Plant roots thet grow in the cracks of rocks can cause physical

a deep steep sided vally formed by weathering and erosionis called a

Plant roots that grow in cracks can cause weathering.
Valleys formed by weathering or erosion can be canyons or ravines.

It is the doctrine that the us had the right and duty to expand throughout north america. it just means they believed they had the right to expand westward and they did! they stated with the 13 colonies and then they kept moving gaining different territories throughout the years