The composition of the earth's interior can be determined as well as distinguishing liquid from solid layers by analyzing the propogation of seismic waves.
The size of the liquid core is measured by seeing where the S-waves disappear, while the nature of the core (liquid or solid) can be studied by seeing how the P-waves are refracted as they enter and pass through it. The inner, solid core is deduced from refraction of P-waves due to their much higher velocity in the solid than in the liquid. The amount of refraction depends on the density of the material.
The earth's crust is actually a two-component layer. The lithosphere is a thin layer of rock (average density of 2.7 grams per cc) and "floats" on top of a plastic-like layer called the asthenosphere. Plastic-like materials are weird - they deform under stress but don't really break. A glacier is a good example of a material that moves and flows plastically. The convective heat currents in the mantle impinge on the asthenosphere causing deformation and subsequent movement of the lithospheric plates.
This process can be simulated in your kitchen by putting some jello in a bowl and putting some peebles on top of the jello. As you shake the bottom of the bowl, the jello deforms but doesn't break and the rocks that float on the jello collide. (apologies to real geologists for this analogy).
The resulting pulling apart or crustal separation results in large scale surface features like the Mid-Atlantic Ridge:
In cross section, the atlantic ocean looks as shown below. Note that broadly similar features are seen on the surface of Venus.
The current structure of the earth, as mapped by seismic waves is fairly complex as shown below. Note that the D" layer, recently discovered, seems to represent some interface between the outer molten core and the lower mantle. It has been speculated that the intense pressures here have produced a new kind of crystalline rock that has unusual properties. This helps to explain why the D" layer doesn't occur at constant depth since the pressure won't be a constant function of depth.
As a result of plate movements, interesting things occur at plate boundaries. In general you don't want to live near a plate boundary as the earth is active there. About 75% of the world's population does live near these boundaries.
There are three types of plate boundaries:
crustal separation two plates
are moving apart in opposite directions
collision of two plates. A collision
of a less dense continental plate with a more dense oceanic plate
creates a subduction zone where the denser plate dives (subducts) beneath
the less dense plate. A collision between two continental plates
results in general uplift. ( animation of Pacific Northwest )
here two plates slide by one another
in opposite directions. The San Andreas Fault is the most well-known
(and potentially most deadly) translational interface. ( animation of southern california )
Local Manifestations of Plate Tectonics:
Find The Fault Lines
The Pacific Northwest is an active tectonic zone. If your interested in monitoring this on a daily basis then bookmark this page
One of the prime hazards of active volcanoes is the heavy mudflows which can result from the sudden melting of their heavily glaciated slopes. Some examples of Northwest Hazards are shown below:
