat.


Heat loss causes contraction and hence increased density. Thus it sinks back to the lower
mantle.
At the same time, hot rocks rises again to balance the downward flow to maintain convective
motions.
Global Structural Landforms
Divergent/constructive plate boundaries


Zones of tension where plates split and move
apart
2 theories:
o Two divergent convective flows in
mantle drag plates apart
o Hot spots in the mantle where heat
flow is above surrounding average
domes the overlying crust, producing a
three-armed rifting pattern (Three-Armed Rifting)
Rift Valleys
Formation


Hot rising plumes causes crustal stretching and formation of tensional cracks
As plates move away from area of upwelling, broken slabs are displaced downward to create
rift valleys.
Features
 Horsts: slabs of crust left upstanding by
subsidence of rift valley floors


Graben: slabs of crust downthrown by rifting
Examples:
o Vosges
o Black Forests
o Rhine Rify Valley
East African Rift Valley
 5500 km long from Jordan in the North to Mozambique
in the South
 In the central part, it splits into 2 branches:
o Western: Albertine in Uganda
o Eastern: Gregory in Kenya
 In between is the downwarped area of the African
plateau
Features in central and Southern Kenya
Faults
Fault
scarps
Step
faulting
Grid
faulting
Rift volcanoes
Rift lakes
Soda lakes
two parallel faults allow valley floor to sink between two inward-facing
scarps forming faults standing more than 600 m above the valley floor
smaller scale faults arranged en echelon
East of Lake Naivasha
even smaller, narrower faults
developed within volcanic rocks underlying rift floor, hundreds or
thousands of meters deep
Mount Longonot, Mount Kilimajaro, Mount Kenya, Mount Lengai
formed where crust has been downthrown to form depressions
lakes rich in dissolved Na salts like sodium carbonate, chloride and sulfate
due to contamination
Lake Magadi, Lake Natron
Lake Natron: 400 mile2 lake crossing the Kenya-Tazania border that is
coloured red by algae. Hot springs with caustic minerals wells up into lake
Freshwater
lakes
Lake Naivasha, Lake Malawi, Lake Victoria
Merits of the concept of three-armed rifting
 Explains the three-armed plans of the
o Rhine Rift Valley: Upper, Lower and Hesse (smaller)
o Red Sea Rift: Red Sea, Aden and Abbyssinian (smaller)
 Explains formation of horsts on either side of the rift
o Vosges on the west and the Black Forest Mountains to the east of the rift
o Actual horsts are tilted blocks rather than plateaus which can be exaplined by the
upwarping
Mid-ocean ridges
 Further spreading causes rift valleys to lengthen and deepen, eventually extending into to
the sea to form a linear sea
o Example: Red Sea
 Crust will continue to form between the divergent plates
 Leads to the formation of MOR
Features
 Length: 10s of 1000s of km
 Width: 100s of km wide
 Height: 0.6km to 3 km or more above the sea floor
o Asthenosphere lies close to surface at MOR
o high temperatures causes thermal expansion of hot mantle rocks decreases its
density and causes it to rise to higher elevation than surrounding seafloor
o As it moves further away due to continuous SFS, it cools, increases in density and
sinks.
 Central rift valley in the middle
o Example: Red Sea central rift valley
 Temperature: 56C
 Salinities: 256 pp 1000 vs 35 pp 1000
 Caused by high geothermal heat flow due to extrusion of lavas which have
a pillow-like appearance due to rapid cooling underwater
 Staggered path like stitches on a baseball due to transform faults
Mid-Atlantic Ridge
 South Atlantic -> Indian -> South Pacific -> East Pacific -> Arctic
Oceans
 Great submarine mountain chain that rises abruptly from the
sea floor
 Runs centrally between Europe and North America and
between Africa and South America
 Formed after the lands were split
Volcanic Islands
 Located on or close to MOR where it reaches above sea level
 Examples:
o Azores
o
o
Ascension Island is located on the ridge and is still active
St Helena is 700 km from the Ridge and is extinct
Case Study: Iceland
 A tholeiitic basalt plateau
 Extrusion of lava formed platform on the Mid-Atlantic Ridge
 Central Icelandic Depression lies in the center which is the belt of the most recent volcanic
activity
 Continues to grow upward as lava well up to seal fissures
 Rocks get older away from the rift and are dominated by fissure eruptions
 Marker by many faults and fissures parallel to main mid-ocean rift
Convergent/destructive plate boundaries




