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Barrier Islands · PlanetGeo: The Geology Podcast

0:03
SPEAKER_00: Welcome to Planet Geo, the podcast where we talk about our amazing planet, how it works, and why it matters to you.
0:13
SPEAKER_01: I'm recording.
0:15
SPEAKER_01: So am I.
0:16
SPEAKER_01: All right, let me get my radio voice on.
0:19
SPEAKER_01: You ready, Kermit?
0:20
SPEAKER_01: I'm ready.
0:23
SPEAKER_02: Kermit.
0:26
SPEAKER_00: I forgot that Jenny called me Kermit.
0:29
SPEAKER_02: That's so funny.
0:30
SPEAKER_02: She did.
0:31
SPEAKER_01: We actually listened to a couple podcasts on the way back.
0:33
SPEAKER_01: We uh just got back from Louisiana on a spring break trip.
0:37
SPEAKER_01: Yeah.
0:37
SPEAKER_01: We listened to some podcasts and she's like, oh, Kermit.
0:40
SPEAKER_01: Kermit's back.
0:42
unknown: Yeah.
0:43
SPEAKER_02: It's gonna take me a while for my ego to recover uh from that one.
0:46
SPEAKER_02: Yeah, somehow I don't buy it.
0:48
SPEAKER_00: How are you doing, Jesse?
0:49
SPEAKER_00: I'm good, man.
0:50
SPEAKER_00: Hey, this is a timely episode because you just got back from spring break, your high school spring break in Michigan.
0:55
SPEAKER_00: And this is a time when a lot of people from the Midwest migrate down to Barrier Islands and spend vacations on, around, or near Barrier Islands.
1:06
SPEAKER_00: And Chris, I you you guys go to Louisiana, right?
1:09
SPEAKER_00: We do.
1:10
SPEAKER_00: Did you see a barrier island at all?
1:12
SPEAKER_00: Or were you mostly around Lake Pontchatrain?
1:15
SPEAKER_01: Yeah, we were on the north side of Lake Pontchatrain, and so we did not see barrier islands, but I've spent a lot of spring breaks on barrier islands before.
1:25
SPEAKER_01: You know, like for instance, you know, Tybee Island in in Georgia near Savannah is a barrier island.
1:31
SPEAKER_01: And it's just it's they're awesome.
1:33
SPEAKER_01: So, Jesse, give us a quick rundown on where we're going today.
1:37
SPEAKER_01: What are we gonna talk about?
1:38
SPEAKER_00: Yeah, we're talking about barrier islands.
1:40
SPEAKER_00: We're gonna sort of briefly intro what they are, what we're talking about here.
1:45
SPEAKER_00: We're gonna then move into the things you need to form a barrier island.
1:49
SPEAKER_00: And really the theme of this episode is what are they geologically and why are they relevant, as usual.
1:54
SPEAKER_00: And so we're gonna talk about like the things you need to form a barrier island.
1:58
SPEAKER_00: We're gonna talk about different models for how they form and how they then therefore evolve, and and these things evolve on human timescales.
2:07
SPEAKER_00: So that's a big topic.
2:08
SPEAKER_00: And then we're gonna sort of end by talking about the human functionality, like the why we care about barrier islands and why you should care about barrier islands.
2:17
SPEAKER_00: And we should care about barrier islands.
2:19
SPEAKER_01: Totally.
2:19
SPEAKER_01: This is totally.
2:21
SPEAKER_01: I think this is a really cool episode idea because it it's relevant.
2:25
SPEAKER_01: I mean, it was really relevant during hurricane season this past fall, with what happened in Florida and you know, which is basically surrounded by barrier islands.
2:35
SPEAKER_01: So yeah, this is it's timely.
2:38
SPEAKER_00: It's a great one, Chris.
2:39
SPEAKER_00: And you just brought up a really good point, and I think we should start there is what are these things?
2:44
SPEAKER_00: Why are they called barrier islands?
2:46
SPEAKER_00: And they serve as a barrier.
2:47
SPEAKER_00: It's an island that is a barrier from you know ocean wave activity to the mainland.
2:52
SPEAKER_00: And so they're ubiquitous around the Gulf Coast in North America.
2:57
SPEAKER_00: They're very prevalent along the Atlantic seaboard of North America as well, at least the southern Atlantic seaboard.
3:03
SPEAKER_00: And these things are all over the world as well.
3:06
SPEAKER_00: They don't always occur right on coastlines, they sometimes occur out away in sort of islands as well, but they're very common globally and really important because they protect the mainland from wave action and storm action.
3:18
SPEAKER_01: All right, Jesse, let me interrupt you a second.
3:20
SPEAKER_01: Let's give a description of what barrier islands look like.
3:23
SPEAKER_01: We want everybody to be able to picture these things because, like you said, dropping big words ubiquitous.
3:29
SPEAKER_01: They're they're very ubiquitous in the eastern part of the United States and they're all around the Gulf Coast and so on.
