How do barrier islands form?
August 27, 2011 8:07 PM   Subscribe

Explain barrier islands to me.

I grew up on the west coast of the US, and out here if you stand on the beach and look out to sea, the nearest land in that direction is Japan.

But I look at a map of the east coast and it seems like half the coast line from Texas all the way to Rhode Island is barrier islands. How do they form? Why haven't they eroded away millions of years ago? Why haven't the waters behind them silted up?

As best I can tell, there isn't anything like that anywhere else on the planet. What's different about that area that forms these things?

My first thought was "the gulf stream" but that doesn't explain Galveston.
posted by Chocolate Pickle to Science & Nature (16 answers total) 3 users marked this as a favorite
Long Island was dropped in place by an ancient glacier. Can't speak for the rest (although I've sailed many of them, so that's embarrassing.)

Good question!

Wait! I took a geology class with field work regarding Manhattan. If you consider that a barrier island - hard rock of various types (you can see the three that make up the island in this park on the upper upper tip of the island) that basically refused to erode.

Not sure about Staten Island, come to think of it.

Upon preview - I'm considering any Island off the coast a barrier island. As for Fire Island off the coast of Long Island - YES - a pile of sand.
posted by jbenben at 8:16 PM on August 27, 2011

Best answer: Is there something specific that the Wikipedia article doesn't cover? Because, while it's not perfect, it seems to cover the basic causes / reasons.
posted by Pinback at 8:24 PM on August 27, 2011

Long Island, Manhattan and Staten Island aren't barrier islands. These are the barrier islands in the New York area — including, yeah, Fire Island, and also Long Beach, Jones Beach, Westhampton, and originally Coney Island (before they put down a bunch of landfill to connect it to the rest of Brooklyn). Glorified sand bars, all of 'em.
posted by nebulawindphone at 8:25 PM on August 27, 2011 [4 favorites]

Response by poster: It didn't occur to me to look at Wikipedia. (I suppose that should have been my first stop.)

Reading that article, the answer seems to be "no one really knows for sure".
posted by Chocolate Pickle at 8:30 PM on August 27, 2011

Thanks nebulawindphone! I think of any island that protects a shoreline as a "barrier Island" but I can see how wrong I am now.

posted by jbenben at 8:32 PM on August 27, 2011

They can change shape somewhat, especially when hurricanes come through.
posted by mareli at 8:37 PM on August 27, 2011

Best answer: On any sandy shore with a significant cross current you get areas of deposition and erosion. The barrier islands are a result of the deposition, but people now try to stabilize some of the bigger ones because they've built things they don't want to have eroded away (or, conversely, things they don't want to be too far from the beach as it grows).

For the record, so far the ocean usually wins in the end. Despite your tax dollars spent on beach augmentation.
posted by ldthomps at 8:49 PM on August 27, 2011

Best answer: Well, that's why I said the article isn't perfect. People do mostly know; as is usual the "don't really know" slant seems to be due to Wikipedia being written by people who learnt the subject from Professor Google ;-)

The barrier islands in the Gulf of Mexico? Mostly river valleys that ran parallel to the coast and have since been submerged. Most of the rest up the US east coast? Spit accretion, like most of the other barrier islands in the world (including the sand islands off the Aus coast that I'm familiar with)

The offshore bar theory? I'm no geo, but it may be a bit adventurous for barrier island formation. Smaller sand islands are known to form that way, but in general they rarely stick around long enough to collect stabilising vegetation and grow high enough that they don't get swamped & eroded with the next king tides.
posted by Pinback at 8:54 PM on August 27, 2011 [1 favorite]

I don't know how they form, but I know why. For the ponies!
posted by Glinn at 9:33 PM on August 27, 2011 [4 favorites]

They can change shape somewhat, especially when hurricanes come through.

Yes, they do, every year, in the part of Florida where my dad lives and races sailboats. Before the race series starts each year, the club has to go through and re-mark a new course, new hazard buoys, etc., because the undersea topology will have changed significantly from last time. Meanwhile the prevailing winds, dead spots due to vegetation, houses, etc., will not have changed much. So every single race series is a brand new level. It is probably no coincidence that they train Olympic sailors there. Also, there are mantees.
posted by pH Indicating Socks at 9:57 PM on August 27, 2011

AAGH - manatees.
Anyway, they beat ponies.
posted by pH Indicating Socks at 9:58 PM on August 27, 2011

Woo finally a geology related question on the Green!! As I am a geologist, I will try to give several possible explanations. I am also typing this on my phone (as am too excited to wait till I have access to a keyboard), hence please forgive any errors. All these answers can br conceptualized and explained if you think about energy levels in the system.

1. Beaches usually have sand dunes which are formed by the intersection of energy levels from any outflow from land (rivers), and the sea.  Rivers have a lot of energy (and hence can carry a lot of sediment), but when they approach the sea, the energy level drops and sediment drops out of the system, forming beach dunes. Barrier islands (and corrrsponding back lagoon) are just much, much larger beach dunes. It gets even crazier if a particular area is hugely influenced by tides. An introductory sedimentology book can explain this better than I can, but there you go.

2. Additionally, the East Coast is a passive margin system, whereas the West Coast is an active margin (subduction, transfer faults, etc). The West Coast has a lot more variation in elevation between different areas, hence has a larger difference in potential energy. There is just a lot more energy to 'wash' barrier islands away.

3. There are also specific local anomalies in surface sediments, texture, currents etc that might explain presence of barrier islands in one place versus the other.

Hopefully this wasn't too garbled! :) Let me know if you want the title of an intro sedimentology textbook.
posted by moiraine at 12:15 PM on August 28, 2011 [3 favorites]

Okay, I wasn't too clear about no 1. Beach dunes form when two systems meet and interfere (rivers meeting sea, destructive interference).
posted by moiraine at 12:20 PM on August 28, 2011

Let me know if you want the title of an intro sedimentology textbook.

Yes, please, do recommend one. And thanks for the introductory lesson!
posted by RRgal at 12:31 PM on August 28, 2011

This was the original scientific paper whuch has beautiful diagrams showing energy levels for different estuary and river models and how these relate to the deposition of features like barrier islands, sand bars, etc. All sedimentology books of any worth will use diagrams from this paper. If you don't read the text, just see the diagrams!

However, the paper is fairly specific, so for a general overview, Principles of Sedimentology and Stratigraphy by Sam Boggs was what I used as an undergrad. Think it's up to the fifth edition, but I used the fourth and I turned out ok :) A general overview would explain how a river works and the maximum sediment it can carry. And it can also tie estuary and river models with features found at a beach.
posted by moiraine at 3:07 PM on August 28, 2011

I do realize that I typed all this in a rush, so may be a little confusing. But if you need just a little more and longer explanation, I will be happy to help as soon as I get access to a laptop (which will be in several days...).
posted by moiraine at 3:20 PM on August 28, 2011

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