Zones of compression where plates collide with
each other
Oceanic-oceanic: denser oceanic plate subducted
and is melted backinto the asthenosphere
Oceanic-continental: denser oceanic plate
subducted and is melted backinto the
asthenosphere
Continental-continental: both are very light, great
uplift where crust is compressed and are fused
together into one block to form great mountain
ranges
Trenches
 Long narrow troughs which mark zones of
subduction
 Most of them are found in the Western part of the
Pacific Ocean
 Can reach 10 km deep (Mariana Trench is about ~
11 km deep)
 Found in two different situations:
o Continental-oceanic: found near continent
 Peru-Chile Trench on the West Coast of South America
 Peru-Chile Trench, coastline and the Andean Cordillera follow parallel
courses
 Earthquake foci underlie the cordillera (which has many active volcanoes)
o Oceanic-oceanic
 Flanked by volcanic island arc
 Semismic activity takes place
beneath island arc
Continental Volcanic Arcs
 Between continental-oceanic plates
 Orogenesis occurs when great wedges of sediments
along coasts are compressed by folding and faulting
to form mountain chains






Oceanic crust is bent, and subducts beneath the continental crust as it is denser
As it descends into the asthenosphere to a depth of about 100 km, partial melting of waterrich oceanic crust occurs
Newly formed magma is less dense than surrounding rocks and will rise when there is
sufficient quantities
Most of the rising (usually andesitic) magma forms an intrusive feature in overlying
continental crust to form granitic batholiths
o Exposed when volcanoes and deeper parts are eroded
o Example: Large elongated batholiths from South California to Northern British
Columbia
Some will be extuded to the surface via explosive volcanic eruptions
o Over time, accumulation of layers of lava will form volcanoes
o ~100-400 km from trench
o Example: volcanic Andes Mountains formed when Nazca plate melted after
subducting beneath the South American plate
At the same time, sediments along the boundary and oceanic crust sheared off are
compressed and deformed and incorporated in the chaotic mass to form mountains
o In shallower regions, brittle rocks rupture
o In intermediate regions, plastic flow develops tight folds
o In deeper zones, intense metamorphism occurs due to high temperature and
pressures
Island Arcs
 Between oceanic plates
 Forms stratovolcanoes parallel to oceanic
trench on overriding oceanic crust
 Denser oceanic crust is bent, and
subducts beneath the lighter oceanic
crust
 As it descends into the asthenosphere to a depth of about 100 km, partial melting of waterrich oceanic crust occurs
 Newly formed magma is less dense than surrounding rocks and will rise when there is
sufficient quantities
 Lava will be extuded to the ocean floor
o Over time, accumulation of layers of lava that rise above sea levels form the island
arcs
o ~100km from trench
 Example: Japanese Islands and the Aleutians
Composition of Lava
 Andesitic lava containing 15% more silica and 3 times of K 2O by weight than ordinary
tholeiitic basalt of ocean floor
 Caused by partial melting of upper mantle under conditions of high pressure and low
temperatures (cold descending slab) in water (subducted wet sediments)
 Hence composition of andesites in island arc volcanoes varies in proportion to depth of the
Benioff Zone
 Heat required to cause melting is due to friction of subducting plate beneath the overriding
plate
Fold Mountains
 Between continental plates
 Continental crust is lighter and less dense than even the hottest regions of the mantle
 Cannot be subducted as it is too buoyant
 Continental crust fused together and pushed upwards to form non-volcanic mountain ranges
like:
o The Alps, the Himalayas and the Appalachians
o The Himalayas was formed by the collion of the Indo-Australian and Eurasian plates
 Collision sweeps up and deforms any sediment along the margin by intense folding and
faulting
o In shallower regions, brittle rocks rupture
o In intermediate regions, plastic flow develops tight folds
o In deeper zones, intense metamorphism occurs due to high temperature and
pressures
Transform plate boundaries




