How can volcanic activity create new landforms?

If a volcanic reaction takes place and the lava hardens, it creates rock. If it flows into the ocean, it can even create new land.

Volcanic activity in simpler terms means a volcano spewing lava, which in turn cools and will either raise the height of a piece of land (could be underwater) or make a whole new land form. The lava cools leaving what a lot of the world is made of. For B i'm not really sure, the ash from the volcano could pollute the air like in Pompeii. I'm guessing for B btw.