Geography · Ch 10 — Distribution of Oceans and Continents
Concept of Sea Floor Spreading
Concept of Sea Floor Spreading
The diagram is a cutaway block of the Earth’s crust and upper mantle, drawn in three dimensions so you can see both the surface features and what lies beneath. It reads left to right as a journey across several different plate boundaries, each one showing a distinct way plates interact.
On the far left sits a convergent plate boundary where an oceanic plate dives beneath another plate. A strato volcano rises above the surface, fed by magma generated as the subducting slab descends. The subduction zone itself is marked by a trench at the surface, and the sinking plate is drawn plunging into the mantle below. Moving right, you encounter a transform plate boundary — here the plates slide past each other horizontally, with arrows showing the opposing directions of motion. There is no creation or destruction of crust at this boundary; the diagram simply shows the lateral shear.
The centrepiece is a divergent plate boundary with an oceanic spreading ridge. This is the heart of sea floor spreading: magma rises at the ridge crest, new oceanic crust forms, and the sea floor is pushed outward on both sides. A shield volcano sits near the ridge, its broad, gentle shape contrasting with the steep strato volcano on the left. The arrows here point away from the ridge, showing the crust moving apart. To the right of this, another convergent boundary appears — an oceanic plate subducts beneath continental crust, with a trench marking the surface expression and the subducting plate shown bending downward into the asthenosphere.
At the far right, the diagram shows a continental rift zone, labelled as a young plate boundary. Here the continental crust is being pulled apart, with volcanic activity accompanying the stretching. This is the early stage of what could eventually become a new ocean basin — the rift is where a divergent boundary begins on land.
Throughout the block, the diagram distinguishes the lithosphere (the rigid outer shell comprising crust and uppermost mantle) from the asthenosphere (the hotter, weaker layer beneath, on which the plates move). It also labels oceanic crust and continental crust separately, and includes a hotspot — a plume of rising mantle material that can create volcanoes independent of plate boundaries. Arrows throughout show the direction of plate motion, making the dynamic nature of the system explicit. …
The story of how the ocean floor actually forms begins where Wegener's continental drift left off. After the idea of drifting continents was proposed, later studies of the ocean floor and of the magnetic properties of rocks revealed a set of facts that demanded a new explanation. These post-drift investigations gave scientists information that was simply not available when Wegener first put forward his concept.
The mapping of the ocean floor and palaeomagnetic studies of oceanic rocks brought out several striking facts. First, volcanic eruptions are common all along the mid-oceanic ridges, and these eruptions bring huge amounts of lava to the surface in those areas. Second, rocks found at equal distances on either side of the crest of a mid-oceanic ridge show remarkable similarities in their period of formation, chemical composition, and magnetic properties. The rocks closest to the ridge have normal polarity and are the youngest, while the age of the rocks increases steadily as one moves away from the crest. Third, the rocks of the oceanic crust are much younger than continental rocks — nowhere on the ocean floor are rocks older than about 200 million years, whereas some continental rock formations are as old as 3,200 million years. Fourth, the sediments on the ocean floor are unexpectedly very thin. Scientists had expected that if the ocean floors were as old as the continents, there would be a complete and thick sequence of sediments accumulated over a very long period. Yet nowhere was the sediment column found to be older than 200 million years. Finally, the deep oceanic trenches are associated with deep-seated earthquake occurrences, while the earthquake foci in the mid-oceanic ridge areas are shallow.
These facts, together with a detailed analysis of the magnetic properties of rocks on either side of the mid-oceanic ridge, led Hess in 1961 to propose his hypothesis known as sea floor spreading. Hess argued that constant eruptions at the crest of oceanic ridges cause the rupture of the oceanic crust, and the new lava wedges into this rupture, pushing the oceanic crust on either side apart. The ocean floor, in this way, spreads. …