Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: While hiking last year, Mike saw a large boulder next to a mountain trail. The boulder had no cracks. While hiking on the trail this year, he saw two large cracks in the boulder. Which of the following most likely caused these cracks to form?
Natural causes, such as weathering or earthquakes.