   
Much of our knowledge of Earth's insides
comes from monitoring 
the thousands of earthquakes that occur every year. 
Five centuries ago the world had mostly accepted that the Earth
 was not only  a sphere, but was thought to be of uniform rock throughout.
Two hundred years later Sir Isaac Newton,
studying our planetary system,
calculated that the interior the earth
must be made of far-denser material 
than the surface rock.
Newton's estimate the overall density of
the Earth
remains essentially unchanged today. 
In the early 1900 scientists discovered they could use data from earthquakes
 as a method for looking deep beneath the surface.
By understanding the travel times of
seismic waves to worldwide stations
scientists were able to calculate where boundaries occurred
and what those boundaries represented.
They thus determined that the Earth has three layers based on chemical composition:
Crust. mantle, and core.
As an analogy of relative scale these layers can be compared to an egg
with the shell representing the outermost layer,
the white the mantle, and the yoke the core .
How did scientists figure out where these layers were?
They used the arrival times of seismic
waves to worldwide seismic stations.
Seismic waves leave the hypocenter of an
earthquake and travel in all directions.
If the Earth had no change with depth,
seismic waves would travel straight paths.
But the earth has composition, density,
and temperature changes that 
cause the seismic rays to reflect and refract along boundaries
as velocity in the mantle increases with depth.
Innovations in computer technology
in concert with a steady beat of
earthquakes, help scientists to continue
to refine our understanding of Earth's interior.
The basic layers of the earth are grouped by their chemical composition.
The crust is made of chiefly 8 major
elements shown by their relative abundance:
Oxygen, silicon, aluminum, iron, calcium, sodium, potassium, and magnesium
At this scale the crust is too thin to
show as more than a line.
The crust ranges from 5–10 kilometers thick in the dense basaltic oceanic crust,
and up to 75 kilometers in the less-dense granitic continental crust
This difference in density and thickness
of these two types of crust
is the reason why the earth has oceans
and continents.
The crust is often mistaken for the
tectonic plates.
However, the crust is just the top part
the tectonic plates.
We will return to the topic in a moment but
first back to the 3 layers.
Below the crust is the mantle, composed of
the same elements but in different proportion,
with increasing amounts of the heavier
elements in the rock.
The chemical composition of the 2,900-km-thick mantle
varies little from top to bottom, but there are distinct physical variations
due to temperature & pressure differences.
The uppermost mantle is relatively cool & brittle
and ranges from 50 to 120 kilometers thick.
Below this zone the upper mantle becomes
notably more plastic & malleable
due to the right combination of heat & pressure.
That ductile zone is known as the asthenosphere and
varies up to 400 kilometers deep
depending mainly on temperature.
The lower mantle is 55% of the planet by volume.
It is denser and hotter than the upper mantle.
At the center the Earth is the core, nearly twice as dense as the mantle 
because it's metallic iron alloy rather than rock.
Unlike the egg yolk analogy,
 Earth's core is made up of two distinct parts:
 the liquid outer core & a solid inner core.
Although the inner core is hotter than
the outer core, 
there is also greater pressure squeezing the atoms, 
changing the material from liquid to solid.
The liquid outer core is convecting vigorously & generates Earth's magnetic field.  
But back to plate tectonics.
As you recall, the cool uppermost part of the mantle is brittle.
How can the top the mantle be brittle when 
the same material in the asthenosphere is ductile? 
A Big hunk candy bar can be used as an analogy.
Like the uppermost cool mantle, when the 
Big Hunk is cold, it is brittle & breaks when bent. 
When you heat it up it becomes ductile, or plastic, and can bend & flow. 
Earlier we mention that the crust is
merely the top of the tectonic plate.
This uppermost brittle mantle behaves much like the overlying crust.
Together they form a rigid layer rock called the lithosphere that moves in unison
The lithosphere ranges from as much as a
100 kilometers thick in the oceanic plate
to 200 kilometers thick in the continental plates.
It is in this brittle zone that earthquakes occur, 
due to compression, extension, & shearing.
Over billions of years the cooled surface
of Earth has been broken up into the
moving planes that are called lithospheric plates (commonly "tectonic plates")
Because they are mostly more buoyant than the asthenosphere, they float above it. 
Convection currents driven by temperature, pressure,  and gravity
provide the mechanism for the process we know as plate tectonics. 
Earthquakes, volcanoes, & the Earth’s magnetic field 
are all the consequence of the Earth trying to lose heat as it converts
some of the thermal energy into mechanical energy in the process.  
Without the tremendous heat being released from the interior of the earth...
we would not have the mechanism to drive plate tectonics.
Without the  earthquakes we may not have had a way to see so deep into the earth.
