Problem with heat transfert in lava

Hello everyone,

I posted a topic here because I encounter a problem with Comsol. Indeed, I try to modelize a lava flow, but I have a problem to fix the temperature of middle flow as you can see on this picture below (:
www.pixenli.com/images/1370/1370356942039372700.png (:

I would like to fix a line of same temperature in the aim to get two gradients : between the floor and the middle of the flow and between the surface and middle of the flow (:

Thank you in advance (:

Alexis, Jordan.

P.S : Sorry my bad english, I'm french >.<