Final answer:
The changes in the boulder observed by Ryder can be attributed to weathering, specifically physical weathering like frost-wedging and chemical weathering from acid rain, as well as erosion from wind and water. The correct answer to why the boulder has gotten smaller and smoother is option b: Wind and water have weathered the boulder, wearing it down over time.
Step-by-step explanation:
Why Has the Boulder in Ryder's Favorite Park Changed Over Time?
The changes that Ryder has observed in the boulder at the park over the last ten years can be attributed to weathering, a natural process that breaks down rocks and other materials on the Earth's surface. There are two main types of weathering: physical and chemical. Physical weathering, or mechanical weathering, involves the breaking apart of rocks without changing their chemical composition. A common type of physical weathering is frost-wedging, where water seeps into cracks, freezes, and expands, causing the rock to break apart. Over 200 days a year at Bryce Canyon, for example, this process repeats daily, exerting tremendous force and eventually shattering rock formations.
Chemical weathering, on the other hand, involves changes to the rock's chemical composition. Rainwater, which is naturally acidic, can dissolve minerals in rocks like limestone, leading to smoother and smaller rocks over time. Furthermore, as seen in Bryce Canyon, this process involves washing away debris, which contributes to the reduction in size of the boulder.
Another cause of such changes is erosion. After weathering breaks down rock, erosion transports the sediment elsewhere. Wind and water are the primary agents of erosion. Consistent wind can carry fine particles away and water can flow over rocks, smoothing them down over time, exactly as Ryder observed with the park's boulder.
To answer Ryder's question, the most likely cause of the boulder's reduction in size and smoother surface areas is option b: Wind and water have weathered the boulder, wearing away at its surface. The other options do not account for the observable evidence of gradual reduction in size and smoothing that results from weathering and erosion processes.
Lastly, geological activities, such as asteroid impacts, while significant in shaping planetary surfaces, as suggested by the history of Earth compared to other celestial bodies, are less relevant to the changes in the size and shape of a boulder in a local park on Earth's surface.