The Rock Cycle
The Rock Cycle above is a simplified model of processes that rocks go through to
change from one to another.  To become an igneous rock, a sedimentary rock must
first melt, then cool.  To become sedimentary rocks, metamorphic or igneous rocks
must undergo weathering.  There are a few more steps and you can find those on the
"click here" link.
For an even larger selection, go to Google.com, type in rock cycle, then click on images.  
There are many from simple to mind-boggling.
A Middle School Level Website
Web-Science.net