3:35
SPEAKER_01: There we go.
3:36
SPEAKER_01: So, anyway, these are just these sand deposits that basically they're disconnected.
3:42
SPEAKER_01: It's not a can it's not this continuous chain of islands that's out away from the mainland shore, but they're parallel, disconnected deposits of sand.
3:54
SPEAKER_01: That's what these barrier islands are.
3:56
SPEAKER_01: So, Jesse, have you been what barrier islands have you been to that are fairly well known?
4:01
SPEAKER_00: Oh man, I've been to a lot of like the smaller ones uh around really on the Gulf side and the Gulf of Mexico.
4:07
SPEAKER_00: You know, Destin's on one, Navarre, there's a barrier island there.
4:10
SPEAKER_00: I forget the names of the actual island, but these are little towns, little sort of vacation towns on barrier islands.
4:16
SPEAKER_00: But but they're sort of all over, and there's lots of little towns, and I always get confused about what the name of the island is versus the little town we're in that's on the island, you know what I mean?
4:24
SPEAKER_01: So yeah, and and you're right about that too.
4:27
SPEAKER_01: It's a cool thing, is you often get these quaint little like pop-ups on these islands that are just so cool, you know, with the tiki bars set up and restaurants, and just they're often very quaint, cool environments there.
4:41
SPEAKER_01: Um, I've been to Tybee Island in Georgia many, many times.
4:46
SPEAKER_01: I love that.
4:47
SPEAKER_01: I've been to Skitaway Island.
4:48
SPEAKER_01: There's a state park there that is absolutely one of my favorite places on the planet.
4:53
SPEAKER_01: The one that made you know headlines big time this past fall with the hurricane season was Sanibel Island in Florida.
5:00
SPEAKER_01: Just absolutely devastated this fall.
5:04
SPEAKER_01: Miami Beach, Atlantic City, these are barrier islands, and these are just some of the famous ones.
5:10
SPEAKER_01: There are literally hundreds, if not thousands, of barrier islands up and down the coastline of the east in the Gulf of Mexico.
5:18
SPEAKER_00: Yeah, Chris, and these aren't just a U.S.
5:20
SPEAKER_00: uh phenomenon.
5:21
SPEAKER_00: In Australia, Morton Bay is a big one, Bribe Bay.
5:25
SPEAKER_00: There's also in New Zealand and in Europe, there are massive barrier islands that are really important ones.
5:30
SPEAKER_00: Importantly, there's some in the Baltic Sea and the Wadden Islands stretch from the Netherlands to Denmark.
5:35
SPEAKER_00: So these are really kind of important things globally from a geoscience perspective, and uh they often exist in somewhat pretty areas and around big cities because they often protect like harbors and things like that.
5:48
SPEAKER_00: So, Chris, I think this is a good time now to transition into how these things form or how we think these things form.
5:55
SPEAKER_00: And so, where do you want to start with that process?
5:58
SPEAKER_00: Because this is again, could be a lengthy discussion, I think.
6:02
SPEAKER_00: Yeah.
6:02
SPEAKER_01: So so you frame it for us.
6:04
SPEAKER_01: It seems like a dumb thing to say that we don't really have a consensus on barrier island formation, but that's that's okay.
6:11
SPEAKER_01: One, I want to say that that's okay, but two, not all barrier islands are the same.
6:16
SPEAKER_01: And so there are going to be little nuances in the formation of different, you know, the geology is gonna determine how individual barrier islands can form.
6:25
SPEAKER_00: So Yeah, and I think let me just step in there really quick, Chris, because I this was an interesting episode to put together from my perspective, because I always assumed that barrier islands were relatively straightforward geologically, and I had one of the ideas or one of these three scenarios in my head as the one that's like universally accepted.
6:44
SPEAKER_00: I think you might have had a different one in your head as thinking universally accepted, and then you know, and building this and learning about this like, oh wait, there's actually a decent amount of debate about this.
6:53
SPEAKER_00: So it was kind of a fun thing to learn about, uh, just putting this episode together.
6:58
SPEAKER_00: So just a side point.
6:59
SPEAKER_00: Sorry to interrupt you there.
7:00
SPEAKER_01: No, no, that's good.
7:01
SPEAKER_01: That's good.
7:01
SPEAKER_01: The thing that we do know, we need three basic things to form a barrier island.
7:07
SPEAKER_01: All right.
7:08
SPEAKER_01: We need to have sand, we got to have a sand supply, and we have to have a broad, gentle sloping coastline, which we cut in geology we call that like a passive continental margin, right?
7:21
SPEAKER_01: There, or in other words, the trailing edge of a continent.
7:24
SPEAKER_01: And that's why the east coast of the United States is ripe for barrier island formation, because the North American continent is moving in the southwest direction, which means the east coast and the Gulf Coast are the trailing edge of the plate.
