Title: Engineering Geology and Seismology
1(No Transcript)
2 Lecture 2
Engineering Geology and Seismology Origin and
Inferiors of the Earth Instructor Dr.
Attaullah Shah
Department of Civil Engineering Swedish College
of Engineering and Technology-Wah Cantt.
3Geology literally means "study of the
Earth. Physical geology examines the materials
and processes of the Earth. Historical geology
examines the origin and evolution of our planet
through time. Engineering geology is the
application of geological data, techniques and
principles to the study of rock and soil
surfacing materials, and ground water.
Seismology is study of the generation,
propagation and recording of the elastic waves
and the source that produce them.
4Importance of engg geology in Civil Engineering
practice
- What is Engineering Geology?
- Engineering geology is the application of
geological data, techniques and principles to the
study of rock and soil surficial materials and
ground water. - This is essential for the proper location,
planning, design, construction, operation and
maintenance of engineering structure.
5Importance of engg geology in Civil Engineering
practice
- What does Engineering Geology study?
- Rock, soil, water and the interaction among these
constituents, as well as with engineering
materials and structures.
5
6Importance of engg geology in Civil Engineering
practice
- Why Engineering geology?
- Serve civil engineering to provide information in
3 most important areas - Resources for construction aggregates, fills and
borrows. - Finding stable foundations
- Mitigation of geological hazards Identify
proplems, evaluate the costs, provide information
to mitigate the problem
6
7Origin of Earth
- Various Theory
- Nebular Hypothesis
- Planetesimal Hypothesis
- Gaseous Tidal Hypothesis
- Binary Star Hypothesis
- Gas Dust Clout Hypothesis
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8Nabular Hypothesis
- German philosopher, Kant and French
mathematician, Laplace - Earth, planets and sun originated from Nebula.
- Nebula was large cloud of gas and dust. It
rotates slowly. - Gradually it cooled and contracted and its speed
increased. - A gaseous ring was separated from nebula
- Later the ring cooled and took form of a planet
- On repetition of the process all other planets
came into being - The central region, nebula became sun.
8
9- Objections to Nabular Hypothesis
- Sun should have the greatest angular momentum
because of its mass and situated in the center,
however, it has only two percent of momentum of
the solar system - How the hot gaseous material condensed in to rings
10Planetesimal Hypothesis
- Chamberlin and Moulton proposed the theory in
1904 - The sun existed before the formation of planets
- A star came close to the sun.
- Because of the gravitation pull of the star,
small gaseous bodies were separated from the sun - These bodies on cooing became small planet's
- During rotation the small planets collided and
form planets
10
11- Objections to Planetesimal Hypothesis
- The angular momentum could not be produced by the
passing star. - The theory failed to explain how the
planetesimals had become one planet
12Gaseous Tidal Theory
- Jeans and Jeffrey proposed the theory in 1925
- Large star came near the sun. Due to
gravitational pull a gaseous tide was raised on
the surface of the sun. - As the star came nearer, the tide increased in
size. - Gaseous tide detached when star move away.
- The shape of the tide was like spindle.
- It broke into pieces-forming nine planets of the
solar system.
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13Interior of earth
- Crust
- Continental crust (25-40 km?)
- Oceanic crust (6 km)
- Mantle
- Upper mantle (650 km)
- Lower mantle (2235 km)
- Core
- Outer core liquid (2270 km)
- Inner core solid (1216 km)
? Values in brackets represent the approximate
thickness of each layer
14Layers of the Earth
- The earth is divided into three main layers
Inner core, outer core, mantle and crust. - The core is composed mostly of iron (Fe) and is
so hot that the outer core is molten, with about
10 sulphur (S). The inner core is under such
extreme pressure that it remains solid. - Most of the Earth's mass is in the mantle, which
is composed of iron (Fe), magnesium (Mg),
aluminum (Al), silicon (Si), and oxygen (O)
silicate compounds. At over 1000 degrees C, the
mantle is solid but can deform slowly in a
plastic manner.
15THE CRUST
- The crust is much thinner than any of the other
layers, and is composed of the least dense
calcium (Ca) and sodium (Na) aluminum-silicate
minerals. Being relatively cold, the crust is
rocky and brittle, so it can fracture in
earthquakes. - The shell of the earth, the crust, can be said to
have two different thicknesses. - Under the oceans, it is relatively thin. It
varies in thickness from 5 to 8 km. Under the
land masses, it is relatively thick. The
thickness of the continental crust varies from 10
to 65 km.
16THE CRUST
- The eggshell analogy for the crust is not an
exaggeration. It is paper thin compared with the
radius of the earth which is approximately 6400
km. - The total weight of the continental crust is less
than 0.3 of the weight of the earth. - Variations in the crust thickness are compensated
by the weight of the water and the differences in
the specific gravities of the crust under the
oceans (3.0 to 3.1) and under the continents(2.7
to 2.8).
17THE CRUST
- If one thinks of the crust as virtually
floating on the mantle, one is less likely to
wonder why the earth does not wobble as it
rotates about its axis. - The weight of the crust plus the mantle has a
reasonably uniform distribution over the globe.
18THE MOHO
- The Moho, or the Mohorovicic Discontinuity,
refers to a zone or a thin shell below the crust
of the earth that varies in thickness from 1 to 3
km.
19THE MOHO
- In seismology, the term "discontinuity" is used
in its general sense. It refers to a change over
a short distance of a material property. In this
case, the "short distance" may be as long as 3
km, a trifle compared with the radius of the
earth. - In that zone, the P-wave velocity has been
observed to increase from approximately 6 to
approximately 8 km/sec. - The Moho is considered to be the boundary between
the crust and the mantle. - The increase in P-wave velocity is ascribed to
change in composition of the medium. Rocks of the
mantle are poorer in silicon but richer in iron
and magnesium
20 THE MANTLE
The mantle can be thought of having three
different layers. The separation is made because
of different deformational properties in the
mantle inferred from seismic wave
measurements. (1) The upper layer is stiff. It is
presumed that if the entire mantle had been as
stiff, the outer shell of the earth would stay
put. This stiff layer of the mantle and the
overlying crust are referred to as the
lithosphere. The lithosphere is approximately
80-km thick
21 THE MANTLE
- (2) Beneath the lithosphere is a soft layer of
mantle called the asthenosphere. - Its thickness is inferred to be several times
that of the lithosphere. - One may think of this as a film of lubricant
although film is not exactly the word for
something so thick. It is assumed that the
lithosphere, protruding (meaning extending
beyond) parts and all, can glide over the
asthenosphere with little distortion of the
lithosphere
22 THE MANTLE
- (3) The mesosphere is the lowest layer of the
mantle. - Considering the vagueness in defining the lower
boundary of the asthenosphere it would be
expected that the thickness and material
properties of the mesosphere are not well known. - It is expected to have a stiffness somewhere
between those of the lithosphere and the
asthenosphere.
23THE CORE
- At a depth of approximately 2900 km, there is a
large reduction (on the order of 40) in the
measured velocity of seismic waves. The boundary
between the mantle and the core is assumed to be
at this depth.
- Because no S-wave has been observed to travel
through the material below this boundary for a
thickness of approximately 2300 km, it has been
inferred that the core comprises two layers. - The 2300-km thick outer layer which is in a
molten state and an 1100-km thick inner layer
which is solid.
24THE CORE
It is known that the pressure increases toward
the center of the earth. So does the temperature.
The liquid outer layer versus the solid inner
layer is rationalized by recognizing that the
melting point of the material increases (with
pressure) at a faster rate than the temperature
as the center of the earth is approached.