Building up and wearing down
Why Does Land Have Shapes At All?
Every shape on a map — every hill, valley, cliff and plain — comes from two processes arguing with each other.
-
Step 1 of 5
Building comes from inside.
Heat deep in the Earth shifts the plates of the crust, which collide, buckle and split. Every mountain range and every volcano comes from that movement.
-
Step 2 of 5
Wearing down comes from outside.
Water, ice, wind and temperature attack anything standing up. They work slowly, they never stop, and they are powered by the sun rather than the Earth's interior.
-
Step 3 of 5
Whichever wins locally sets the shape.
Where uplift wins you get sharp high ground. Where wearing wins you get plains and rolling hills. Where they roughly balance you get steady, long-lived slopes.
-
Step 4 of 5
The rock underneath decides the detail.
The same rain makes a cliff out of hard rock and a gentle slope out of soft. Most of the detail in a landscape is the pattern of what is underneath, uncovered.
-
Step 5 of 5
Which is why shapes repeat everywhere.
Deltas, dunes, gorges and hills look the same on every continent, because the same few processes made all of them out of whatever was there.
The same shapes turn up on other planets. Mars has river valleys, dunes and vast volcanoes, made by the same processes with different ingredients.
The short version
Every landform is a contest between forces lifting the land and weather wearing it down, shaped by whatever rock is underneath.
Try it yourself
Look at any hillside and ask which is winning there. Bare rock and cliffs mean one answer; deep soil means the other.