7:41
SPEAKER_01: All the actions on the west coast.
7:43
SPEAKER_01: Very little geologic tectonic activity is going on in the east, and so you get this broad, gently sloping coastline.
7:51
SPEAKER_01: So that's the second thing that we need.
7:53
SPEAKER_01: And then the third thing that we need, we need waves, we need tidal currents, but and they have to be strong enough to move a substantial amount of sand.
8:02
SPEAKER_01: And there is a like a sub-bullet to this, too, that we need it to be a wave-dominated environment and rather than a tidal-dominated environment.
8:14
SPEAKER_00: Yeah, and I think Chris, we need to just just touch on this a little bit and we'll come back to this point.
8:19
SPEAKER_00: But sand is the key component here.
8:21
SPEAKER_00: Like, this is not barrier islands, they are not mud, they're sand.
8:23
SPEAKER_00: This is sand dunes, these are sandy beaches.
8:26
SPEAKER_00: Part of the reason they're so beautiful, and people build little cities on them, and and big cities is because it's so beautiful.
8:31
SPEAKER_00: The sand, that's a higher energy movement of water that you need to move a sand grain than you do to move a mud particle.
8:38
SPEAKER_00: So, sand, we consider sand as sort of a moderately high energy water will move sand, and low energy water will not move sand around.
8:47
SPEAKER_00: So that's why we have to have this sort of wave-dominated area because we need the waves to be high energy enough to move sand around.
8:54
SPEAKER_00: Like that's fundamentally what's going on as we're moving sand up and down to do this.
8:58
SPEAKER_00: So I just wanted to sort of touch on that.
9:01
SPEAKER_01: No, that's a that's a really good point.
9:03
SPEAKER_01: I mean, to just maybe reiterate this, the three common rocks that are made up of these like classed particle sizes.
9:11
SPEAKER_01: You have shale, which is made up of clay size or mud-sized particles.
9:16
SPEAKER_01: Then you have sand, which you just described, which is a moderate to higher energy deposit, waves and rivers that have a good swift current to them and so on.
9:25
SPEAKER_01: And then you have conglomerate, which is made up of gravel-sized pieces, which is much, much higher energy.
9:30
SPEAKER_01: So the the particle size that make up the rocks is really indicative geologically of the environment in which formed them.
9:39
SPEAKER_01: So that's a that's a really good point to make.
9:41
SPEAKER_01: All right, Jesse, here we go.
9:43
SPEAKER_01: Let's get into the three ideas, the three ways in which barrier islands can form.
9:50
SPEAKER_01: So, can I start off with the first one because this is more of like an older school, and you know, you always rip on me for being old guy, and not that you're wrong.
9:59
SPEAKER_02: Yeah, you are old.
10:01
SPEAKER_02: So, I mean yes, why don't you take the first one?
10:05
SPEAKER_00: The old school Chris going back to the 1930s, bringing us scientific theories from the 30s.
10:10
SPEAKER_02: Here we go.
10:11
SPEAKER_02: All right.
10:12
SPEAKER_01: That's right, that's right.
10:13
SPEAKER_01: This idea, which which does happen, I think it's important to again reiterate that all three of these do come into play.
10:21
SPEAKER_01: This used to be the common idea.
10:24
SPEAKER_01: Here's the idea.
10:24
SPEAKER_01: Let me describe it and then then I'll I'll go with where I was gonna go.
10:28
SPEAKER_01: This theory is basically it's beach ridge submergence.
10:33
SPEAKER_01: In other words, where we have these barrier islands, this is where the old shoreline was during times when sea level was lower.
10:43
SPEAKER_01: Okay, like for instance, during the last ice age, at the peak of the last ice age, there was so much water locked up in ice that sea level lowered dramatically.
10:52
SPEAKER_01: And so the rivers were bringing their sand to where that older shoreline was, forming deltas and dunes, these beach ridges, right?
11:02
SPEAKER_01: And they would be, you know, the shape of them would be basically parallel to where the shoreline is today, just offshore from where we have it now.
11:12
SPEAKER_01: So it makes a lot of sense.
11:14
SPEAKER_01: If you look at a map of barrier islands, you're like, oh, well, yeah, I can see where sea level would be lower, dunes would form here, deltas would form here, lots of sediment supply because of the ending of the last ice age.
11:27
SPEAKER_01: It was very wet, and you know, rivers were rigorous and moving a lot of stuff.
11:31
SPEAKER_01: And then since that time, when sea level rises, all of this deposit just gets reworked into these barrier islands.
11:40
SPEAKER_01: So that's the basic gist of this theme.
11:44
SPEAKER_00: Let me Chris, but I think this is a good point.
11:46
SPEAKER_00: Let me just uh interject real quick there and say, you know, as you noted, this is that you still see this in textbooks, right?
11:52
SPEAKER_01: And this kind of goes back to it's in mine.
