HELP with a Geological Crossword .

5 letters -
E - - - e
(hint) To wear down the surface of the earth
6 letters -
- - - - a -
hint ) Northern or of the Arctic
8 letters
- e - - o - - -
hint ) To do with the structure of the earths crust
6 letters
- u - - e -
hint ) the usual shape of a tornado : a tube of winds spiralling at high speeds .

Thanks just do the best you can to help !!! THANKS

Erode
Polar (but it doesn;t fit, so its not right)
Tectonic
Funnel

The other answerer is quite right on most, but the one for "Northern or of the Arctic" should be "boreal".