Zone of crust conservation as plates slide past
each other
Forms transform fault
Zones of intensely shattered rock
Commonly found at mid-oceanic ridges and rift
valleys (less obvious)
The San Andreas Fault
 Fault system found in North America between North
American and Pacific plates
 Extends NW for 1300 km through W. California
 Central: simple and straight
 Extremities: many branches that fault zone may exceed 100
km in width
 Major branches:
o Hayward fault in central California
o Elsinore fault in southern California
 Responsible for earthquakes
o Slow creep: little noticeable seismic activity
o Regular slips: small earthquakes
o Some store elastic energy for hundreds of years: great earthquake
o Example: San Francisco earthquake of 1906 caused 5 m of displacement
Hot Spots





Regions where flow of geothermal heat is higher than average
Common sites of volcanism with lava rich in alkali metals
Can cause doming up crust up to 200 km across
Remain relatively stationary
Possible origin:
o Abnormally high temperatures at core-mantle boundary initiates hot spot volcanism
on the surface
The Hawaiian Chain of Islands
 Islands mark progress of Pacific plate
across a plume
 As seafloor moves, volcano can only
remain in contact with magma source for ~
1 million years
 Cone directly overlying plume is active but
becomes inactive when it moves away
 Volcanic chain from Hawaiian Islands to
Midway Island then toward the Aleutian
trench
 Volcanoes increase in age with increasing distance fromm Hawaii
 Age of each volcano indicates the time it was situated over mantle plume when Pacific plate
moved over it
 Pitcairn and Macdonal are active volcanoes which suggest presence of 2 more plumes
 Kinks in the chain mean change of direction of Pacific plate movement
Other hot spot activities
 50-120 hot spots exist
o ~12 at divergent plate boundaries
o A sixth under the African continent
 Hot spot beneath Iceland thought to be responsible unusually large accumulation of lava at
that part of the Mid-Atlantic Ridge
 Hot spot beneath Yellowstone National Park responsible for large outpourings of lava and
volcanic ash in the area
Summary
Extrusive Volcanism

Volcanism is the movement of magma and it’s cooling within or above the surface and the
rock formation created


Extrusive volcanism is when magma reaches the earth’s surface and causes volcanic
eruptions that form various volcanic features
Distribution
o Subduction zone
 Majority of vocanic and seismic activity
 10-13% of magma reaching the surface
 84% of known eruptions and 88% of eruptions with fatalities
 Pacific Ring of Fire
o Hot Spots
 Hawaiian Islands in the middle of the Pacific Plate
o Divergent plate margins
 Iceland along the Mid-Atlantic Ridge
Components
Lava Flows
 Magma is molten rock material in the mantle
 Less dense than solid rock and hence rises to the surface of earth to form lava
Type of Magma
Basaltic
Andesitic
Rhyolitic
Silica Content (%)
50
60
70
Aphanitic (fine)
Basalt
Andesite
Rhyolite
Phaneritic (coarse)
Gabbro
Diorite
Granite
Distribution (%)
80
10
10


Rhyolite and Gabbro are rare.
Most basaltic rocks are fine grained as they are less viscous (low silica content) and reach
surface where cool temperatures allow cooling and solidification to take place quickly hence
crystals formed are fine.
 Most rhyolitic rocks are coarse grained as they are more viscous (high silica content) and cool
and solidify slowly at hot underground temperatures. The process is slow are allows large
crystals to form.
Viscousity Type of lava
Description of lava flow
of lava
flow
Lowest
Pahoehoe flow
 Twisted and ropy structure
(basaltic)
 Relatively smooth skin that wrinkles due to advacement of
the subsurface lava that is still molten while the top layer has
started to congeal
Aa flow


Surface of rough, jagged blocks with sharp edges and spiny
projections
As the suface flow cools to form a crust, the interior remains
molten and continues to advance, breaking hardened crust
into a jumbed mass of angular blocks


Initial stage: very fluid lava forms thin pahoehoe flows
Cooling with time and distance and loss of gases: viscousity increases to form Aa
flows
Pillow lava
 Extruded underwater along MOR


