Yes. It is the midpoint because the line would be GHJ and H is halfway between the 14 difference.
You probably went to far south
A thermocline is a transition layer between warmer water at the surface of a body of water and the cooler deep water that lies below. It is caused by the effect of the sun on the water, in that it keeps the upper parts of the body of water warm whilst being unable to reach the lower, deeper parts.
The volcanic intrusion rule is relative dating
