Hidden in the Earth are huge forces and fires that release their might
through earthquakes and volcanoes. Before we can hope to predict these often
catastrophic events we need to know what they are and why they happen
VIOLENT earthquakes and volcanic eruptions are among the most spectacular
of all natural phenomena. A single eruption can wield the explosive force of a
nuclear bomb, pulverising as much as 15 cubic kilometres of rock. Big
earthquakes shift huge volumes of rock in seconds, sending strong shock waves
through the planet.
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Earthquakes and volcanoes do not occur randomly; they fall into a simple
pattern. The theory of plate tectonics states that the Earth鈥檚 outer shell,
the lithosphere, is divided into sections or plates, which are constantly
moving relative to each other. Almost all of the active volcanoes and
earthquakes on Earth lie on the boundaries of these plates.
An earthquake is the result of some sort of shock within the Earth that
releases energy as tremors or seismic waves. The shock may be a sudden rupture
on a fault 鈥� a fracture running through layers of rocks. These seismic waves
radiate from the source or focus of the quake. The epicentre is the point on
the surface of the Earth immediately above the focus. This is where the
effects of the quake are usually most apparent. A big quake can be preceded by
smaller tremors, known as foreshocks, and followed by aftershocks. Both can
cause devastation, but particularly when aftershocks hit buildings that are
already damaged by the main shock.
Seismic waves travel both along the surface and through the body of the
Earth. Surface waves may move vertically and horizontally, like waves on the
sea, or they can oscillate solely horizontally. After a quake, there are two
types of body waves. The first type, called primary or P waves, are always the
first to arrive at a particular point. These are pressure waves, like sound
waves. They consist of a sequence of compressions and rarefactions moving in
the direction in which the waves are travelling. The second body waves to
arrive are transverse waves. These S waves are a series of oscillations at
right angles to the direction of movement.
There is a sizeable earthquake roughly every day, somewhere in the world.
Tiny tremors are much more frequent. Quakes of magnitude 7 and larger (see
Box), which are about the size of the devastating quake in Azerbaijan in 1988,
happen every week. But such large quakes tend to be noticed only when they
happen on land, near populated areas, or if they are submarine and create
large destructive 鈥渟ea waves鈥� or tsunami. Most earthquakes are so small or
originate so deeply that they are detected only by seismometers, instruments
which amplify amd record the movement of the ground resulting from quakes.
By mapping out the intensity of ground movements during quakes, researchers
have come to realise that the properties of rocks and sediments immediately
below the ground influence the pattern of vibrations during a quake. Sediments
such as sand oscillate in a less predictable way than solid rock, doing far
more damage to the foundations of buildings. Pressure waves from quakes can
also disrupt subsurface water, turning solid sediments into fluids, like
quicksand. This process, called fluidisation can wreck the foundations of
large buildings and other structures, causing them to collapse.
Seismologists are also learning that the type of quake depends on the
processes happening at plate boundaries. In California, the San Andreas fault
zone marks the boundary between the Pacific plate, moving north, and the North
American plate, heading south. Earthquakes on the San Andreas and related
faults tend to have the same sort of movement, mainly horizontal slip, with
the west side moving north relative to the eastern rocks. These quakes tend to
be shallow, originating in the top 10 or 15 kilometres of the Earth鈥檚 crust.
At greater depths, pressure and temperature are so high that the rocks tend to
flow rather than fracture (see Inside Science No 6).
Mid-ocean ridges, the huge volcanic mountain ranges that lie on the world鈥檚
ocean floor, where the Earth鈥檚 crust is continually growing as the plates
drift apart, also tend to have shallow quakes. The two sides of the ocean move
apart along the ridge. The quakes happen on shallow faults that extend across
the ocean floor. Deep earthquakes occur at another type of plate boundary 鈥�
subduction zones. The ocean floor that forms at mid-ocean ridges is
compensated for by two plates moving towards each other in another region of
the Earth. The ocean floor slides beneath the buoyant continent into the
mantle, in a process known as subduction. This is happening today along the
west coast of South America. Subduction can also take place away from
continents.