Most lavas on oceanic crust are pillow lavas
Jumbled pile of sand bags






Higher







Cool underwater conditions cools basaltic lava quickly
Brittle chilled surface cracks to allow molten magma inside to
ooze out like toothpaste
Similarly, it cools quickly while the surface cracks, etc.
Lava flows are not very fast
Fluidity ensures they follow existing low areas, allowing determination of path it
will take to be simple
Evacuation can easily take place
Continental edges and strings of islands (Carribean ad Japan)
Explosive but like stiff toffee is gas content is low
Thick lava flows which moves slowly often piling up over and around the vent as
a lava dome
o Made of rhyolite
o May reach several hundered meters tall
Lava look like toothpaste squeezed out a tube
More mobile pasty lava will flow down slope and spread out over surrounding
land at a speed of ~ few meters a day
Hundrerd of meters thick
Shorter than a few km
Pyroclasts
 Gases in very viscous magma cannot escape easily
 Lead to build up of internal pressure which creates a
violent eruption
 Superheated gases expand by 1000x and blow
pulverised rock and lava vent (pyroclasts) deposited
as tephra
Ash
Big
particles
Ash fall



Ejection into atmosphere
Spreads over a very wide area
Example: Eruption of Mount Hekla, Iceland in 1947, ash fell 3800
km away on Helsinki, Finland
Ash flow


Coherent clouds of ash and gas
Flow along or close to land surface
Bombs


Twisted, streamlined shapes
Erupted as globs of fluid that cooled and solidified during flight in
the air
Blocks


Angular pieces of rocks
Pieces of solidified crust of magma or from the volcanic conduit


Usually not as widespread as ash deposits
Immediate area of eruption only
Lahars (Mudflows)
 Saturation of volcanic ash and debris on slopes
 Increase in weight and reduction in cohesion
 Flow down the steep volcanic slopes
 Sources of water:





o Ice and snow melt: Nevado del Ruiz, Columbia
o Torrential rain: Mayon Volcano, Philippines
Can attain high speeds of 100 km/h, enough energy to uprrot trees, etc.
Flows narrowly in valleys but spreads over large area
Length: 100 km is common
Can reach 5000 km2 in ancient times
Example: Mout St Helens eruption in May 1980
o Lahars were > 30 km/h down the valley of the Toutle river
o Destroyed all homes and bridges
chlorine,
nitrogen
5%
Gases


sulfur
5%
Gases are held in molten magma by confining
pressure
1-5% of magma by weight, very large volume
hydrogen,
argon, etc.
5%
carbon
dioxide
15%
water
vapour
Nuee Ardente (Pyroclastic Flows)
70%
 Combination of pycroclasts and hot gases
 Formation:
o Large volumes of pyroclast like ash
and pumice erupted upwards as a billowing eruption column
o Column collapses about vent
o Flows of ash and larger fragments spread out from the centre as hot avalanches
o Hot buoyant gases creates a nearly frictionless environment
 Allow nuee ardente-s to flow at high speeds of > 200 km/h
 Travel very far to > 100 km/h from source
 Unsorted fragments of fine ash to large blocks
 May build up a level plateau
ANNEX D FOR CASE STUDIES
Type of Volcanic Eruptions
Viscousity
Percentage of
silica



Higher percentage leads to higher viscousity
Networks of silica tetrahedral retards flow
Strong networks must break for flow


Rhyolitic: short and thick flows
Basaltic: long and thin flows
Temperature


Higher temperature leads to lower viscousity
As lava cools and begins to congeal, mobility decreases until it halts
Gas content


More dissolved gases increases fluidity
Provides force to propel molten rock from vent
Nature of eruption
 Pressure controls amount of gas magma can dissolve
o High pressure: more
o Low pressure: less
 Confining pressure greatly reduced as magma moves from mantle to near the surface

