Continental Drift and Plate Tectonics - Background

What's inside the Earth?  How do we know the composition of its inner core?  What does the Earth's interior have to do with volcanic explosions (like that of Mount St. Helen) and earthquakes (like the famous 1906 quake which struck San Francisco)? 

This clip, from a program which explores those questions and more, gives us a bit of background.

Credits

Clip, online courtesy Phoenix Learning Group Channel at YouTube.