STRUCTURES
AND COMPOSITION
OF THE EARTH
Three major divisions of the Earth:
1. Crust – this is the upper layer of the solid portion
of the earth. This portion is composed of rock
with a thin weathered upper layer which we call
coil. It is estimated to be 20 miles deep and
constitute about ½% of the earth radius. This
upper layer maybe subdivided into sub – layers.
►sial – is composed of granitic rocks mostly of
silicon and aluminum, and
► sima – predominantly of basaltic rocks of silicon
and magnesium. The crust appears to be a thin
shell when viewed to scales with the rest of the
lithosphere.
An analysis of the composition of this layer showed that it
contains 92 naturally appearing elements combined with each
other forming compounds.
These elements are distributed as follows:
Oxygen
46.7%
Calcium
Silicon
27.7%
Sodium
Aluminum
8.0%
Potassium
Iron
5.0%
Others
3.7%
2.8%
2.6%
3.5%
At the base of the crust is the Mohorovivic discontinuity
which separates the crust from the underlying layers. The
rocks above this discontinuity have a specific gravity of 3.0
while those below it have a specific gravity of 3.3.
2. Mantle – is the layer just below the Mohorovivic
discontinuity which extends to a depth of 1,800 miles
below the earth’s surface. Studies have inferred that
this region is the dense arrangement of silicate
materials that are dark – colored originating from rocks
of ocean and old mountain zone regions. Studies
made on this layer through seismic readings indicate
the presence of plastic zones responsible for:
► lava
formation by converting sold mantle material into
liquid rock material.
► furcating continents
► rising and sinking of portions of continents; and
► the breaking up and drifting of the continents;
3. Core or Nucleus – is that part of the earth which has
a diameter of about 4,300 mile, more than ½ the total
diameter of the earth. Through the seismic readings this
layer is believed to be composed of an outer layer and
inner solid part.
Temperature and pressure rises as we go deeper into the
core of the earth. The inner core is estimated to have a
temperature of about 40,000 to 90,000 0F. Studies
suggest that the core of the earth contains iron and nickel
due to the following:
► Certain meteorites are composed of iron and nickel which
may have the same origin as the earth.
► The magnetic field of the earth can be explained if the field
originates from movement in a current conducting liquid
core.
► The estimated specific gravity for about 9.4 to 15 is a
result of denser packed iron and nickel.
Land Features of the Earth’s crust
1. Mountains. Are regions of undisturbed or deformed rocks
with steep slopes and considerable heights.
Types and origin of mountains:
► Erosional
mountains – are remnant landscapes
formed
by selective erosion by streams or glaciers.
► Folded mountains – are formed by a sidewise
pressure
which causes the earth’s crust to fold or bend upwards
for
thousands of meters.
► Fault or Block mountains – are lifted upwards or
titled at
different angles when large block if rocks move along
cracks.
► Volcanic mountains – form as a result of extrusive
Mammoth Mountain
Mount Mckinley
2. Volcanoes and Earthquake
Volcanoes are opening on the earth’s surface where molten
materials, gases and solids come out. It builds up different
features on the earth’s surface. Plenty of steam and gases like
carbon dioxide, hydrogen, hydrochloric acid, hydrogen sulfide
and sulfur dioxide are shoot out.
Types of Volcanoes:
Volcanoes may be classified according to eruption and activity.
According to eruption, the types of volcanoes are:
► Explosive volcanoes – throw out violently big amounts of lava,
gases and steam through the opening of the crater.
► Quiet volcanoes – the molten lava flows without violent
reactions, builds a gently sloping lava cone and produces little
gas or solid materials.
► Intermediate volcanoes – sometimes erupts violently but
sometimes just give out lava quietly.
► Fissure eruptions – quiet lava flows out not only from craters
but also along crustal cracks or fissures and forms lava plains
or plateaus.
According to activity the types of volcanoes are:
► Active volcanoes – they show signs of eruption or are known to have
erupted within the recent past and has the probability of erupting again.
Examples: Taal, Mayon, hobok-hibok, Canlaon and Bulusan in the
Philippines. Paricutin in Mexico; Vesuvius in Italy;
Mauna Loa in Hawaii; and Lassa in California.
► Dormant volcanoes – show sign of activity but have not erupted fro
several centuries.
Examples: Mounts Arayat, Banahaw, Makiling and San Cristobal in
the Philippines; Mt. Rainer in Washington D.C., USA.
► Extinct volcanoes – do not show signs of activity and not have
erupted since recorded history. Their forms are being changed by
weathering and erosion. Mt. Makiling in Laguna; Biliran in Leyte;
Malindig in Marinduque are not completely extinct Volcanoes for there
may be no completely extinct volcanoes. They just omit very hot water
and mud which are continuously bubbling and gushing out of many
openings.