11:54
SPEAKER_00: It's okay.
11:55
SPEAKER_00: It's still in mine.
11:56
SPEAKER_00: Yeah, I it and it kind of points to this really unfortunate lag that we have between modern thinking at the deep levels of topics and what's represented in the textbook still.
12:08
SPEAKER_00: Because like textbooks, you know, even though they get rewritten, the the ideas don't change necessarily that much in the textbook view.
12:13
SPEAKER_00: Anyway, all right, number two.
12:15
SPEAKER_02: All right, go ahead, Jesse.
12:16
SPEAKER_00: This is okay.
12:17
SPEAKER_00: New school here.
12:18
SPEAKER_00: New school.
12:20
SPEAKER_00: So this is not really new, but newer, I guess, yeah.
12:23
SPEAKER_00: Uh the offshore bar theory.
12:25
SPEAKER_00: This is the second scenario that you can form barrier islands with.
12:29
SPEAKER_00: And basically the way this works is as waves sort of move into shallow water, they start to churn up sand.
12:37
SPEAKER_00: We talked about this energy in that wave action can really move sand quite a lot.
12:41
SPEAKER_00: And so when the waves break, though, they lose energy.
12:44
SPEAKER_00: So the waves are coming in, they're moving sand, they hit something, they lose all their energy, and they lose the ability to move sand.
12:50
SPEAKER_00: So the sand gets dumped there.
12:52
SPEAKER_00: And they can deposit a submarine sandbar, which most people are familiar.
12:58
SPEAKER_00: If you've walked out into the ocean anywhere really, you know that there's a sandbar somewhere.
13:02
SPEAKER_00: Like it gets deep right offshore, and then it gets shallower, and you can like stand up and waste deep water way out there.
13:08
SPEAKER_00: That's a sandbar.
13:09
SPEAKER_00: And if big waves are coming in and they hit that sandbar, they lose energy and they dump sand, that grows the sandbar.
13:16
SPEAKER_00: And then this can kind of become an emergent above-sea level offshore bar that just sort of grew up.
13:24
SPEAKER_01: Yeah, it's basically you're getting a bunch of sand that's gonna pile up because as the waves come in and they're moving this sand, it's higher energy.
13:34
SPEAKER_01: We talked about one of the necessary ingredients being wave-dominated, right?
13:39
SPEAKER_01: Because that's the stuff that has the sufficient energy to move these heavier sand grains.
13:44
SPEAKER_01: When the waves break, they lose energy and deposit the sand.
13:48
SPEAKER_01: And so as long as sea level remains, you know, stagnant for a while or stationary, then the waves are gonna break fairly consistently at this zone offshore.
14:00
SPEAKER_01: And therefore, as they lose energy, sand gets deposited in a fairly consistent place.
14:06
SPEAKER_01: Okay.
14:07
SPEAKER_01: Are we good with number two then?
14:09
SPEAKER_01: I think that's good.
14:10
SPEAKER_01: Is that sufficient?
14:11
SPEAKER_01: Pretty simple.
14:11
SPEAKER_01: Okay.
14:12
SPEAKER_00: Yeah, I think we I think we nailed it.
14:13
SPEAKER_01: Uh all right, Jesse.
14:14
SPEAKER_01: Let's get in then to number three, the third idea.
14:18
SPEAKER_01: And we know this happens too.
14:19
SPEAKER_01: Like I said before, all three of these do play a role here.
14:23
SPEAKER_01: This one's called the spit extension theory.
14:27
SPEAKER_01: And we need to define this because this is uh what is this spit?
14:34
SPEAKER_00: We have some terms that we need to define.
14:36
SPEAKER_00: Geology's full of stupid terms, and spit is just a few.
14:40
SPEAKER_01: I know, but I do kind of like this one.
14:41
SPEAKER_01: I I it is silly, but I like it.
14:43
SPEAKER_01: This is very difficult to do in a podcast format because it's very you know, shoreline features are very visual.
14:51
SPEAKER_01: But imagine a a shoreline that has a bay or uh like a lagoon, okay?
14:57
SPEAKER_01: In other words, an indent toward the mainland of the shore.
15:01
SPEAKER_01: What happens when water enters into that?
15:06
SPEAKER_01: The water's dispersed and it loses energy.
15:09
SPEAKER_01: And just as we said before, whenever water loses energy, deposition is a result of this.
15:14
SPEAKER_01: Okay.
15:15
SPEAKER_01: Well, this is one part in terms of how you get spits.
15:18
SPEAKER_01: It's the sandbar that extends partway across a bay or a lagoon kind of area of a shoreline.
15:25
SPEAKER_01: Okay.
15:26
SPEAKER_01: So that's the first thing.
15:27
SPEAKER_01: Now, we also, though, can determine what's called the longshore current by looking at the geometry of the spit.
15:35
SPEAKER_01: So, Jesse, I'm gonna leave it to you because I've been doing a lot of talking.
