briefly introduction about Volcanoes,Fault Zone And Earthquakes,,Seismograph,Body Waves,Features Of Volcanism,Volcanic EruptionsVolcanic Products,Locating Earthquakes,Measuring The “Size” Of Earthquakes,World Earthquake Distribution,Effects Of Earthquakes

1. WELCOME• Volcanoes
• Structure Of A Volcano
• Volcanic Products.
• Volcanic Eruptions
• Features Of Volcanism
• Earthquakes
• Fault Zone And Earthquakes
• Body Waves
• Seismograph
• Locating Earthquakes
• Measuring The “Size” Of Earthquakes
• World Earthquake Distribution
• Effects Of Earthquakes

2. VOLCANOES
• The interior of the in the earth is quite hot, and at certain
places, the interior crustal rocks may exist molten state. This
molten material or liquid rock when heavily charged with
gases and volatile substances existing below the earth’s
surface, is called MAGMA.
• The process of movement of magma within the earth’s curst
and eruptions is called volcanism or volcanic activity.
• The eruptions through long fissures are normally quite, are
called fissure type eruption.

3. STRUCTURE OF A VOLCANO
• In addition to emission of gases and molten lavas , vast quantities of
fragmental materials (pyro clasts) are also produced during volcanic
eruptions. This material accumulates around the vent.
• Conical-hill like masses formed in this way are called volcanic cones.
Volcanoes often have side vents as well . The smaller cones formed around
these side vents are called parasitic cones.
• The erupted products spread in such a way that a typical circular
depression or pit is formed at the top of the hill, which is known as crater.

4. VOLCANIC PRODUCTS.
• Solid products
• Enormous quantities of solid materials are thrown out by volcanoes
during an eruption. They consist of fragments of rocks or pieces of
already cooled lava.
• The ejection of the solid materials are usually accompanied by
violent explosions. The solid materials, during the initial stages of
volcanism, mostly contain the fragments of the crustal rocks
through which the pipe of the volcano passes; but at later stages
they consist mostly the fragments of solidified lava, resulted from
the partial solidification in the molten reservoir beneath the
surface as well as the solidified lava of earlier eruptions.

5. The rock fragments ejected during volcanic-eruptions are called pyroclasts or tephra. Generally, larger
fragments fall at the edge of the crater and slide down its inner and outer slopes, while smaller ones are
thrown into the surrounding plains or pile up at the foot of the cone.
According to their size and shape the pyroclastic materials are classified as follows:
(i) Volcanic blocks
These are the largest masses of rock blown out. These are either the masses of the solidified lava of
earlier eruptions or those of the pre-existing rocks. They are usually angular and the diameter of the
fragments is always above 32 milimeter. Thus they are the huge solid fragments ejected during a
volcanic activity.
(ii) Volcanic bombs
These are rounded or spindle-shaped masses of hardened lava, which may develop when clots of lava
are blown into the air and get solidified before reaching the ground.
Their ends are twisted, indicating rapid rotation in the air while the material was plastic. Because of their
somewhat rounded appearance, they are known as volcanic bombs. The diameter of these fragments
are always above 32 millimeter.

6. Bread-crust bombs are those volcanic bombs which present a cracked surface, may be due to the approxi-
mately solid state of the material from which they have been formed, which gives the appearance of the
crust of a bread.
(iii) Cinders or lapilli
The size of the fragments is between 4 mm to 32 mm, and are shaped very much like bombs. The term
'lapilli' is used when the fragments are not conspicuously vesicular; and in case of vesicular fragments they
are known as cinders. Still smaller fragments are called volcanic-sand.
(iv) Ash
These particles range in size from 0.25 mm to 4mm and as such, are the fine particles of lava.
(v) Fine-ash or volcanic dust
These are the minute pyroclas- tic materials, and their diameter is always less than 0.25 mm. In many
instances volcanic dust was carried by wind to enormous distances and scattered over a vast territory
forming volcanic dust layers.