The movement of slabs into subduction zones has produced some of the
deepest earthquakes recorded 鈥� from more than 600 kilometres below ground.
When the slab of oceanic lithosphere slides into the hotter, fluid mantle it
takes time to warm up. As the slab descends, it distorts and eventually cracks
to create earthquakes. But subduction is relatively fast, so by the time the
slab cracks it has slid several hundred kilometres down in the mantle.
Predicting quakes
Creeping faults
AT its simplest, earthquake forecasting says that people living in active
fault zones should expect a quake if there has not been one for a long time.
At a plate boundary, the movement, or slip, in all the earthquakes over
decades or even hundreds of years roughly matches the average displacement of
the plates. Plates move at a few centimetres a year 鈥� about the same rate as
fingernails grow. A big quake involving a few metres of slip could be expected
every hundred years or so, on a single active fault zone. Alternatively, a
fault could have much smaller quakes every few decades.
In this simple model, earthquakes result from forces generated by plate
movements. The plates are constantly moving, increasing the stress on the
fault zone. The rock deforms a little, but eventually it fractures to create
an earthquake. The distorted rocks spring back like a stretched rubber band
after it snaps. Afterwards, each side of the fault has moved, but the elastic
distortion of the rocks has disappeared.
The notion that strain builds up around a fault before an earthquake has
inspired an array of methods to detect such warnings. Distortion of fences,
roads and buildings are obvious, but more subtle signs of strain require
careful observation. 快猫短视频s can detect small distortions by resurveying
networks of triangulation points used for map making. They can also set up
instruments such as strain gauges and tiltmeters in appropriate areas. Because
strain can alter the water-holding ability, or porosity of rocks by closing or
opening tiny cracks, the level of water in boreholes can also provide useful
information. And now that satellites can be used to measure the position of
points on the surface of the Earth to within a few centimetres, many of the
existing triangulation points can be resurveyed from space.
All these methods can reveal when a quake is likely to happen. But as data
from active fault zones mounts, it is clear that predicting quakes is not that
simple. The first problem is the length of time between major quakes. When
they occur hundreds of years apart, it is difficult to find information about
past quakes. Researchers have to depend on signs of quakes in the geological
record. These include signs of fluidisation in ancient sediments and of
streams suddenly changing course along a fault as a quake altered the
landscape.
The shape of the fault can affect the quake. Fault zones, such as San
Andreas, are made up of many fault surfaces, roughly parallel. A quake on one
can affect the chances of slip on another, or a big quake might join two
strands of the fault to take up even more slip.
The picture is made even more complicated when different parts of a fault
behave in different ways. In the simplest case, this can make forecasting
easier: if one segment of the fault has regular moderate quakes, and the
neighbouring strand has had none in recorded history, then the quiet segments
of the fault 鈥� called seismic gaps 鈥� are likely spots for quakes.
But not all segments of faults behave in this way: some active faults never
produce noticeable earthquakes. Instead, they continually slip quickly enough
to keep pace with the plate movements by a combination of infinitesimally slow
deformation of the rocks and earthquakes too small to be felt at the surface
(except by sensitive seismometers). This process is called creep and faults
that move in this way are said to be creeping. Such faults do not pose a
significant earthquake hazard unless the creep stops. By monitoring creeping
faults and seismic gaps over many years, researchers are able to map out
active faults and to identify the most likely sites for future quakes. But
earthquakes are by no means predictable.
Volcanic eruptions
Active fronts
THE ability of volcanologists to predict the eruption of volcanoes has
advanced in a similar pattern to the seismologists鈥� ability to predict
earthquakes.
Volcanoes, like earthquakes, cluster along the boundaries of the Earth鈥檚
lithospheric plates. Although the deeper levels of the Earth are much hotter
than the surface, the rocks are not usually molten because the pressure is so
high. But along plate boundaries there is molten rock 鈥� magma 鈥� to supply
volcanoes.
At mid-ocean ridges, the sheet of ocean floor on each side of the ridge
moves away from the centre as if on a conveyor belt. Hot fluid rock from deep
in the mantle flows up, as if moving to fill the central gap, warming
shallower parts of the mantle, which begins to melt. Suddenly basalt lava
erupts, to make new ocean crust.
