Creating a realistic world map - Underwater geography

Question

This Query is part of the Worldbuilding Resources Article.

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Creating a realistic world map - Erosion covered the various factors that govern the topography of a world. Rivers, wind, heat, and plants are some of the most important factors.

Underwater, though, things are different. At the bottom of the ocean, most of the important factors on land do not exist. Different processes must play a part in shaping the ocean floor.

• What processes are responsible for underwater mountains, valleys, and other features far from land? The focus here is on geography far out in the ocean, beyond the continental shelf.

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Note:

This is part of a series of questions that tries to break down the process of creating a world from initial creation of the landmass through to erosion, weather patterns, biomes and every other related topics. Please restrict answers to this specific topic rather than branching on into other areas as other subjects will be covered by other questions.

These questions all assume an earth-like spherical world in orbit in the habitable band.

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See the other questions in this series here : http://meta.worldbuilding.stackexchange.com/questions/2594/creating-a-realistic-world-series

Answer 1

They're kind of boring. Vast and boring.

There is a nice image (from Merriam Webster) to match my other answer on coastlines.

• Continental Margin - Underwater extension of the continent; it comprises the continental shelf, the continental slope and the continental rise.
• Continental shelf - Section of the continental margin extending from the coast of the continent to the continental rise; its depth is no more than 660 feet.
• Guyot - Ancient volcano whose summit has been cut off by erosion and then submerged.
• Magma - Molten rock and gas under very high pressure that can reach extremely high temperatures.
• Seamount - Isolated mountain of volcanic origin featuring a pointed summit. Or a large seahorse ridden by mermen.
• Trench - Extremely deep elongated depression bordering a continent or island arc; it occurs when one tectonic plate moves under another.
• Volcanic island - Volcano whose summit rises above sea level.
• Island arc - String of volcanic islands formed when two tectonic plates meet. Abyssal hill - Rounded underwater rise of low elevation.
• Sea level - Mean water level observed for a given duration (day, month, year); it is used as a reference to define coastal features and calculate the elevation of topographical elements.
• Mid-ocean ridge - Group of underwater mountain chains criss-crossing the oceans; it is formed by an outpouring of magma.
• Continent - A collective term for the vast landmasses and their submerged margins.
• Submarine canyon - Deep valley that is frequently the extension of a river; it ends in a sediment buildup.
• Continental slope - Slope of a few degrees that extends from the continental shelf; it is 660 to 6,600 feet deep.
• Continental rise - Gently sloping section of the continental margin; it connects the continental slope to the abyssal plain.
• Abyssal plain - Zone located at a depth of 6,600 to 20,000 feet; it covers most of the ocean floor.

The main components we care about (the ones beyond the continental shelf) are in the flat boring part in the middle of the map. There are some seamounts, a mid-ocean ridge, trenches, and the vast abyssal plain. Though the abyssal plain is shown as a small section of this picture, it actually covers more than 50% of the Earth.

In drawing a map, the main detail will go into the mid-ocean ridge where a divergent plate boundary exists. This is where you will find volcanic vents, mountains, hills, and something other than flat deserts of ocean floor.

An occasional seamount will be spotted as it has broken off from the mid-ocean ridge and heading for the abyss at the break neck speeds of continental drift. Lookout seamount! You're going to go over the edge!

An abyss (or trench) can also be quite interesting, though less populated than the ridge, its depth can be mysterious.

Answer 2

I'm assuming there is no surface land on your planet?

Geology

Your world will still have the same geology as Earth, in terms of processes - volcanoes, rifts, underwater/shifting hot spots. The differences include the lack of a Neritic Zone (the area of sediment above the continental shelf) and all shelves and slopes will have been formed as simple piles of sediment and volcanic action.

Features would include:

• Ridges: long stretches of mountain and trench
• Mountains (and volcanoes & hot spots)
• Depths: depth can be a graded or shown as features (e.g.- a polygon showing the Photic Zone of a region, etc.)

Erosion

Unlike the land surface of the Earth, there will be no aeolian erosion, removing the stark geographies found in dry areas of erosion on land. Underwater currents will most likely be a series of thermoline ocean current loops, which could consolidate sediments along the poles as warm water carries more stuff northwards than cold water does southwards. So maybe two lumpy hills at the poles, deep underwater. This could mean more exposed bedrock along the equator.

(Mer)man-made Features

There won't be any man-made features, I take it (lakes from dams, reclaimed land, etc.)? So it depends on what your civilization looks like. Maybe large, long stone walls will collect sediments creating stretches of sloped hills upon which Echinoderms and Sea Grass can be raised and harvested.

Exposed Surface

I believe there will be seasonal surface ice during the year. The reason I don't expect it to be year-round is because water moderates weather and temperatures; our poles, surrounded by land or physically on-land aren't as exposed to the moderating currents as your poles will be.

Temporal

There may be use in temporal maps, describing places by the process of time: short term (tides) and long-term (seasons).

This will be a very boring planet, except for the areas that are formed by volcanic & tectonic activity.