In which way does nicotine cause addiction?

Nicotine addiction causes changes in the brain that lead to further addiction and the desire to smoke.  Indirectly, nicotine causes the release of dopamine in the pleasure and motivation areas of the brain.

Hope this helps! :D

the plates spread at mid-ocean ridges (they are divergent plate boundaries) which allows magma to rise through the weakness between the plates. the magma then emerges as lava from the boundary and cools, creating a new crust.