Another geologicalish Q

Ok, say you had a planet with roughly 20-30% liquid water, right? So we'll use the earth for example. Say instead of 2/3 of earths surface being covered in oceans, what if it were 1/4? Now from what ive researched thats abt the minimum that cud sustain enough of a breathable atmos to stick to the planet. So anyway,, geologically, obviously the shoreline globally would be much lower, and mid ocean peaks would be exposed, but how would it be around continental plate boundaries? I mean obviously based on rock make up, huge cliffs would only be standing in some places over time..? Or with enough erosion would there just be basically a smooth ascent upward from coastal areas until flattening out into continental plains, with various mountain chains on top of that in places? And then in other places would it just be towering peaks straight to sea level in places such as the alaska range? And what do you think that would mean for inland rivers and such? Or what if there was a supercontinent with those new sea levels?

Okay, now say this planet is 3x earths mass, with same relative gravity (possibly due to relatively smaller iron core, as well as a slight increase in centrifugal rotation over that of earths, and slightly lower air pressure?) now say that on this planet there is one, largely equatorial ocean stretching maybe 2/3rds of the way around the planet, okay? And that made up the bulk of the planets' liquid water, along with 2 connected shallow seas, and what would be called 4 separate oceans on earth's scale, but we will call them seas, making up the rest of the salt water.

Now apply all of the same questions and help me project a little bit.

Go! Spill ur noodles out!

I guess just skip all the questions abt the earth, i guess just apply what you know about earth's geology or science in general really - to anyone - to the scenario i presented, and the same Qs i had about topography and such.

Pun wrote:Ok, say you had a planet with roughly 20-30% liquid water, right? So we'll use the earth for example. Say instead of 2/3 of earths surface being covered in oceans, what if it were 1/4? Now from what ive researched thats abt the minimum that cud sustain enough of a breathable atmos to stick to the planet.

Is that so? I've never looked into the relation between liquid water and breathable atmosphere. Interesting.

So anyway,, geologically, obviously the shoreline globally would be much lower, and mid ocean peaks would be exposed, but how would it be around continental plate boundaries? I mean obviously based on rock make up, huge cliffs would only be standing in some places over time..? Or with enough erosion would there just be basically a smooth ascent upward from coastal areas until flattening out into continental plains, with various mountain chains on top of that in places? And then in other places would it just be towering peaks straight to sea level in places such as the alaska range? And what do you think that would mean for inland rivers and such? Or what if there was a supercontinent with those new sea levels?

The geography would depend entirely upon local topography, tectonics, and climate. If the sea level were to drop dramatically in the Gulf region you would likely see a gradual slope (much like that which is currently exposed throughout the South/Mid West. In most areas along the Californian coast, where the topography consists largely of steep terrain and deep submarine canyons, sea level decline would expose some nasty, rugged terrain. Along the Northern California, Oregonian, and Washington coast you would have rugged terrain leading to a very deep canyon roughly parallel to the modern coast line (this would be the trough associated with the subduction of the Falleron plate beneath North America).

How this topography would change over time is dependent on the topography, climate, lithology (rock type), and tectonics. All these factors play together to control erosion.

Topography sets the amount of energy a) that the sediment has and b) the water bodies flowing through the area have to erode or transport sediment. If you walk up to the base of a tall cliff, you will likely find a pile of loose rock on the bottom (geologists call this colluvium). These rocks were once perched somewhere up slope, but due to their gravitational instability fell and accumulated on the bottom of the cliff (where the slope is considerably lower). Likewise, if you go to the Mississippi delta and dug a really deep (1000's of feet) hole, you would find large amounts of sediment. The gradient of the river in that area (and this holds true for most rivers near deltas or alluvial fans) is so low that the water looses the ability to transport some sediment and is forced to deposit (drop) it. As you may expect, we see the highest rates of erosion in tall, steep mountain ranges (there is lots of erosion in the Andes and Himalayas) and the lowest rates in flat lands (like the great plains). In extreme topography landslides can become an important process, even under the ocean (submarine landslides are called turbidites, there has been some interesting work done looking at the relation between slopes and flank collapse at Hawaii that you may be interested in).

Climate is important in understanding erosion. In the most basic view, without water you aren't going to get much erosion and you wouldn't be able to transport sediment very far. If you look closer, you will find that not only is water important in transporting sediment, it often plays a crucial role in other processes. Water helps break down rocks by dissolving/reacting with certain minerals and causing rocks to crumble (its easier to transport sand, made mostly of non-reactive SiO2, than it is to transport a mountain!). Freeze/thaw cycles can cause water in fractures in rocks to expand and contract, wedging rocks until they split apart. Several types of vegetation chemically interact with the rocks/soil they grow in, altering their substrate (and plants tend to be very sensitive to climate). Landslides tend to be very sensitive to water conditions (the pressure of ground water on the sediment around it, called pore pressure, is one of the key factors to understanding most mass slope movement events).

