David also has an SGI and time to play, and has made some cool closeups . Here are some basic Earth facts which you will need to know for this class. Earth is differentiated. Differentiation happened when heat from impacts and radioactivity caused part of Earth to melt, probably during or shortly after Earth's formation. Dense material sank to center, light material floated to surface.
Structure of Earth is inferred from many sources, including seismology and gravity.