7. Pyroclastic materials accumulating on the slopes and adjoining areas of a volcano with gradual
compaction and cementation gives rise to rocks called Volcanic-tuffs.
These tuffs when consist of angular fragmental materials, they are known as Volcanic- breccia; when
volcanic bombs are predominant in the tuffs, they are referred to as Volcanic-agglomerates.
The diameter of these fragments are always larger than 20 mm. The welded tuffs are commonly known
as Ignimbrites. In certain instances, a great cloud of superheated vapours and incandescent rock material
and volcanic ash are violently emitted during the eruption. These are called Nuees ardentes and are
sometimes referred to as glowing avalanches.
(b) Liquid products
Lavas are the major and the most important liquid product of a volcano. As we know, the magma that has
flowed out on to the surface is called lava. All lavas contain gases, but because of the high pressure that
prevails in the interior of the earth the content of gases and vapours in the magma is more.
According to the composition and the gas content, the temperature of lavas during eruptions usually
ranges between 900°C to 1200°C. Like magma, lava is also divided in to three types viz. acidic, medium
and basic, depending on the silica content
Acid lavas contain a high proportion of silica, have a high melting point and are usually very viscous and
therefore their mobility is low. They cool very slowly and contain many gases in a dissolved state.
They congeal at relatively short distances from the crater. Rhyolites, composed of orthoclase feldspar and
quartz are the examples of acid lavas.

8. The lavas of intermediate or medium composition have the silica content between 55 to 60%. Andesite lavas are
the best examples of the lavas of intermediate nature and they mostly characterize extrusions around the margins
of the Pacific.
The basic lavas contain low percentage of silica, which is usually 50% or less. These lavas melt at lower
temperature, and have a high density as well as liquid consistency. They cool quickly and contain little gas.
These lavas are highly mobile and spread over large distances, forming flows or sheets. Basalts are the best
examples of the basic lava.
Since the lava behave differently depending on their chemical composition they give rise to different
configurations when consolidated, as described below:
(i) Lava tunnels
Sometimes the outer surface of the lava flows; cools and solidifies first forming a crust while the lava is still in a
liquid state inside. This enclosed liquid may drain out through some weak spots of the solidified flow forming a
tunnel called a lava-tunnel.

9. (ii) Block lava
It is also known as aa-lava. In this case, the gases escape explosively from the partly crystallized flows thus break the
congealing crust in to an assemblage of rough and uneven blocks.
The escape of gases increase the viscosity of the lava and helps in rapid cooling, giving rise to a solidified lava flow with
spiny, rubbly surface. It is therefore the Hawaiian name, aa (pronounced ah-ah meaning rough or spiny) is applied to this
type of lavas.
(iii) Ropy-lava
Lavas with low-viscosity remain mobile for a longer period. These lavas usually contain much entrapped gas and cool very
slowly.
The lava spreads out in thin sheets and congeals with a smooth surface which wrinkles or twisted into ropy form like that
of a stream of flowing pitch. It is also called Pahoehoe-structure.
(iv) Pillow lava
Lava erupted under water-logged sediments in sea-water, beneath ice-sheets, or in to rain soaked air, characteristically
emerges as a pile of rounded bulbous blobs or pillows. Basic lava of spilitic type often presents pillow structure.