Lithology is very important to understanding an area's geomorphology (the shape of the surface of the earth). Some rocks can hold far steeper slopes than others. As you are from Florida, I presume you are familiar with some of the characteristics of limestone. If you were to go to a limestone quarry you would likely see some steep sided walls. Most carbonates (limestone and dolomite) tend to have high internal strength and be good at resisting mechanical erosion (even if they are incredibly susceptible to chemical erosion). What would happen if you tried to build a similar pit in dry sand? If you have a continental margin with steep slopes but weak lithology you will likely see rapid (geologically speaking) change in topography (from steep to gentle), where as if you have steep slopes but strong lithology erosion should be relatively slow. That said, this is a gross oversimplification. There are several other factors that can come into play.

Tectonics is the ultimate end-all-be-all of understanding an areas topography. Tectonic processes can create huge mountains and gaping valleys. While erosion might be trying to make the earths surface a level plain, tectonics keeps jacking it up. Thus, if you were to drop the sea level near the Cascades (a very tectonically active area) the mountains would still keep growing and would likely do so until subduction ceases. In areas like the Gulf of Mexico this may not be a huge factor, but along the Pacific coast its a bid deal.

All of these factors come together and play off each other. What does this mean for the inland rivers and waterways? That is incredibly hard to say. Water availability is dictated by ocean circulation (which changes with both water temperature and land distribution) and wind patters (controlled by temperatures and topography). To come up with a reasonable projection of what the water situation in parts of the world would look like if the sea level dropped that much (why did it drop? Climatic reasons could have huge implications for where gets water and where doesn't) would require very complex modeling that I am not capable of executing. The only thing that I could readily predict is that the rivers, if they still had enough water, would begin to down cut into the earth due to the change of base level (ie change of sea level).

Okay, now say this planet is 3x earths mass, with same relative gravity (possibly due to relatively smaller iron core, as well as a slight increase in centrifugal rotation over that of earths, and slightly lower air pressure?) now say that on this planet there is one, largely equatorial ocean stretching maybe 2/3rds of the way around the planet, okay? And that made up the bulk of the planets' liquid water, along with 2 connected shallow seas, and what would be called 4 separate oceans on earth's scale, but we will call them seas, making up the rest of the salt water.

Now apply all of the same questions and help me project a little bit.

Go! Spill ur noodles out!

If gravity is the same, I would assume the erosion processes would be about the same.

The only caveat I could think of would be the relation between gravity and tectonics. If the gravity was the same as earth, the planets density (average of all rocks) would have to be much lower than the Earth's. This would presumably suggest that most of the planet would be made out of silicic material (ie high in SiO2). This would mean that the planet's composition would be mostly crustal with a small mantle/core. I would assume (perhaps incorrectly?) that this would suggest that there is minimal tectonic activity?

I think I see where you are going with this. While I find it admirable that you would pay such attention to details, I fear my level of expertise on climatology is far too weak to provide any real help. There are lots of factors that play off each other, and even given all the information needed to project climatic interpretations, my lack of background in the subject would still render me useless. I hope my long, rambling post was of some help. If you have any specific questions, I will do my best to try to provide coherent answers.

Right, no, im just trying to give birth to a planet here and figure out what is both possible, and hypothetically plausible. Think i may need to shrink the size a bit and up the water volume a tad. Basically this a desert planet, but obvioulsy it cant all be bone dry like tattooine. So im trying to get a sense of what a desert planet would be like, with just enough water to sustain life.

And as for the relation of liquid water and water vapors in the atmos that makes it thick enough to stick to the planet and not be blown away by stellar winds. A good example would be mars. A very thin atmosphere, with virtually no liquid water, except for maybe some in some craters that thaws every now and then but i digress.

Anyway, i dont want tattooine, i want a diverse desert planet as any habitable planet would be. I do want vast dune seas, but i also want some river deltas, high desert, dry savannahs and scrub forests, steppelands, frozen polar deserts like antarctica which gets no LIQUID water. I want arid plains, lush valleys and all the variation we would realistically expect to find of such an inhabitable planet.

Guess my main misconception was that just shrinking the core size could offset a great deal gravity, and didnt realize that would be indicative of rock density outside of the core

So give me some topographical and geological examples of a lot of the things we would find on such a planet, whatever the final size and water % may be. I mean i envision the capitol city as essentially being the size of the san juaquin (sp?), nestled in a valley roughly the same size, created from anancient landslide, with steep near verticle walls on three sides, descending to an ocean or sea on the fourth, but what makes the city so important is that it sits atop the planets largest freshwater aquafer so i need to know if all or any of those things are conflicting.