This appears to be a frost crack. Especially if it is on the south side of the tree. The sun warms the bark in winter in the day time and then the fluids just under the bark freeze when the temperature goes down at night. Freezing and expanding water causes the crack. Leave the tree alone for now, but monitor it for any insect damage in the future. The exposed area is heartwood and will keep the tree stable as long as it is not further damaged. The edges of the bark may show compartmentilization which is a sign of the tree sealing off the area from the living part of the bark. 
https://www.canr.msu.edu/news/frost_cracks_in_trees
https://homegarden.cahnr.uconn.edu/factsheets/frost-cracks-and-sunscald-on-trees/