Gases are released suddenly as bubbles
o Low viscousity basaltic magma: gases migrate upward and escape easily
 Quiet eruptions: Hawaii
o High viscousity magma: upward migration is impeded, collected as bubbles that
increase in size and pressure until they explosively eject onto surface
 Violent eruptions
 Sometimes viscous magma solidifies and clogs up vent to inhibit further
rise of magma
Slica content (%)
Viscousity
Degree of violence
Tendency to form lavas
Tendency to form
pyroclasts
Basaltic
50
Least
Least
Highest
Least
Andesitic
60
Intermediate
Intermediate
Intermediate
Intermediate
Rhyolitic
70
Highest
Highest
Least
Highest
Types of Eruptions
Type of lava
Basaltic
Gas/Eruption
Quiet, fissure eruption
Hawaiian
Runny, basaltic
lava flows
Strombolian
Runny lava
Gases escape easily
leading to quiet
eruption
Frequent gas
Icelandic
Pyroclast
(landform)
Horizontal plains (Deccan/Columbia
Plateau)
Occasional
Large quantities of lava blasted to form
Vesuvian
with white
cloud of
stream
emitted from
crater
Plugs of sticky,
cooled lava
Very viscous
lava cools
rapidly
Lava flows
Plinian
Sticky lava
Vulcanian
explosions leading to
very explosive
eruptions
blocks
Violent gas explosions
leading to violent
eruptions
Fragments build up into ash and pumice,
blocked vent is cleared, emission of large
quantities of volcanic ash
Very powerful blasts
of gas that it is even
more violent than
vulcanian eruptions
Gas rushes through
sticky lava
Ash clouds are pushed high into the sky and
ash fall covers surrounding areas
Features of extrusive volcanism
Ash and fragments blasted into sky in huge
explosion, immense clouds of gas and
volcanic debris several km thick, gas clouds
and lava also rush down the slopes, part of
volcano may be blasted away
Shield Vocanoes & Composite Volcanoes (Stratovalcanoes)
Type of lava
Shape
Base
Composition
Slopes
Examples
Shield Volcano
Basaltic
Broad, dome shaped
Wide base of > 100km as lava can travel
great distance
Few pyroclastic material (quiet
eruptions) and thin uniform sheets of
successive flows of basaltic lava
Convex slopes
Slopes near summit is slight as magma is
hot and fluid and will run readily down
slight slope
Further down, magma cools and is more
viscous, slope must be steeper for it to
flow
Oceanic areas
Hawaiian Islands and Iceland
Kilauea, Mauna Loa in Hawaii
Composite Volcano/Stratovolcano
Andesitic
Large and nearly symmetrical
Narrower base
Inter-bedded lavas and pyroclastic
deposits (due to violent eruptions) where
tephra can exceed volume of lava as well
as lahars
Volcano may extrude lava/pycroclast for
long periods before switching to the other,
creating volcano with alternating layers of
lava and pyroclast
Concave slopes
Steeper summit (30) and gently sloping
flanks (6-10) as finer pyroclasts found
further away have lower angles of rest
Pacific Ring of Fire
Continental and island arcs
Mount Mayon, Philippines
Fujiyama, Japan
Vesuvius erupted in AD79, killing more
than 2000 inabitants of Pompeii
Extremely rhyolitic magma may create a
plug which chokes the vent, solidifying to
form a resistant rock even after flanks are
worn away
Cinder Cones
 Volcanic peaks made of pyroclastic
materials ejected into the atmosphere
that fall back to the surface, accumulating
around the vent
 Small, steep-sided (~33)
 Small (<300m) with large, bowl-shaped
crater
 Form as parasitic cones or in calderas of
larger volanoes in groups
 Appear to be the final stages of activity
 Example: Wizard Islandin Crater Lake, Oregon
o Cinder cone formed after summit of Mount Mazama collapsed to form the caldera
Basalt Plateau
 Formed by fissure eruption where lava is
extruded along an extended fracture
 Formation
o Very fluid basaltic lava
o Great volumes of magma rises
rapidly to create floods of basalt as
there is little melting of crust
during its upward journey
o Can be as thick as 50m, burying
landscape to form a lava plain
o Can be as long as 150 km as it is very fluid
and water-like
 Basalt formed is resistant to erosion compared to
surrounding uncovered land
 Land around it reduced by erosion, basalt acts as
cap-rock and area protected by flow is left
upstanding as a plateau
 Examples:
o Deccan Plateau, West India
o Atrim Plateau, NE Ireland
Calderas
 Circular depressions on summits that exceed 1km
in diameter
 Summit collapses into partially emptied magma
chamber
 Example: Crater Lake, Oregon
Volcanic Hazards


