Mid Winter: Magma Search Unsuccessful

I dug all the way down to the core (160 floors below the surface) and didn't hit any magma on my way. I did hit two different underground caverns on the way. It took me a while to figure out how to seal up the hole created when you breach a cavern. What I do is build an upward stair next to the breach with a matching downward stair on the floor above. Remove the up stair with the designate tool, then construct a floor over the downward stairs. This blocks off the breach preventing any flying creatures from entering your fortress from below.

The bottom floor has "semi-molten rock" that I had never seen before. It's different from warm stone in that there's no guarantee that the tile borders magma, and you can't dig through it. I'll be doing some exploratory digging on the level above to see if I can find a magma pool. To tide over until I get magma (I've never actually built a magma forge yet,) I built a wood furnace to make charcoal, two smelters, and a metalsmith's forge. I've got an abundance of wood so I should be able to equip my first squad with some iron weapons and armor.

It most definitely doesn't show here. Middle of winter and the plants and the foliage looks like it's still autumn.