10. (v) Vesicular or Scoriaceous structure
When lavas heavily charged with gases and other volatiles are erupted on the surface, the gaseous constituents
escape from the lava, due to the decrease of pressure, giving rise to a large number of empty cavities of variable
dimensions on the surface of the lava-flows.
Due to the presence of vesicles or cavities, the resulting structure is known as vesicular- structure. These cavities
when filled up subsequently with secondary minerals, the structure is called amyg- daloidal structure and the
infillings as amygdales.
A highly vesicular rock, which contains more gas space than rock, is known as 'Scoria'. In more viscous lavas, when
the gases cannot escape easily and the lava quickly congeals, it forms Pumice or 'Rock -froth', which contains so
much void space that it can float in water.
(vi) Jointing
As a consequence of contraction due to cooling joints are developed in the lava flows, which may be manifested in
the form of sheet, platy or columnar structures, (c) Gaseous Products Volcanic activity is invariably associated with
emanation of steam and various gases from the volcanoes.
Water vapor constitutes about 60 to 90% of the total content of the volcanic gases. Second in abundance to steam
among volcanic gases is carbon-di-oxide.
Amongst other gases which have been detected in considerable quantities, hydrochloric acid, sulphuretted
hydrogen, Sulphur-dioxide, hydrogen, nitrogen, boric-acid vapours, phosphorous, arsenic vapour, argon,
hydrofluoric acid etc. are the most important
The vents emitting sulphurous vapours are called Solfataras' when carbon-dioxides are emitted they are called
'Mofettes' and in the case of emission of boric-acid vapours, they are known as Saffioni.

11. Volcanic Eruptions
The most common type of volcanic eruption occurs when magma (the
term for lava when it is below the Earth's surface) is released from a
volcanic vent. Eruptions can be effusive, where lava flows like a thick,
sticky liquid, or explosive, where fragmented lava explodes out of a
vent. In explosive eruptions, the fragmented rock may be accompanied
by ash and gases; in effusive eruptions, degassing is common but ash is
usually not.
Volcanologists classify eruptions into several different types. Some are
named for particular volcanoes where the type of eruption is common;
others concern the resulting shape of the eruptive products or the
place where the eruptions occur. Here are some of the most common
types of eruptions:
VOLCANIC ERUPTIONS

12. Hawaiian Eruption
In a Hawaiian eruption, fluid basaltic lava is thrown into the air in jets from
a vent or line of vents (a fissure) at the summit or on the flank of a
volcano. The jets can last for hours or even days, a phenomenon known
as fire fountaining. The spatter created by bits of hot lava falling out of the
fountain can melt together and form lava flows, or build hills called spatter
cones. Lava flows may also come from vents at the same time as
fountaining occurs, or during periods where fountaining has paused.
Because these flows are very fluid, they can travel miles from their source
before they cool and harden.
Hawaiian eruptions get their names from the Kilauea volcano on the Big
Island of Hawaii, which is famous for producing spectacular fire fountains.
Two excellent examples of these are the 1969-1974 Mauna Ulu eruption
on the volcano's flank, and the 1959 eruption of the Kilauea Iki Crater at
the summit of Kilauea. In both of these eruptions, lava fountains reached
heights of well over a thousand feet.

13. Strombolian Eruption
Strombolian eruptions are distinct bursts of fluid lava (usually basalt or basaltic
andesite) from the mouth of a magma-filled summit conduit. The explosions
usually occur every few minutes at regular or irregular intervals. The explosions of
lava, which can reach heights of hundreds of meters, are caused by the bursting
of large bubbles of gas, which travel upward in the magma-filled conduit until they
reach the open air.
This kind of eruption can create a variety of forms of eruptive products: spatter, or
hardened globs of glassy lava; scoria, which are hardened chunks of bubbly lava;
lava bombs, or chunks of lava a few cm to a few m in size; ash; and small lava
flows (which form when hot spatter melts together and flows downslope).
Products of an explosive eruption are often collectively called tephra.
Strombolian eruptions are often associated with small lava lakes, which can build
up in the conduits of volcanoes. They are one of the least violent of the explosive
eruptions, although they can still be very dangerous if bombs or lava flows reach
inhabited areas. Strombolian eruptions are named for the volcano that makes up
the Italian island of Stromboli, which has several erupting summit vents. These
eruptions are particularly spectacular at night, when the lava glows brightly.