Lava flows: confined to slopes and often take known paths, well understood by locals
Ash fall: extends thousands of km from volcano, at most brings about total darkness for
hours, suffocating animals and smothering plant life and preventing use of machinery

Pyroclastic flows and mudflows: tens or hundreds of km from volcano which develop and
move rapidly are responsible for almost all deaths as it can wipe out towns in a matter of
minutes
Volcanic Hazard Management
Predictions
 Can detect first outburst, but direction and intensity cannot be determined
 Mostly can only detect fluid magma which does not threaten life
 Violent eruptions of viscous magama cannot be predicted
o Nevado del Ruiz, Columbia in November 1986: final eruption could not be predicted
leading to 20 000 deaths in heated mudflows
 False alarms are expensive and weaken credibility
o Guadeloupe, West Indies on 12 April 1976: 75 000 people evacuated for 15 weeks
for a small, harmless eruption on 8 June at a cost of US$500 million
Land
deformation
measurement
Seismic
Activity
Monitoring

Movement of magma causes tilting of earth’s surface measured by
tiltmeters
o Large sudden tilts: violent eruption
o Slight tilts: movement of magma closer to Earth


Usage in Hawaii forecasted eruption of Kilauea
Dramatic inflation of 0.5-1.5m a day on Mount St Helens before eruption

Flow of magma apply stress of rocks which can fracture and set off seismic
activity at less than 10km depth at low magnitudes
2 types
o Long period vibrations due to resonance of magma flowing
through cracks used to forecast impending eruption
o Regular vibrations that indicates origin and nature of magma
Can also represent collapse of rock into emptying magma chambers,
meaning the cessation of volcanic activity rather than an impending
eruption


Analysis of
Geomagnetic
and
Geoelectric

Long period vibrations predicted eruption of Mount Redoubt in Alaska on 2
January 1990

Contains a lot of ferromagnetic minerals which can change local magnetic
field as volcanoes have their own geomagnetic fields
Rising temperatures reduce magmetisation (200-600C reduced magnetic
field)



Effects


Gas Analysis
Magnetic field dpends of proximity and temperature of molten magma
BUT, piezomagnetism where increasing pressure and stress exterted by
flowing magma to the surface enhanced magnetic field
Geoelectric measurements of resitivity of subsurface layers and changes in
telluric currents give an image of behaviour of magma at depth
Resistivity id dpendent on nature of rock, water content, salinity and
temperature


Analysis of gas erupted by volcano is a good technique
Need to analyse the gas costituents immediately as it is vented

Prediction of Moun Pinatubo
Response
Lava
flows


Often follow existing valleys
Diverted by building barriers or flowed with cold water
Explosive
eruptions


Cannot be contained or directed
Evacuation plans
o Morgue facilities
o Local hospital emergency facilities for burns or respiratory irritation
o Face masks and breathing apparatus
o Advice for local communities on how to cope if cut off for several days
Ash fall





Example: Mount St Helens in 1980
Sweep ash from roofs to prevent collapse
Measure levels of toxic gases and ash particle size
Alternative sources of drinking water
Alternate transport and communication methods as trains may not be able to
travel, planes are grounded and tranmission signals may be intefered
Athmosphere
Structure and Composition of the Atmosphere
Vertical Stratification of the Atmosphere







