hundred years ago the famous scientist Isaac Newton calculated,
from his studies of planets and the force of gravity, that the
average density of the Earth is twice that of surface rocks
and therefore that the Earth's interior must be composed of
much denser material.
now know that the earth is made up of 4 layers:
--The thin, outermost layer of the earth is called the
crust. It makes up only one percent of the earth's mass.
This consists of the continents and ocean basins. The
crust has varying thickness, ranging between 35-70 km
thick in the continents and 5-10 km thick in the ocean
basins. Within the crust, intricate patterns are created
when rocks are redistributed and deposited in layers through
the geologic processes. The crust is composed mainly of
-- The mantle is a dense, hot layer of semi-solid rock
approximately 2,900 km thick and is composed mainly of
ferro-magnesium silicates. This is where most of the internal
heat of the Earth is located. .Large convective cells
in the mantle circulate heat and may drive plate tectonic
- Below the mantle is the core. It makes up nearly one
third the mass of the earth. The Earth's core is actually
made up of two distinct parts: a 2,200 km-thick liquid
outer core and a 1,250 km-thick solid inner core. The
outer core is made of iron and is very dense. As the Earth
rotates, the liquid outer core spins, creating the Earth's
magnetic field. The inner core is made of solid iron and
nickel. Many scientists believe it is kept in the solid
state because of the extreme pressure from the other layers.
we know about the different layers of the earth?
today comes from studies of the paths and characteristics of
seismic waves from earthquake waves traveling through the Earth,
as well as from laboratory experiments on surface minerals and
rocks at high pressure and temperature and studies of the Earth's
motions in the Solar System, its gravity and magnetic fields,
and the flow of heat from inside the Earth.
and strength of seismic waves gives us a picture of the
interior of the earth.
waves gradually bend and change speed as the density of
are the different types of Seismic Waves?
waves are waves of energy that travel through the earth, for
example as a result of an earthquake, explosion, or some other
process that imparts low-frequency acoustic energy.
wave studies have allowed scientists to construct a model of
the earth's interior.
are two types of seismic waves, body wave and surface waves.
WAVES --- S AND P WAVES
fastest wave, and therefore the first to arrive at a given
location, is called the P wave. The P wave, or compressional
wave, alternately compresses and expands material in the
same direction it is traveling. Think
of a slinky being compressed.
waves propagate through the Earth with a speed of about
15,000 miles per hour and are the first waves to cause
vibration of a building.
S wave is slower than the P wave and arrives next, shaking
the ground up and down and back and forth perpendicular
to the direction it is traveling.
waves arrive next and cause a structure to vibrate from
side to side. They are the most damaging waves, because
buildings are more easily damaged from horizontal motion
than from vertical motion.
waves follow the P and S waves. A surface wave is a seismic
seismic wave that is trapped near the surface of the earth.
waves are analogous to water waves and travel along the
Earth's surface. They travel slower than body waves. Because
of their low frequency, long duration, and large amplitude,
they can be the most destructive type of seismic wave.
There are two types of surface waves: Rayleigh waves and
waves -- sometimes called long waves, or simply L waves
-- are responsible for most of the damage associated with
earthquakes, because they cause the most intense vibrations.
Surface waves stem from body waves that reach the surface.
that reaches the surface of the earth generates waves
that travel outwar from the epicenter. These waves are
call surface waves and move by giving particles an elliptical
motion as well as a bak and froth swaying motion. These
are the waves that cause most of the destruction from
waves, also called ground roll, are surface waves that
travel as ripples with motions that are similar to those
of waves on the surface of water
waves are surface waves that cause circular shearing of