Module 1&2 Flashcards
(76 cards)
study of the
generation, propagation and measurement of seismic waves through earth and the sources that
generate them.
Seismology
The word seismology originated from Greek words,
‘seismos’ meaning
earthquake and ‘logos’ meaning science.
earth’s shape is an oblate
spheroid with a diameter along the equator of
about
12740 km
polar diameter as
12700km.
The higher diameter along equator
is caused by the
higher centrifugal forces
generated along the equator due to rotation
of earth
pecific gravity of
materials that constitute the surface of earth
is only about
2.8,
e average specific gravity
of earth is about
5.5
very heavy materials towards interior of earth.
The interior of the earth can be classified into
three major categories
Crust, Mantle and
Core
or the lithosphere, is the outer part of the earth is where the life exist. The
average thickness of crust beneath continents is about 40km where as it decreases to as much
as 5km beneath oceans. The oceanic crust is constituted by basaltic rocks and continental part
by granitic rocks overlying the basaltic rocks. Compared to the layers below, this layer has high
rigidity and anisotrop
Crust
e is a 2900 km thick layer. The mantle consists of 1) Upper Mantle reaching a
depth of about 400 km made of olivine and pyroxene and 2) Lower Mantle made of more
homogeneous mass of magnesium and iron oxide and quartz. No earthquakes are recorded in
the lower mantle. The specific gravity of mantle is about 5. The mantle has an average
temperature of about 2200degree Celsius and the material is in a viscous semi molten state.
The mantle act like fluid in response to slowly acting stresses and creeps under slow loads. But
it behaves like as solid in presence of rapidly acting stresses, e.g. that caused by earthquake
waves.
Mantle
has a radius of 3470 km and consists of an inner core of radius 1370 km and an
outer core (1370 km < R < 3470 km). The core is composed of molten iron, probably mixed
with small quantities of other elements such as nickel and sulfur or silicon. The inner solid core
is very dense nickel-iron material and is subjected to very high pressures. The maximum
temperature in the core is estimated to be about 3000 degree Celsius. The specific gravity of
outer core is about 9-12 where as that of inner core is 15.
Core
proposed the hypothesis that the continents
had once formed a single landmass before breaking apart and drifting to their present locations.
German scientist Alfred Wegener, in 1915
He proposed that a large
continent termed Pangae existed in earth around
200 million years ago
The theory of plate tectonics, presented in early
y 1960s
lithosphere is
broken into seven large (and several smaller) segments called
plates
The upper rigid layer,
lithosphere
, is about 100 km
thick below the continents, and about 50 km under the oceans, and consists of Crust and rigid
upper-mantle rocks.
lithosphere
The lower layer
asthenosphere
extends down to about 700 km
depth.
asthenosphere
are areas along the edges of plates move
apart from each other
Spreading ridges or divergent boundaries
are located beneath the
oceans
spreading ridges
are formed where the two plates move toward each other.
In this process, one plate could slip below the other one or both could collide with each other.
convergent boundaries
These boundaries are created when either oceanic lithosphere subducts beneath oceanic
lithosphere (ocean-ocean convergence), or when oceanic lithosphere subducts beneath
continental lithosphere (ocean-continent convergence)
Subduction boundaries
The junction where the two
plates meet, a trench known as
oceanic trench
