Answer: weathering due to freezing and thawing
Explanation:
The reason that caused these cracks to form is weathering due to freezing and thawing.
Weathering simply means the breaking down of rocks. In this situation, the boulder is freezed and thawed and due to this, as time progresses, it'll gradually be broken down. This is the reason why Mike saw the cracks even though there were no cracks the last time he went hiking.