15:38
SPEAKER_01: What is longshore current?
15:41
SPEAKER_00: Longshore current is this really interesting phenomenon, and you can watch this.
15:47
SPEAKER_00: If you ever go sit at a beach and it's wavy, just watch this happen.
15:50
SPEAKER_00: The waves very rarely hit the beach.
15:53
SPEAKER_01: Jesse, I'm gonna interrupt you for a second.
15:55
SPEAKER_01: I'm gonna interrupt you.
15:56
SPEAKER_01: I'm sorry, keep your train of thought, but I do want to say sitting on a beach and watching this kind of stuff to me is amazing.
16:05
SPEAKER_01: Watching geology on a shoreline is it's better than a campfire.
16:10
SPEAKER_00: Okay, it's it's I agree, Chris.
16:13
SPEAKER_00: It's a great point.
16:14
SPEAKER_00: I mean, the best is to have a campfire and watch longshore current going on, so a beach campfire going on.
16:19
SPEAKER_00: But I think you're right.
16:20
SPEAKER_00: This is one of the few phenomena in geology that happens in human timescales, and you can actually watch it happen.
16:26
SPEAKER_00: And so longshore current is what what's happening here is the waves rarely ever hit the beach head on.
16:31
SPEAKER_00: So they rarely like come at the beach and hit it square.
16:36
SPEAKER_00: They're usually hitting it at a slight angle.
16:38
SPEAKER_00: And you can often see this as a breaker, right?
16:40
SPEAKER_00: The wave is not breaking all at once, it kind of rolls across the horizon in one direction or the other.
16:45
SPEAKER_00: That's the wave energy hitting at an angle.
16:47
SPEAKER_00: And so what happens when wave energy hits at an angle is it'll wash sand grains up at that angle.
16:55
SPEAKER_00: So as the wave hits the shore, it washes sand grains up at an angle, so not straight up the beach, but slightly to the left or slightly to the right.
17:03
SPEAKER_00: But then as the wave recedes, it's just gravity.
17:06
SPEAKER_00: So it just moves straight down the face.
17:09
SPEAKER_00: So what you get is a sand grain that is brought up at an angle and then straight down, and then up at an angle.
17:16
SPEAKER_00: It's a zigzag.
17:18
SPEAKER_00: And this you can watch this happen.
17:20
SPEAKER_00: If you just find one sand grain or one little shell and watch that get moved up and down, you'll see it migrate downshore and to the left or to the right as you're sitting there.
17:28
SPEAKER_00: And if you add that movement up billions and billions of times as billions and billions of sand grains, and they're moving meters per day sometimes, depending on the wave energy, that's a lot of movement.
17:38
SPEAKER_00: That's a lot of sand being moved in one direction or the other.
17:41
SPEAKER_00: And because weather patterns change, it'll vary day to day, but there will be a consistent trajectory or consistent trend of waves hitting at an angle on average in one angle different than another.
17:54
SPEAKER_00: So sending the sand grains to the right or to the left more frequently than to the other direction, basically.
17:59
SPEAKER_00: So we get this on average yearly directionality to it.
18:03
SPEAKER_01: The bottom line is that you generate this current that runs in the direction parallel to the shore in the direction that the sand is moving in that kind of zigzag fashion.
18:14
SPEAKER_00: It's totally cool.
18:14
SPEAKER_00: Okay.
18:14
SPEAKER_00: It's totally cool.
18:15
SPEAKER_01: It is very cool.
18:16
SPEAKER_01: It is very cool.
18:17
SPEAKER_01: Well, and so what happens then is sand will move down the shoreline in the direction of longshore current.
18:25
SPEAKER_01: And when it moves across this bay or or lagoon or whatever, then it loses energy and it stops moving and it gets deposited then.
18:36
SPEAKER_01: And that deposit then that comes partway across the lagoon is called a spit.
18:42
SPEAKER_01: And what it'll do is it'll actually form a little hook that kind of curves into the lagoon.
18:47
SPEAKER_01: And that's that hook on the end of the spit is just a classic, oh, that's a spit.
18:52
SPEAKER_01: Like every geology 101 student is looking for that classic little hook that extends into the lagoon or the bay.
19:00
SPEAKER_01: All right, Jesse, we've defined what a spit is, we've defined what longshore current is.
19:04
SPEAKER_01: Finish this theory off.
19:06
SPEAKER_00: It gets really easy from where you've gotten us from a spit.
19:09
SPEAKER_00: Okay, there's all this sand is moving, let's say it's moving down a shore.
19:12
SPEAKER_00: The shore is straight, it's moving down the shore.
19:15
SPEAKER_00: If you're looking at a map view, hits some bay, forms a little spit, you get this hook going out.
19:20
SPEAKER_00: It's really easy to see how if you just continue this, you build the spit bigger and bigger and bigger and bigger, and eventually it's going to break off.