At subduction zones, a different process triggers melting. As the slab of
ocean floor 鈥� basalt, chert and limestone 鈥� slides down the subduction zone,
it warms up slowly. Volatile compounds such as water and carbon dioxide leave
the slab and move upwards into the mantle so that it melts. The hot magma
rises to make volcanoes.
Subduction zone volcanoes form a line parallel to the plate boundary, above
the descending slab. Most of the volcanoes of the 鈥淩ing of Fire鈥�, which bounds
the Pacific Ocean, are related to subduction, beneath either oceanic or
continental crust. Subduction in the oceans produces chains of volcanic
islands known as island arcs. The Aleutian Islands, a chain of islands
extending southwards from the Alaska Peninsula, are a classic example of an
island arc. Subduction beneath a continent produces ranges of mountains, such
as the Andes, that are notable for their active volcanoes.
The exceptions to the rule that volcanoes cluster at plate boundaries are
isolated chains of volcanic islands in the middle of the oceans. Hawaii,
famous for the spectacular eruptions of its active volcano, Kilauea, is just
one in a line of increasingly older volcanic islands stretching northwest
across the Pacific Ocean. The chain continues as a series of even older
volcanoes, which are now underwater, the Emperor Seamount Chain.
These volcanoes also result when a plate moves over an especially hot part
of the fluid mantle. A mantle plume, or jet of hot material, rising from deep
in the mantle is responsible for such a hot spot. If you imagine a piece of
paper sliding above a candle, the volcanoes are the equivalent of scorch marks
on the paper. These island chains provide important clues about the speed and
direction of movement of the plates. Hot spots beneath continents can produce
isolated volcanoes. And plumes play a part in the break-up of continents to
form new oceans. This involves speedy eruptions of hundreds of thousands of
cubic kilometres of lava, known as flood basalts, in as little as 2 million
years.
Volcanoes around the world erupt in different styles. Some, such as Etna in
Sicily, smoke and steam but can produce lava flows and showers of pulverised
rock. Hawaiian volcanoes will often produce lakes of lava; Iceland has
fountains of fire that spurt many metres into the air. An explosive eruption
results from something that blows the rock, both solid and molten, into tiny
fragments of rock and glass, called ash. Some explode and pulverise huge
volumes of rock into very fine ash and some coarser debris 鈥� rock fragments
and pumice 鈥� collectively called pyroclastics.
Huge explosions occur whenever water meets hot rock. The water vaporises,
increasing the pressure until the rock explodes. Gases from within the molten
rock can also build up high pressures. However, the likelihood of a really big
explosive eruption depends largely on the viscosity of a magma, and hence its
composition. Rocks are made of silicate minerals, which bond together to form
molecular clumps, chains, sheets and scaffolding in crystals. When rocks melt,
the more silica there is, the more bonds form and the greater the viscosity.
Broadly speaking, subduction zone volcanoes, which produce more viscous lava
with a high silica content, tend to erupt explosively and produce a lot of
ash. In contrast, the volcanoes associated with a mid-ocean ridge or hot spot
tend to produce relatively fluid, basaltic lava 鈥� low in silica 鈥� as in
Iceland and Hawaii.
Gases dissolve easily in molten rock deep underground, where the pressure is
high. As the magma rises to the surface, the pressure drops and some of the
gas becomes insoluble and forms bubbles, like the bubbles that appear in a
bottle of fizzy drink when you loosen the stopper. In relatively fluid magma,
the bubbles rise to the surface, or freeze within the rock as it solidifies.
But viscous magma can trap the gas, until it builds up enough pressure to
blast the molten rock into smithereens.
The style of eruption is greatly influenced by the processes operating at
different plate boundaries, which produce magma of different, but predictable
compositions. Rocks do not melt all at once. Some minerals melt before others,
in a process called partial melting. As the rocks heat up, different minerals
melt, altering the composition of the molten rock produced. Partial melting of
the Earth鈥檚 mantle produces basalt with much the same composition no matter
where in the world the volcano lies.
