Coastal Mountain Ranges--In Which the Mountains are the Coast

Question

One of the ways to create a major mountain range is a process called subduction.

Here, heavy oceanic rock sinks beneath the lighter continental rock. The result--coastal mountain ranges like the Andes, the Aleutians, the Japanese and the Southern Alps.

But look closer...

Between the Pacific Ocean and the Andes Mountains are lowlands. Narrow, undeniably, but still closer to sea level. So the question is this--is it possible for a mountain range to be the beach rather than at least a few miles inland? And if the answer is yes, then what force of geology would I need to make that possible?

Answer 1

"Beaches" are a result of erosion

Erosion wears down huge cliffs into smoother descents into the ocean. First you have this:

where the cliffs drop directly into the water. Then, after some time and sufficient rain, eroded material is deposited at the bottom of the cliffs, until you have this:

Given a few tens of millions of years, the process continues until the Andes themselves start several miles back from the beach, which is now a coastal plain.

Conclusion

Young mountains will rise directly out of the water. But beware, it takes a long time for a mountain to rise. By the time it has gotten to the height of the Andes, some millions of years have passed; plenty of time for a coastal plain to have developed.

The best way to slow down this erosion is to have a lack of water. But the west coast of South American from Lima, Peru south is about as dry a place as exists on Earth, and you can see on a map how much coastal plain has formed.

Answer 2

Short answer: You can have cliffs like this, but they're not going to be as high as the Andes.

Long Answer: You have several options for creating cliffs like this, but none of them are going to be as high as the Andes.

Let's talk about mountain formation

Option one: Fault Block Mountains

Maybe you're thinking of something like this:

https://commons.wikimedia.org/wiki/File:Telascica-Cliff.JPG

https://commons.wikimedia.org/wiki/Category:Telašćica_Nature_Park#/media/File:Telašćica_Cliff.jpg

...but with mountains. Welcome to the Telascica Cliffs in Kornati National Park, Croatia. Let's zoom out and take a broader look:

http://crodestinations.com/telascica-one-of-the-most-popular-havens-for-yachtsmen-in-the-adriatic/

In the pics on that page, you can see how it's just the western face of the cliffs that rise dramatically out of the water. The eastern side of the island gently slopes down. That's because the cliffs are a tilted block on a fault line.

You can scale up tilted blocks to the size of mountains, and what you get is the Big Sur:

https://en.wikipedia.org/wiki/Big_Sur#/media/File:Central_Californian_Coastline,_Big_Sur_-_May_2013.jpg

Another variation of fault block mountains is are horst and graben cliffs. The scaled-up version of these is a rift valley with a narrow sea between them (as kingledion points out, if you have enough time for an ocean to form, beaches will have formed).

https://en.wikipedia.org/wiki/Horst_(geology)

In both cases, these cliffs are usually about a mile high before erosion.

Option two: extreme erosion

In this case, you're looking at something like the Na Pali coast.

https://en.wikipedia.org/wiki/Nā_Pali_Coast_State_Park#/media/File:Na_pali.jpg

Whether or not you get this is dependent on what type of rock you have and how it erodes. It's also going to be about a mile high.

Another way to do extreme erosion is fjords.

https://en.wikipedia.org/wiki/Fjord#/media/File:MilfordSound.jpg

Now let's talk about the other half of this: beach erosion

The easiest way for you to not have beaches is to have something that gets rid of beaches. These processes exist. To quote this lecture: http://www.iupui.edu/~g115/mod13/lecture05.html

Eroded coastlines are defined by wave energy erosion exceeding sediment deposition.

In other words, you want your waves to carry sand away faster than it can deposit. This can be done with relatively swift offshore and nearshore currents and riptides.

It helps if your mountains are made of relatively hard rock, like gneiss, that erodes slowly compared to how quickly the ocean washes sand away. The west coast of the US also has submarine canyons that give the sand a place to go as the current carries it away from the cliffs.

Last but not least...

You might be disappointed about your mountains realistically being a mile high. Keep in mind the Velebit in Croatia are about a mile high.

https://www.flickr.com/photos/trancevelebit/5023784382

Also, this is high enough to cause a pressure differential in winter that results in strong winds. Your ocean immediately adjacent to your mountains might not be navigable for part of the year.

https://en.wikipedia.org/wiki/Bora_(wind)

Answer 3

Very young mountains not raised by subduction would seem to be the best bet.

The volcanoes produced by subduction are always a ways in from the edge of the continental plate which is usually pretty close to shore. So that's not ideal.

Crumple zone mountains (like the Himalayas and Alps) are caused by two continental plates hitting each other. Those types of mountains can't help but be inland.

Even a very, very new mountain like Mauna Loa on the Big Island of Hawaii has a narrow flattish "beach" -- basically, when eroded material hits the ocean, it slows down and forms a flattish area around sea level. Likewise lava hits the ocean and tends to stop. This comes pretty close, but only when they're young.

Another approach (if you don't demand ultra-high mountains) would be to have a high tableland which is eroded to form peaks separated by deep valleys. (Think the fjord country of Norway or Alaska.) These can go right to the ocean's edge.

Another approach -- again, the "mountains" are only a thousand or so feet high -- is when you have an island volcanic caldera which explodes, leaving sheer faces over the sea. (Think Santorini.)