ACTIVE VOLCANOES
TAAL
MAYON
VESUVIUS
DORMANT VOLCANOES
MOUNT ARAYAT
MOUNT MAKILING
MOUNT RAINER
Volcanoes in the Philippines
The Philippines has many volcanoes. Volcanic cones hot on the
land from Batanes to Sulu. They throw ashes, gases and big solids.
Though they have destroyed plenty of lives and property, people who live
in volcanic areas are reluctant to leave because of rich harvest they reap
from the soil made of weathered volcanic materials. At present there are
46 recorded volcanoes in the Philippines, 13 are active and the rest are
dormant. The active volcanoes are:
Mayon in Albay
Canlaon in Negros Oriental
Musuan in Valencia
Didicas in Babuyan Island
Pinatubo in Zambales
Taal in Batangas
Bulusan in Sorsogon
Hibok-hibok in Camiguin
Banahaw in Laguna
Smith in Babuyan Island
Sto.Tomas in Benguet
Makaturing in Lanao del Sur
Babuyan Claro in Cagayan
These volcanoes are neither wholly of the “explosive” type nor
wholly of the “quite” type but have eruptions in which both lava flows and
explosions figure dominantly.
Mt. Canlaon
Hibok-Hibok
Pinatubo
Taal
Why Volcanoes Erupt – Volcanoes act as “safety
valves” in the earth’s crust are pockets of magma.
When its pressure builds up to a certain point that can
force upon the rocks that lie over them; the volcano
erupts. Eruptions continues until the pressure becomes
weak and the volcanoes become quite. As long as
there is enough heat and pressure volcanoes erupt. As
previously mentioned; geologists believe that great
internal pressure cause heat. Recently, radioactivity
was discovered to produce heat when radioactive
elements disintegrate and transmulate into other
substances. It is said that radioactive changes that take
place within the earth can produce enough as long as
there is enough pressure inside the magma pocket to
force out volcanic.
Types of natural earthquake are classified as tectonic,
volcanic, and plutonic, according to the origin of
disturbance:
earthquakes – are due to sudden
displacement
of the earth’s crust along a line weakness or fault.
► Plutonic or deep- seated – are those that have the
same
cause as tectonic or normal with the foci reaching as
deep
as 500km. Most Philippine earthquakes are tectonic in
origin.
► Volcanic earthquakes occur when volcanoes become
active. Plutonic earthquake are better recognized by
seismograph.
► Tectonic
Earthquakes intensity and magnitude.
Earthquake are described by the destruction caused and energy
released. Scales have been made to describe the effects observed.
They include sound, visible and felt vibration, type and severity of
damage. Different scales are used in different places such as:
For instance, in the United States, the Modified Mercalli Scale
is used. In Europe, the Rossi – Forel from which the modified Mercalli
scale was developed, is used. In Japan, the Japan Meteorological
Scale is used.
Earthquake intensity mainly on the distance between the
observer and the epicenter or place of the earth’s surface directly
above the focus, the point where damage is maximum.
Charles Richer developed a scale for determining the
magnitude of an earthquake. A magnitude is the amount of energy
released by an earthquake and is determined by instrumental
measurements, unlike intensity, which can be made by subjected
observations.
Earthquake magnitudes may be a little over zero but not more
than 8. If the magnitude is 8, it is 108 times as strong as the smallest
earthquake, in terms of energy released.
The following is the Rossi – Forel Scale of Earthquake Intensities (Adapted to
the Philippines by Rev.W.C. Reptti)
► Hardly perceptible Shock: felt only by an experienced observer under the
favorable condition.
► Extremely Feeble Shock: Felt by small number of persons at rest
► Very Feeble Shock: Felt by small number of persons at rest. Duration and
direction maybe perceptible. Sometimes dizziness or nausea experience.
► Feeble Shock Felt generally indoors, outdoors by few. Hanging objects swing
slightly. Creaking of frames of houses.
► Shock of moderate intensity: Felt generally by everyone. Hanging objects
swing freely. Overturn of all vases and unstable objects light sleepers awaken.
► Fairly Strong Shock – General awakening of those asleep. Some frightened
persons leave their houses. Stopping of pendulum clocks. Oscillation of hanging
lamps. Slight damage in very old or poorly built structure.
► Strong Shock – over turn of movable objects. General alarm, considerable in
old or poorly built structures, old walls, etc. Some landslides from hills and steep
bank. Cracks in roads surfaces.
► Very Strong Shock – People panicly. Tress shaken strongly. Changes in the
flow of springs and wells. Sand and mud objected from fissures in soft ground.