Mid-ocean ridge volcanoes erupt basalt to produce anything from small cones
clustered around the ridge to huge lava plateaus. Subduction zones can erupt
basaltic rocks, but they also produce a range of rock types. The older and
deeper the slab, the longer that partial melting of the mantle has gone on and
the more silica-rich the magmas produced. In established subduction zones,
such as the Andes, the volcanoes erupt different types of rock depending on
the depth of the subduction zone below.
When will it erupt?
Telltale signs
SO if volcanologists know what to expect and where, the next question is
when will a volcano erupt?
The rise of magma beneath a volcano is a telltale sign that an eruption
could be imminent. Often, the magma stops rising and collects a few kilometres
below the volcano; this volume of molten rock is known as a magma chamber.
Magma rising into a shallow magma chamber can distort the shape of the volcano
at the surface, as happened before the eruption of Mount St Helens in
Washington State, US, in 1980. Researchers use surveys and monitoring
instruments such as tiltmeters to assess changes in the shape of a volcano.
Seismological studies can also indicate when a volcano is preparing for
action. Seismometers on and around volcanoes pick up tiny earthquakes that
represent the growth of fissures filled with magma. Often, the foci of the
quakes move nearer to the surface as an eruption approaches. And these
鈥渕icroearthquakes鈥� happen more frequently before an eruption, until all the
seismometer can register is a distinctive vibration, slight but continuous,
known as a harmonic tremor.
Something in the air
Avalanche danger
ANOTHER sign of impending eruption in some active volcanoes comes from
monitoring the gases that seep from fissures in the surface, known as vents or
fumaroles. Researchers monitoring El Chich谩n in Mexico noticed a
distinct change in the proportion of chlorine to sulphur in the gases it
emitted. That was at the end of March 1982, just before the volcano erupted.
When an eruption has enough energy, a column or plume of ash rises above
the crater, sometimes as high as 8 kilometres, reaching the stratosphere. The
plume tends to spread out as it rises, giving a characteristic umbrella shape.
Ash and debris fall steadily from the cloud to blanket the ground with a
deposit called a pyroclastic fall. Ash layers can be traced around the world
after a big eruption.
Pyroclastic falls can be dangerous because fine ash particles can damage
the lungs. Also, ash is very heavy. A layer only a few centimetres thick on a
flat roof can be enough for a building to collapse. Dust and aerosols that
reach the stratosphere will spread around the world, bringing strange colours
to sunsets and sunrises for some weeks afterwards, and sometimes they can
cause unusual weather patterns.
However, the dangers of ash can be far more immediate. If the rising column
of ash loses energy, perhaps because the eruption slows or stops, then the
column will collapse. Ash, pumice and fragments of rock move down and away
from the volcano, with the potential energy gained in the column transformed
to kinetic energy. This is one type of pyroclastic flow.
The fine particles of ash help to make pyroclastic flows move quickly and
easily. Turbulent flow mixes enough air with the fine particles of ash to make
it behave as a fluid, like avalanches of snow. A simple avalanche of debris
from the slopes of a volcano can flow long distances down a hill-side, into a
valley and hundreds of metres up the other side. Flows with extra sources of
energy 鈥� from explosive eruptions, for example 鈥� can move faster.
Explosive eruptions can take place with the force of a nuclear bomb 鈥� and
often more 鈥� and they share with such explosions the formation of a ring-
shaped cloud of debris that sweeps along the ground away from the blast. This
is called a base surge. The combination of blast and fast-moving debris can be
lethal.
Pyroclastic flows are powerful enough to knock down trees and leave a trail
of destruction in their wake. Some of them are very hot 鈥� 400 掳C or more 鈥�
making them particularly deadly. The flows may glow as they thunder away from
the volcano, earning them the name nu茅es ardentes, French for glowing
clouds.
Pyroclastic flows can also form when unstable parts of the volcano
collapse. This happened during the 1991 eruption of the Unzen volcano in
Japan. Unzen is a subduction zone volcano that erupted highly viscous lava
which grew into a dome. When it collapsed, the lava exploded to form a
pyroclastic flow. This happened several times, with the flow usually following
the same path. But just once a flow took an unusual path and reached an area
considered safe, killing a number of people including three experienced
volcanologists.
