What causes a breaking wave as it approaches shore?

What causes a breaking wave as it approaches shore?

• A. The bottom slows the wave while the top keeps moving, causing instability.
• B. The water temperature drops.
• C. The wind increases.
• D. The wave crest grows taller due to depth.

Answer: (A)

As a wave moves into shallower water, the seafloor slows the bottom part of the wave while the crest above keeps moving at a similar speed. This differential slowing makes the wave rise up and become steeper. When the height and steepness get too large, the water can no longer support the crest, and it spills forward or crashes—this is breaking. So the key idea is shoaling: depth-related slowing at the bottom causes instability in the wave, producing the break. Temperature changes or wind shifts aren’t the direct cause of breaking as shoreward depth decreases, and while the crest can grow taller as depth decreases, it’s the bottom friction and resulting steepening that lead to the break.

As a wave moves into shallower water, the seafloor slows the bottom part of the wave while the crest above keeps moving at a similar speed. This differential slowing makes the wave rise up and become steeper. When the height and steepness get too large, the water can no longer support the crest, and it spills forward or crashes—this is breaking. So the key idea is shoaling: depth-related slowing at the bottom causes instability in the wave, producing the break. Temperature changes or wind shifts aren’t the direct cause of breaking as shoreward depth decreases, and while the crest can grow taller as depth decreases, it’s the bottom friction and resulting steepening that lead to the break.