Geography · Ch 10 — Distribution of Oceans and Continents
Ocean Floor Configuration
Ocean Floor Configuration
The diagram is a hypsographic curve — a single graph that compresses the entire vertical profile of the Earth's solid surface into one picture. The horizontal axis measures cumulative area of the Earth's surface, given both as a percentage and in square kilometres. The vertical axis measures height and depth in kilometres, with sea level fixed at zero. Positive values rise above sea level; negative values plunge below it.
Reading the curve from left to right, you trace the surface from the highest point on land down to the deepest point in the ocean. The curve begins at the summit of the highest mountain, then descends through the continental landmass. As it approaches sea level, the line steepens dramatically — this is the continental shelf, the gently sloping submerged edge of the continent. Just past the shelf, the curve drops almost vertically: that is the continental slope, the true edge of the continental block where the seafloor plunges toward the deep. Below the slope, the curve flattens out into a long, nearly horizontal plateau — the ocean basin floor, which occupies the vast majority of the graph's width. Finally, the curve tails off to the deepest trench on the ocean floor.
What makes this graph so striking is its step-like shape. The continental shelf and slope form a sharp "step" near sea level, and the ocean basin floor forms a long, flat "tread" far below. This is not an accident of drawing — it reflects a fundamental fact about the Earth's crust. The continents stand high because they are made of lighter, thicker granitic material that floats higher on the denser mantle. The ocean basins sit low because they are floored by heavier, thinner basaltic crust. The hypsographic curve is therefore visual proof that the Earth's surface is bimodal — it does not grade smoothly from high to low, but clusters at two distinct levels: the continental plains around sea level and the abyssal plains several kilometres down. …
The ocean floor is not a flat, featureless plain. Its configuration — the shape and structure of the land beneath the sea — holds important clues about how continents and oceans have come to be distributed the way they are. A basic grasp of this underwater relief helps you connect the movement of plates to the features you see on a map. The full detail of ocean floor relief is taken up later in the course; here, the aim is only to note the broad framework.
Based on depth and the forms of relief, the ocean floor is divided into three major divisions: continental margins, deep-sea basins, and mid-ocean ridges. Each division has a distinct character, and together they make up the entire submerged surface of the Earth.
Abyssal Plains
These are extensive, flat plains that lie between the continental margins and the mid-oceanic ridges. They occupy a vast area of the deep ocean floor. The key process at work here is sedimentation: continental sediments that move beyond the margins get deposited on these plains. Over time, this deposition builds up the flat, smooth surface that gives the abyssal plains their name.
Mid-Oceanic Ridges
This is the most striking feature of the ocean floor. A mid-oceanic ridge forms an interconnected chain of mountains running through the ocean. Though submerged under oceanic waters, it is the longest mountain chain on the surface of the Earth — longer than any mountain range found on land.
The ridge is not a simple, uniform crest. Along its entire length, it is characterised by three distinct parts:
- a central rift system at the crest
- a fractionated plateau
- a flank zone …