Troposphere
Tropopause
Stratosphere
Stratopause
Mesosphere
Mesopause
Thermosphere
Mass
Characteristics/
Presence of
weather
Temperature
Troposphere
80%
Vertical mixing of
air, weather
Decrease with
increasing
altitude
(6.5C/km)
15C near ground
and -57C near
the top (direction
of energy
emanates upward
from surface)
Thinning gas, less
mloecules per
unit volume to
absorb solar
energy
Thickness
Varies,
dependent on
average
temperature
Thickest in the
Stratosphere
19.9%
Absence of
precipitation:
particulates from
volcanic eruptions
remain in layer
for many months
and strong winds
cause it to be
distributed across
the globe,
creating veil of
material which
affect penetration
of sunlight
1991 eruption of
Pinatubo, caused
redder-thannormal sunrises in
the Northen
hemisphere
Constant until
20km
Increases sharply
until 50km
(Stratopause)
Concentration of
ozone which
absorbs UV from
sun
Increasing
temperature with
increasing
altitude due to
less UV radiation
abailable as
energy
penetrates down
Mesosphere
Thermosphere
No well-defined
upper limit
Pressure is 10-6
pressure at sea
level
Decrease with
increasing
altitude until 90C as the
density of
atmosphere
decreases
Increases with
increasing
altitude due to
absorption of
very shortwavelength solar
energy by atoms
of oxygen and
nitrogen
tropics (16km)
due to high
temperature
which leads t
more developed
thermal mixing
Thinest in polar
regions (<9km)
due to low
temperature
Atmosphere’s Gaseous Compounds
Permanent gases
 Constant proportion of atmospheric mass
 Nitrogen and Oxygen (99% of clean, dry air)
 Argon (0.93% of clean dry air)
 Hydrogen and Helium (the rest)
 Does not really affect weather
Variable gases
 Dristribution varies in time and space
 Small % but affects weather
 Water vapour
o Most of all variable gases
o Ranges from 1% (deserts/polar) to 4% (tropics)
o Concentration falls with height and rare at high altitudes (usually in lowest 5km)
o Source: evaporation from earth’s surface
o Greenhouse gas which forms clouds that absorb energy emitted by Earth’s surface
 Carbon dioxide
o 0.037%
o Source: respiration, decay of organic material, volcanic eruptions, natural and
anthropogenic combustion
o Removed: photosynthesis
o Greenhouse gas: absorbs radiation emitted by Earth’s surface
Earth’s Energy Budget and Regional Temperature Variations
Earth’s Energy Budget



Earth only intercepts a minute percentage of energy given off by sun (1/2 billion)
Solar radiation is more 99.9% the energy that heats the earth and causes weather and
climate
Unequal heating of Earth causes circulation of atmospheric and oceanic currents
Global Energy Balance
 Insolation: comes from the sun (short wave due to high temp of sun)
 Terrestrial radiation: emitted by earth (long wave due to lower temp of earth)






Wavelengths of emitted radiation are inversely proportional to the temperature of the
mitting body
Net radiation is zero to maintain temperature of earth
Energy budget: annual balance of incoming and outgoing energy
Amount of solar radiation intercepted at top of atmosphere is more than amount absorbed
by the atmosphere and earth
The rest is scattered by dust and gas particles, reflected by clouds and the earth’s surface.
When returning to outer space, it is radiated from the atmosphere, surface at night and by
wind and condesation
Scattering



Reflection



Gases and dust particles in the atmosphere redirects energy in a different
direction as insolation travels in a straight line
Some insolation that passes through will be scattered in many directions until
it reaches the earth’s surface or returns to space
Dependent on wavelengths, scattered more if shorter
o Blue has a shorter wavelength, more scattering, blue sky
Biggest component in loss of short-wave radiation
Radiation returned to space in short-wave form
Dependent on:
o Colour: light colours reflect better
o Type: smooth and shiny surfaces reflect better
o Albedo (reflectivity of a surface)
 Water has great variance
 Low albedo when sun is high (sun rays penetrates
through at high angles
 High albedo when sun is at low angles, reaching