19:28
SPEAKER_00: There's, you know, big storm comes in and cuts through the spit a little bit, and then all of a sudden it's an island.
19:33
SPEAKER_00: It's a barrier island.
19:34
SPEAKER_00: And it's just sand moving in the longshore current direction.
19:37
SPEAKER_00: And so you can see how this would get extended for great distances fairly rapidly on a geologic time scale, fairly rapidly just by building these big spits, and then they get broken by wave action or a big storm comes in and just cuts through that spit, and now it's an island.
19:55
SPEAKER_00: And so I think it's a relatively simple thing to understand.
19:58
SPEAKER_01: I don't know.
19:58
SPEAKER_01: We've introduced a lot of terms though, Jessica.
20:00
SPEAKER_01: That's true.
20:01
SPEAKER_01: Good point.
20:02
SPEAKER_01: So but where these are really common is in an estuary environment.
20:08
SPEAKER_01: And then so here we go with an introduction of a new term.
20:11
SPEAKER_01: An estuary is a flooded or drowned river mouth.
20:16
SPEAKER_01: Think of the I think the most famous one in the United States is the Chesapeake Bay in Virginia.
20:22
SPEAKER_01: And and so what happens then is rising sea level can flood this river mouth and leaving this spit really stranded quite a long ways away from the mainland.
20:35
SPEAKER_01: And like you said then, as these hurricanes and these events come in and they really just dissect that linear geologic feature called a spit.
20:44
SPEAKER_01: So that's the third way that you get barrier islands.
20:49
SPEAKER_00: Yeah.
20:50
SPEAKER_00: And Chris, this is the way that I am more familiar with.
20:53
SPEAKER_00: And I think, you know, before putting this episode together, I would have said, oh, this is the dominant way.
20:57
SPEAKER_00: Like longshore current-dominated processes generate barrier islands.
21:02
SPEAKER_00: Now, one exercise that's totally fun to do is to look at the Google Earth image of especially eastern North America, and then you can view spits and barrier islands and see how these things kind of interact and see where these three models make sense and sometimes don't make sense for how a particular barrier island formed, for instance.
21:24
SPEAKER_01: Because I would disagree a little bit with you on that.
21:26
SPEAKER_01: I would say that that the offshore bar theory, number two, that we described, is I think the most common when you look at Google Earth and you look at the map, you don't see all these estuaries that are leading to barrier islands.
21:43
SPEAKER_01: You just see a relatively straightforward parallel shoreline and then these barrier islands off from it.
21:50
SPEAKER_01: You know, and so you can like look at it and and see which one plays kind of bigger into that geography.
21:58
SPEAKER_00: Yeah, that brings up a really good point.
22:00
SPEAKER_00: Chris, and this is an interesting discussion to that I think is is useful for understanding why there's a bit of debate about barrier islands is you know, the sand supply, for instance, you think of Cape Cod or like Martha's Vineyard or these types of things that have a a lot of sort of barrier islands or or they them are themselves barrier islands, a lot of that sand is like glacial sand that was deposited by the glaciers.
22:21
SPEAKER_00: And actually, why there's very few barrier islands up in Maine is there's not a lot of sand.
22:26
SPEAKER_00: It was all moved south by the glaciers.
22:28
SPEAKER_00: And so we can have, at least the way I sort of learned it, and I think the way I would have interpreted this before this episode at least is that the glacial sand got dumped there and then it was moved around by longshore current.
22:41
SPEAKER_00: Doesn't need to be right near a river mouth to form this.
22:44
SPEAKER_00: But like you pointed out, there's a bunch of other ideas to how these things form.
22:49
SPEAKER_00: And these three processes all play a different role, different proportions in formation of any given barrier island, probably.
22:56
SPEAKER_00: So I think we got those three kind of across, Chris.
23:00
SPEAKER_00: But I think the last part of this episode, we wanted to focus on like why are these super important?
23:05
SPEAKER_00: And they're beautiful, we've kind of hit on that, but why are they really important, geologically important?
23:10
SPEAKER_00: What what stands out to you in this part?
23:13
SPEAKER_01: Um what stands out to me is the name.
23:16
SPEAKER_01: I think the name is for once appropriate.
23:18
SPEAKER_01: They're called barrier islands, they act as a natural barrier for our main coastline.
23:25
SPEAKER_01: The barrier, barrier to what?
23:27
SPEAKER_01: Well, they take the brunt of storms, they take the brunt of hurricanes.
23:34
SPEAKER_01: Because when these storms, a storm surge slams into these ridges that are off from the mainland, those ridges take the energy.
23:43
SPEAKER_01: They're the ones that get reworked and just pounded by these events, and that then protects the mainland from taking the brunt of that energy.
23:54
SPEAKER_01: So I think geologically, from a human standpoint, they're very important.
23:59
SPEAKER_01: I don't know, does that make sense?
24:00
SPEAKER_00: Yeah, just in protecting.