14. Vulcanian Eruption
A Vulcanian eruption is a short, violent, relatively small explosion of viscous
magma (usually andesite, dacite, or rhyolite). This type of eruption results
from the fragmentation and explosion of a plug of lava in a volcanic conduit, or
from the rupture of a lava dome (viscous lava that piles up over a vent).
Vulcanian eruptions create powerful explosions in which material can travel
faster than 350 meters per second (800 mph) and rise several kilometers into
the air. They produce tephra, ash clouds, and pyroclastic density currents
(clouds of hot ash, gas and rock that flow almost like fluids).
Vulcanian eruptions may be repetitive and go on for days, months, or years, or
they may precede even larger explosive eruptions. They are named for the
Italian island of Vulcano, where a small volcano that experienced this type of
explosive eruption was thought to be the vent above the forge of the Roman
smith god Vulcan.

15. Plinian Eruption
The largest and most violent of all the types of volcanic eruptions are Plinian
eruptions. They are caused by the fragmentation of gassy magma, and are usually
associated with very viscous magmas (dacite and rhyolite). They release enormous
amounts of energy and create eruption columns of gas and ash that can rise up to 50
km (35 miles) high at speeds of hundreds of meters per second. Ash from an eruption
column can drift or be blown hundreds or thousands of miles away from the volcano.
The eruption columns are usually shaped like a mushroom (similar to a nuclear
explosion) or an Italian pine tree; Pliny the Younger, a Roman historian, made the
comparison while viewing the 79 AD eruption of Mount Vesuvius, and Plinian
eruptions are named for him.
Plinian eruptions are extremely destructive, and can even obliterate the entire top of
a mountain, as occurred at Mount St. Helens in 1980. They can produce falls of ash,
scoria and lava bombs miles from the volcano, and pyroclastic density currents that
raze forests, strip soil from bedrock and obliterate anything in their paths. These
eruptions are often climactic, and a volcano with a magma chamber emptied by a
large Plinian eruption may subsequently enter a period of inactivity.

16. Geysers, fumaroles (also called solfataras), and hot springs are generally
found in regions of young volcanic activity. Surface water percolates
downward through the rocks below the Earth's surface to high-
temperature regions surrounding a magma reservoir, either active or
recently solidified but still hot. There the water is heated, becomes less
dense, and rises back to the surface along fissures and cracks.
Sometimes these features are called "dying volcanoes" because they
seem to represent the last stage of volcanic activity as the magma, at
depth, cools and hardens.
Erupting geysers provide spectacular displays of underground energy
suddenly unleashed, but their mechanisms are not completely
understood. Large amounts of hot water are presumed to fill
underground cavities. The water, upon further heating, is violently
ejected when a portion of it suddenly flashes into steam. This cycle can
be repeated with remarkable regularity, as for example, at Old Faithful
Geyser in Yellowstone National Park, which erupts on an average of
about once every 65 minutes.
FEATURES OF VOLCANISM

17. Fumaroles, which emit mixtures of steam and other gases, are fed by
conduits that pass through the water table before reaching the surface of
the ground. Hydrogen sulfide (H2S), one of the typical gases issuing from
fumaroles, readily oxidizes to sulfuric acid and native sulfur. This accounts
for the intense chemical activity and brightly colored rocks in many
thermal areas.
Hot springs occur in many thermal areas where the surface of the Earth
intersects the water table. The temperature and rate of discharge of hot
springs depend on factors such as the rate at which water circulates
through the system of underground channelways, the amount of heat
supplied at depth, and the extent of dilution of the heated water by cool
ground water near the surface.

18. EARTHQUAKES
•An earthquake is a trembling or shaking of the ground
caused by the sudden release of energy stored in the rocks
beneath Earth’s surface
–Tectonic forces within the Earth produce stresses on rocks that
eventually exceed their elastic limits, resulting in brittle failure
•Energy is released during earthquakes in the form of
seismic waves
–Released from a position along a break between two rock masses
(fault)
•Elastic rebound theory - earthquakes are a sudden release
of strain progressively stored in rocks that bend until they
finally break and move along a fault

