Final answer:
The dark lines on the map appearing jagged are caused by transform plate boundaries. Transform boundaries occur when two plates slide past each other horizontally.
Step-by-step explanation:
The dark lines on the map appearing jagged are caused by transform plate boundaries. Transform boundaries occur when two plates slide past each other horizontally. This movement can result in friction and the formation of faults, causing the earth's crust to crack and create the jagged lines observed on the map.