24:01
SPEAKER_00: I I agree completely with that.
24:03
SPEAKER_00: I uh geologically, that's the best.
24:05
SPEAKER_00: And you and I have this sort of uh well, we always have a confrontational approach to biology, I think, but they are really important for biology too, because they basically do this protection thing, which also protects a whole bunch of rural habitats behind the barrier island, in the estuaries or in the bays behind the barrier island, between the barrier island and the mainland.
24:30
SPEAKER_00: These are like super, super biologically productive regions.
24:35
SPEAKER_01: That's right.
24:36
SPEAKER_01: Let's get into some of these specific habitats then.
24:39
SPEAKER_01: So these barrier islands they have a beach side to them, and the beach side of that is the ocean side of the barrier island.
24:47
SPEAKER_01: In other words, the beach is what takes the full brunt of tropical storms, storms, hurricanes, all of that kind of stuff.
24:56
SPEAKER_01: That's what gets hit the first and and takes the most of the energy.
25:00
SPEAKER_01: Behind the beaches, you get these dunes that have like they're they're very topographically diverse, right?
25:08
SPEAKER_01: I mean, some of them can be very high, you get a lot of relief, and some of them are fairly low line and so on.
25:14
SPEAKER_01: But what they do, like you just said, is they offer protection.
25:17
SPEAKER_01: These dunes offer protection and they allow soil to develop on the other side of the island, on that protected side, the side of the island that faces the mainland.
25:28
SPEAKER_01: Okay.
25:29
SPEAKER_01: And this is one of the coolest things about these barrier islands, I think, is the that diversity of flora and fauna that you can get in these very protected environments.
25:40
SPEAKER_00: Yeah, they're so, so, so, so cool.
25:44
SPEAKER_00: And I think that the sort of back barrier side on the mainland side, I guess, the back barrier.
25:50
SPEAKER_00: So you're you're you're facing the mainland, your back is to the ocean.
25:54
SPEAKER_00: Storms can wash water and sand over the whole thing.
25:58
SPEAKER_00: So they can go over the barrier island, wash a ton of crap into that lagoon there, and disturb the ecosystem.
26:05
SPEAKER_00: And this is part of this dune migration thing, but it really protects, like you, you don't do that very frequently, you don't do that very often.
26:13
SPEAKER_00: When it does happen, it's kind of catastrophic.
26:15
SPEAKER_00: But you know, that's a relatively uncommon phenomenon, and so it's just this protective barrier to all of the biological diversity that really likes the shallow, muddy water.
26:26
SPEAKER_01: And on that other side, on the mainland side of the barrier island, you get one of my favorite places on the planet, really, uh, which is the salt marsh environment.
26:36
SPEAKER_00: Um you you do strike me as a salt marshy guy a little bit.
26:39
SPEAKER_00: You like wading around crabbing in the salt marsh or something, don't you?
26:43
SPEAKER_01: I I love it because you get the smell of it.
26:46
SPEAKER_01: Have you like, I don't know.
26:47
SPEAKER_01: I to me, this is comfort food, honestly.
26:49
SPEAKER_01: It really is.
26:50
SPEAKER_01: The time that I spend, like in the early, early morning hours going for a walk in the salt marsh is one of my favorite things to do, actually.
26:59
SPEAKER_01: If you're in, you know, Louisiana or Georgia, where I've done a lot of salt marsh walking before.
27:05
SPEAKER_01: This is where you have lots of alligators and you get snakes and pelicans and just this huge diversity of exotic life.
27:14
SPEAKER_01: I mean, I'm from Michigan, okay.
27:15
SPEAKER_01: This is this is not the common kind of stuff, but I I absolutely I I love it.
27:21
SPEAKER_01: And that's on the mainland side of the Barrier Island where you get this environment.
27:25
SPEAKER_01: And you know what else too, Jesse, is that they serve a very geologically important function too, is that they protect our wetlands on the mainland.
27:35
SPEAKER_01: Okay.
27:36
SPEAKER_01: And in other words, these wetlands are they're it's made up of loose, unconsolidated material as well, just like the barrier island is itself.
27:43
SPEAKER_01: And so if you didn't have the barrier island protection that takes the brunt of the energy, then our wetlands, they would be the ones that get reworked by all of these storms and and they'd just be rapidly disappearing, even more so than they already are.
27:59
SPEAKER_00: Absolutely.
28:00
SPEAKER_00: And I think so.
28:03
SPEAKER_00: One thing to kind of close this loop, I think, Chris, is to talk about the reason that these things are protecting is because they have this big emergent sandbar.
28:13
SPEAKER_00: They're a big sand dune that's sticking up above sea level and it runs parallel to the mainland.
28:18
SPEAKER_00: And so the value is that it's sticking up and that it's sand, it's kind of harder to move.
28:23
SPEAKER_00: It does not mean that they're permanent.