Absorption







Greenhouse
effect






almost 80%
Land generally has higher albedo so more radiation is
reflected over land
Atmosphere gains heat and warms up
Gases, clouds, dust and haze absorb certain wavelengths
Gas molecule transforms energy absorbed into internal molecular motion,
temperature increases
Short-wave radiation converted to long-wave radiation to warm atmosphere
Oxygen removes shorter UV radiation
Ozone absorbs longer UV rays in stratosphere
With water vapour, absorbs most of the 19% of total solar radiation absorbed
Gases like carbon dioxide and water vapour help to warm the earth by
absorbing and re-emitting infrared radiation
Re-emitted radiation will be passed between surface and the gases
Some will be lost at night to account for energy loss
Earth has a lower surface temperature than the sun, hence terrestrial
radiation is emitted in longer wavelengths between 1 -30 micrometers
(infrared range)
Largely transparent to short-wave and more absorptive to longwave radiation
Incoming solar radiation reaches earth unhindered while outgoing terrestrial
radiation readily absorbed
Poleward Heat Transfer
 Net radiation for the planet is zero
 Energy gain is greatest at the tropics (2.4x) compared to polar regions where
o Angle of the sun is low
o Greater albedo of snow and ice
o Thicker atmosphere (more scattering and reflection)
 Transfer of heat by air movement which creates winds and drives the ocean currents
 Unending cycle to balance heat
Latitudinal Difference in Insolation Amount
 Beam spreading
o Tropics: less beam spreading as
sun is directly overhead, insolation
is concentrated and radiation
received per unit is greatest
o Polar region: more beam spreading
as angle of sun is lower, insolation
is spread out and radiation
received per unit is less
 Thickness of atmosphere to pass through
o Polar region: angle of sun lower,
passes through more atmosphere, more reflection and scattering, less insolation
 Albedo
o Polar regions: high albedo of snow and ice and low angle of sun hitting water leads
to more reflection
Latitudinal Radiation Balance
 Long-wave radiation
emission is less variable than
insolation is proportional to
the absolute temperature of
the surface
 Higher in lower latitude
regions compared to polar
regions
 Difference between
incoming and outgoing
radiation is the net radiation
o Surplus in the
tropics
o Deficit in the polar
regions
 Need to redistribute heat from areas of surplus to areas of deficit via advection of heat from
equator to poles by winds (75%) and ocean currents (25%).
Regional Temperature Variations
Proximity to the Sea
 Maritime locations experience smaller temperature variations than continental locations
Specific
heat
capacity

Penetration
of radiation


Water has higher (5x of land) specific heat capacity (amount of heat
needed to raise the temperature of 1 kg of a subtance by 1C)
More energy is needed to proudce a temperature change

Radiation received at the surface of water can penetrate many meters in,
allowing energy to be distributed throughout the mass
Insolation is only absorbed at the the thin, opaque surface layer of land
Evaporation


Vast supply of water available in seas available for evaporation
Energy used in evaporation leads to less energy for warming
Mixing

Energy surpluses from one area can flow to regions of lower temperature
as water can be easily mixed vertially and horizontally
Ocean Currents
Direction
Location
Effect
Examples
Warm
Poleward
Western part of oceans near
the east coast of continents in
the middle latitudes
High water temperatures
promote higher air
temperatures
Gulf Stream and North Atlantic
Stream flow N then E across the
Atlantic to keep winters in the
British Isles mild
Cold
Equatorward
Eastern part of oceans near the
west coast of continents in the
middle latitudes
Low water temperatures
promote lower air
temperatures
Labarador Current NE of N
America reduces summer
temperatures to produce cool
summers
Altitude
 Temperature falls with altitude (10C/1km)
 Low altitude: dense air contains dust and water which retains/traps heat, heat escapes
slowly
 High altitudes: thinner air contains little dust and water to retain/trap heat
Cloud Cover
 Decreases amount of insolation reaching the surface and leaving it
 No cloud cover would mean the maximum range of temperature (no hindrance in entry and
exit of insolation)
 Thick cloud cover: Rainforest areas have a small range of 20-30C
 Little cloud cover: Desert areas have a large range from freezing to 40C
Aspect





Direction a place faces
More noticeable in temperate regions
N hemp: S facing slopes (adret) are warmer than N facing slopes (whac)
S hemp: N facing slopes (adret) are warmer than S facing slopes (whac)
Opposite sope receives midday sunlight at a more direct angle, more insolation and hence
higher temperatures
Atmospheric/Oceanic Circulations and the Occurrence of
Seasons
Global Atmospheric Circulation
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2012 Physical Geography Notes

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