rift
This is called a graben. It may ultimately form a rift valley.
Some asteroids split into peices and breaking apart as they enter the atmosphere
Putting your legs apart from each one
pull your legs apart and sit down
When a spreading boundary develops on land, two slabs of Earths' crust slide apart. :D :ID (c:
metaphase
It is called a fissure
The rocks cracks on everything
they are spread farther apart, causing the air to get thinner, making it harder to breathe. ;P
rift valley
You could say it had plane cleavages. What happens is that bonds between the molecular planes are much weaker than those in the plane of the cleavage. Graphite and mica would be good examples.