Green dragons are also sometimes called "earth" dragons. They usually live deep within caves, and have power over the earth. They are capable of launching boulders at high speeds, and can cause earthquakes by slamming down onto the ground.
When an earthquake is caused in Ashkheir's cave, the dragons know that Gryfrot is frustrated. Whenever the earth dragon can't complete what he wants to finish, he screams in anger and causes quakes from his roars. His roars cause more terrible earthquakes than he does when he slams himself into the ground. It takes Ashkheir and two of her powerful dragons to calm Gryfrot's anger. The roars of Gryfrot are ear splitting, but the dragon would calm down after he sees all the trouble he caused with his frustration. He'd become quiet for days, and then strike up on a new, hard project he'll think up of when he becomes bored.