Small landslides.
► Extremely Strong Shock – General panic. Partial or total destruction of some
buildings. Fissures in ground. Landslide and rocks falls.
Destruction brought about by earthquake: Tsunami
and Fires
Tsunamis or seismic waves are waves caused by
earthquakes. They are popularly called tidal waves
although they are not related to tides. They are brought
about by the up an down of down movements of rocks
or by sliding mud under the sea or by huge volcanic
eruptions materials to the surface. Dropping even the
biggest bombs can only change the direction of flow of
lava but cannot affect the eruption.
How Volcanoes are Distributed – Volcanoes are found in
definite belts on the earth’s surface. As they are related to
earthquakes, volcanic belts are also earthquake belts, and
belts of young, growing mountains. It is believed that
volcanic belts shows margins of very large, slowly moving
earthquake slabs of the lithosphere. The volcanic belts are
the following:
► The
Circum Pacific Belt which is the major volcanic belt
that border the sides of the Pacific Ocean. Its major
volcanoes are in the Philippines, Japan, Aleutian Islands,
Alaska, South American Andes, Hawaiian Islands and New
Zealand.
► The Meditteranean belt which parallel to and boarders the
Mediterranean Sea. It includes the Alps. Asia Minor, and the
Himalayas eastward to the Indonesian states.
► The third volcanic belt which is submerged along the
system of the mind – ocean ridges where many small
volcanoes are arranged in line with crustal fractures.
Life History of Volcanoes
Like mountains and rivers, volcanoes pass through a
life history in which the volcano is tall straight. Erosion does
not have a chance yet to do its work. All active volcanoes and
extinct volcanoes which have not been affected yet by
erosion are young. Examples are Mayon and Mariveles. In
Maturity, the volcano, now extinct, has gullies and valleys on
its slopes due to erosions. Examples are Taal and Makiling in
the Philippines and Mount Hood in the US. In old age, all that
remains of the volcano is its core of more resistant rock,
called the volcanic neck. Old aged volcanoes have necks like
the Devil’s Towe in Wyoming and Mount Royal in Montreal
Canada.
DEVIL’S TOWE IN WYOMING
Earthquakes
The shaking of the ground is called the earthquake. It may be
too weak to be noticed or too strong to be destructive. Nobody feels
the weak daily earthquakes. Relatively stronger ones are noticed by
experienced observers and by recording instruments called
seismographs. The most destructive ones destroy property and do all
the things mentioned above. Why the earthquakes is not yet fully
known, the seismologists suggests the “elastic rebound theory” to
explain it. The theory says that the slowly builds up stress. When this
goes beyond the breaking point of the rocks involved, they yield or
break and everything around shakes. The great amount of energy
released during an earthquake causes a series of tremors composed
of one major shock followed by lesser ones called aftershock s.
Sometimes foreshocks precede the big tremor. This means a slow
build up strain energy in the earth before the earthquake. An
earthquake may last only a few seconds or maybe as long as 75
seconds, as in San Francisco, California earthquake which reached
its greatest intensity in 40 seconds, stopped for 5 to 10 seconds and
went on stronger than the previous for about 25 seconds.
Seismologist suggest that minor earthquakes maybe
caused by volcanic eruptions and underground
shifting of magma and that major ones are caused
by the passage of a group of elastic waves
produced by a quick movement of parts of the
earth’s crusts and mantle along the fault surfaces.
Classification of earthquakes as to depth:
► Shallow earthquake – originates from the crust and
has depth of 0 -33 km.
► Intermediate earthquake – originates from the
upper mantle and has a depth of 33 -300 km.
► Deep earthquake – originates from the lower mantle
and has a depth of 300 – 700 km.
Big or small earthquakes occur in the crust, along fractures. Occasional
earthquakes originate from the mantle. Where earthquakes happen,
although they can occur anywhere, anytime, they are most common in
belts. They swarm around regions where mountain building and crustal
movement are active. Earthquakes and volcanoes occur in belts. The
circum –pacific belt is composed of young mountains and chains of
volcanic mountains. It extends from Chile along the Western borders of
North and South America, Alaska, Japan, Philippines, Indonesia, New
Zealand and some other Pacific Islands.
The other earthquake belt, the Mediterranean and trans-Atlantic
belt, includes the Carribean area through the Himalayas, the Alps, Spain
Italy, Greece and Norhtern India. About 80% of earthquakes come from
circum –pacific belt, 15% from the Mediterranean and Trans – Atlantic
and 5% from other parts of the earth. In the Philippines, which is in the
circum-Pacific belt, earthquakes swarm in the eastern part, along the
western Pacific Ocean, where young mountains and volcanoes are
commonly found.