Another dangerous type of flow is a lahar, or volcanic mud flow. Wet mud or
ash can slip downhill as a dense fluid, carrying off boulders, cars and
houses. Volcanoes are prone to lahars immediately after an eruption, when
unstable ash slopes and rain make a menacing combination.
Both earthquakes and volcanoes combine a range of hazards to people living
near active sites. Human ingenuity can do little to stop them happening, but
plenty to ensure that their dangers are minimised.
Active volcanoes now figure on hazard maps, which show the areas threatened
by different aspects of the likely eruption. Places near the peak are at risk
from large fragments of debris, ash falls and poisonous gases. Farther away,
pyroclastic flows pose the greatest threat; mud and debris flows threaten
towns and villages in valleys at greater distances from the peak. Many
volcanoes have left traces of ancient eruptions in the geological record,
which show where pyroclastic flows and lahars have reached in the past. Hazard
maps can also be useful in areas at risk from earthquakes. Seismic hazard maps
relate to likely quakes on particular faults, and take into account the
physical features that can affect the intensity of the ground movement.
We cannot stop earthquakes and volcanic eruptions, but we can do our best
to be alert to the dangers they pose. Better forecasting and better
understanding of what happens during one of these awesome natural disasters
have already saved lives: they will save more in the future.
The Richter and Mercalli scales of earthquakes
IN 1935, Charles Richter, of the California Institute of Technology at
Pasadena, suggested that seismometer traces could be used to assess the energy
released by a quake 鈥� a term he called magnitude. He devised the Richter
scale, a logarithmic scale like the one used for measuring the brightness of
stars.
A magnitude 5 quake releases about thirty times the energy of one of
magnitude 4, for example. But magnitude and other more modern assessments of
the energy released at the focus, such as the seismic moment, which is based
on estimates of the size of the fault, the amount of slip and properties of
rocks, do not always indicate how dangerous a quake may be.
However, seismologists widely use the physical effects of a quake to chart
earthquakes, and apply the so-called Modified Mercalli Intensity Scale. This
relates ground movement to commonplace observations about light bulbs
and bookcases. It has the advantage that the testimony of ordinary eye-
witnesses during quakes can be used by seismologists to find out where the
shock was worst:
1 Rarely felt.
2 Felt by people who are not moving, especially on upper floors of
buildings. Hanging objects may swing.
3 The effects are noticeable indoors, especially upstairs. The vibration is
like that experienced when a truck passes.
4 Many people feel it indoors, a few outside. Some are awakened at night.
Crockery and doors are disturbed, and standing cars rock.
5 Felt by nearly everyone, most people are awakened. Some windows are
broken, plaster becomes cracked and unstable objects topple. Trees may sway
and pendulum clocks stop.
6 Felt by everyone, many are frightened. Some heavy furniture moves,
plaster falls. Structural damage is usually quite slight.
7 Everyone runs outdoors. Noticed by people driving cars. Poorly designed
buildings are appreciably damaged.
8 Considerable amount of damage to ordinary buildings, many collapse, well-
designed ones survive with slight damage. Heavy furniture is overturned and
chimneys fall. Some sand is fluidised.
9 Considerable damage occurs to even buildings that have been well
designed. Many are moved from their foundations. Ground cracks and pipes
break.
10 Most masonry structures are destroyed, some wooden ones survive. Railway
tracks bend and water slops over banks. Landslides and sand movements occur.
11 No masonry structure remains standing. Bridges are destroyed. Broad
fissures occur in the ground.
12 Total damage. Waves are seen on the surface of the ground. Objects are
thrown into the air.
Further reading
Inside the Earth, by Bruce Bolt. Freeman, 1978; Earthquakes, by Bruce Bolt,
Freeman 1988; Global Tectonics, by Phillip Kearley and Frederick Vine,
Blackwell Scientific Publications, 1990; Mountains of Fire, by Robert and
Barbara Decker, Cambridge University Press, 1991; Volcanoes: a Planetary
Perspective, by Peter Francis, Oxford University Press, 1993.