Which phenomenon provided the key basis for developing plate tectonics?

• A. Seafloor spreading
• B. Ocean currents
• C. Glacial movement
• D. Volcanic eruptions

Answer: (A)

Seafloor spreading provides the mechanism by which new oceanic crust forms at mid-ocean ridges and pushes the plates apart. This idea ties together several crucial observations: the youngest ocean floor sits near the ridges and gets progressively older away from them; rocks on opposite sides of a ridge show matching, symmetric patterns of magnetic reversals preserved as paleomagnetism; and deep trenches mark where old crust is returned to the mantle by subduction. Together, these lines of evidence form a coherent process that explains how continents move and why earthquakes and volcanoes cluster at plate boundaries. Other options describe related phenomena but don’t by themselves supply the global mechanism for moving the lithosphere. Ocean currents are surface flows, glacial movement relates to climate episodes, and volcanic eruptions reflect magmatic activity without explaining the broad, coordinated motion of Earth's outer shell.