Here's why:
* Prior to Hess: While Alfred Wegener proposed the Continental Drift theory in the early 20th century, he lacked a solid explanation for how continents could move. His evidence (matching coastlines, fossil distributions, etc.) was intriguing, but not enough to convince the scientific community.
* Hess's Contribution: Hess proposed that new oceanic crust is generated at mid-ocean ridges and then spreads outwards, carrying the continents along. This explained how continents could drift apart and also provided a mechanism for the formation of new ocean floor.
* Evidence for Sea-Floor Spreading: Hess's theory was supported by a variety of evidence, including:
* Magnetic striping on the ocean floor: The ocean floor exhibits alternating bands of normal and reversed magnetic polarity, which perfectly mirrored the known magnetic reversals in Earth's history. This indicated that the ocean floor was spreading and creating new crust at mid-ocean ridges.
* Age of ocean floor: The oldest ocean floor is found farthest from the mid-ocean ridges, with younger floor closer to the ridges. This pattern aligns with the idea of sea-floor spreading.
* Subduction zones: The discovery of subduction zones (where ocean floor dives back into the mantle) provided a mechanism for how the old ocean floor is recycled.
The combination of sea-floor spreading and the evidence that supported it finally convinced the scientific community that continents do indeed move, leading to the acceptance of the theory of plate tectonics.