19. Fault zone and earthquakes

20. Body Waves
- Travel through the earth
- 2 types, have different motion:
Primary (P) waves
•Particle motion is parallel to wave direction
•Travel fastest (arrive first)
•Travel through solid or fluid
Secondary (S) waves
•Particle motion is perpendicular to wave direction
•Travel slightly slower
•Only travel through solid

21. Surface Waves
•Slowest type of seismic waves produced by earthquakes
•Love waves - side-to-side motion of the ground surface
–Can’t travel through fluids
•Rayleigh waves - ground moves in an elliptical path
opposite the direction of wave motion
–Extremely destructive to buildings

22. Measuring Earthquakes
•Seismometers - used to measure seismic waves
•Seismographs - recording devices used to produce a permanent
record of the motion detected by seismometers
•Seismograms - permanent paper (or digital) records of the
earthquake vibrations
–Used to measure the earthquake strengths

23. Seismograph
•Measures horizontal
motion (P waves)

24. Locating Earthquakes
•P- and S-waves leave earthquake focus at the same time
•P-wave gets farther and farther ahead of the S-wave with distance and
time from the earthquake
•Travel-time curve - used to determine distance to focus
–based on time between first P- and S-wave arrivals

25. Locating earthquakes
•Plotting distances from 3 stations on a map, as circles with radii equaling the distance
from the quake, locates earthquake epicenter
•Depth of focus beneath earth’s surface can also be determined
–Shallow focus 0-70 km deep
–Intermediate focus 70-350 km deep
–Deep focus 350-670 km deep

26. Measuring the “Size” of Earthquakes
•Earthquake “size” measured two ways -
intensity and magnitude
•Intensity - a measure of the effects an
earthquake produces (on both structures
and people)
–Modified Mercalli scale

27. •Size of earthquakes measured in two ways -
intensity and magnitude
•Magnitude is a measure of the
amount of energy released by
an earthquake
–Richter scale
•Moment magnitude - more
objective measure of energy
released by a major earthquake
–Uses rock strength, surface area
of fault rupture, and amount
of movement
–Smaller earthquakes are more common than larger
ones

28. Effects of Earthquakes
•Earthquakes produce several types of effects, all of which can cause loss of
property and human life
–Ground motion is the familiar trembling and shaking of the land during
an earthquake
•Can topple buildings and bridges
–Fire is a problem just after earthquakes because of broken gas and
water mains and fallen electrical wires
–Landslides can be triggered by ground shaking, particularly in larger
quakes
–Liquefaction occurs when water-saturated soil or sediment sloshes like
a liquid during a quake

29. World earthquake distribution
•Most earthquakes occur in narrow geographic belts which
mark tectonic plate boundaries
•Most important concentrations in circum-pacific and
mediterranean-himalayan belts
•Shallow-focus earthquakes common along the crests of
mid-oceanic ridges
•Nearly all intermediate- and deep-focus earthquakes occur
in benioff zones
–Inclined seismic activity associated with descending oceanic plate
at subduction zones)

30. Earthquakes and plate tectonics
•Earthquakes are caused by plate inter-actions along
tectonic plate boundaries
•Plate boundaries are identified and defined by
earthquakes
•Earthquakes occur at each of the three types of plate
boundaries: divergent, transform, and convergent
–At divergent boundaries, tensional forces produce
shallow-focus quakes on normal faults
–At transform boundaries, shear forces produce
shallow-focus quakes along strike-slip faults
–At convergent boundaries, compressional forces
produce shallow- to deep-focus quakes along
reverse faults

31. Earthquake prediction and seismic risk
•Accurate and consistent short-term earthquake prediction
not yet possible, three methods assist in determining
probability that an earthquake will occur:
–Measurement of changes in rock properties, such as
magnetism, electrical resistivity, seismic velocity, and
porosity, which may serve as precursors to
earthquakes
–Studies of the slip rate along fault zones
–Paleoseismology studies that determine where and
when earthquakes have occurred and their size
•Average intervals between large earthquakes and
the time since the last one occurred can also be
used to assess the risk (over A given period of time)
that A large quake will occur