Originally Posted by wmtire
Geocaching has a type of cache called "Earth Caches" as you described. It's just one of the many types of things the game offers.
There are several types of geocaches:
Multi (has more than one "stage" to it)
Earthcache - learn about geology
Mystery - puzzles and challenges
Virtual - you'll identify something but there is no log to sign
Event - place to meet geocachers in your area
These are the more popular. There are also webcam caches, letterbox caches, Mega events (more than 500 attendees), and more.
I like the earthcaches and hold an Earthcache Master Bronze rank. It's fun to learn about the geology of the Earth. Makes you look at an area in a different way.