28:24
SPEAKER_00: These things move all the time, they're really kind of ephemeral.
28:28
SPEAKER_00: And so I think we should end with talking about nourishment of these things and maintenance of these things.
28:32
SPEAKER_00: Because there's two, I would say, categories of risk to barrier islands.
28:38
SPEAKER_00: Well, maybe three.
28:39
SPEAKER_00: The first one is kind of natural that they move a lot, and we can touch on that at the end.
28:43
SPEAKER_00: But the other two, the ones that are like human-induced changes to barrier island activity, one would be limiting sand movement, and the other one would be sea level rise.
28:53
SPEAKER_00: Real quickly, how do you view these two?
28:56
SPEAKER_01: Yeah, that's a really good point, first of all.
28:59
SPEAKER_01: I I think an appropriate time to bring this in.
29:02
SPEAKER_01: When you think about how would we have affected the sand supply because that's what they need, right?
29:08
SPEAKER_01: That's the number one thing that we said that all barrier islands need is a sufficient sand supply.
29:13
SPEAKER_01: How would we have possibly have affected this?
29:16
SPEAKER_01: Well, think about what we do to rivers almost always is we levy rivers and we levy them out into the ocean, in fact.
29:26
SPEAKER_01: Which means then that the the wetlands and these barrier islands, they're not gonna get the gift of the river, right?
29:32
SPEAKER_01: Geologically, the gift of the river is sediment.
29:35
SPEAKER_01: And we're directing the river by levying it where to push the sediment.
29:41
SPEAKER_01: Okay.
29:42
SPEAKER_01: And so we can then impact the future of barrier islands by maybe diverting the sediment supply from one place to another.
29:52
SPEAKER_01: I think that's where my mind goes.
29:53
SPEAKER_00: What do you think?
29:54
SPEAKER_00: Absolutely absolutely, I agree completely with the sort of sediment supply side.
29:58
SPEAKER_00: Whenever we put a big pier out, that sand piles up on one side and is depleted on the other side, and that's just a very small representation of what you're describing, cutting off the sand supply from the river, really, all the way out along the coast, and sand that would eventually be moved over thousands of years, over hundreds of years, maybe even all the way along the coast by this longshore current.
30:21
SPEAKER_00: So that's definitely, I would say, the first human-induced sort of sand supply issue.
30:27
SPEAKER_00: The second one is sea level rise.
30:28
SPEAKER_00: And the way to think about this is that average wave energy is super important for the movement and maintenance of barrier islands.
30:36
SPEAKER_00: And so where that average wave energy is, like we talked about before, if you raise sea level a little bit or drop it a little bit, you're changing the height of that wave energy.
30:46
SPEAKER_00: And since sand is so sensitive to where the waves are crashing and how high they're crashing, basically, if we just increase sea level a little bit, it'll start pushing sand in more.
30:56
SPEAKER_00: More wave energy is crashing higher up on those dunes, which is moving it, and it's more likely a big storm will come and crest over the barrier island and move a ton of sand towards the mainland.
31:07
SPEAKER_00: And so these barrier islands are quote unquote retreating away from the ocean side and towards the mainland side at a decent clip because of sea level rise, even a little bit of sea level rise.
31:18
SPEAKER_01: And that leads to what you you mentioned, which again is another term.
31:22
SPEAKER_01: You said beach nourishment.
31:24
SPEAKER_01: If we take barrier islands and and we build on them, then we can't just have the barrier island migrating and moving like you just described.
31:32
SPEAKER_01: And so the most important feature of a barrier island to human development is the beach, right?
31:40
SPEAKER_01: I mean, we if we don't have a beach, then we don't have anything to have structures on.
31:44
SPEAKER_01: And so when storms take sand and move it, and they can dramatically affect a barrier island with one event, okay.
31:53
SPEAKER_01: I mean, it can be a huge dramatic effect.
31:55
SPEAKER_01: We saw this this fall in Sanibel Island in Florida, just devastating.
32:00
SPEAKER_01: We need to renourish that sand.
32:02
SPEAKER_01: And and what we're really talking about is just dredging sand with the huge pipes and pumps and so on, and taking sand from a little bit further offshore and literally dumping it back onto the where the beach should be, where we think the beach should be, and then just reworking it with heavy equipment.
32:20
SPEAKER_01: It's called beach nourishment.
32:21
SPEAKER_01: We're just uh filling the beach back up essentially.
32:25
SPEAKER_00: Yeah, exactly.
32:26
SPEAKER_00: It's an interesting problem, and I think this really points to it's a it's a tough one for me because the Barry Island is such a naturally active environment.
32:36
SPEAKER_00: We as humans kind of want to force it to do what we want it to do, but it's naturally such an active environment that we kind of have to we have to work with it a little bit, and we have to figure out this balance of how much do we artificially maintain, how much do we let nature take its course, how much are we inducing by climate change and sea level rise, and how much do we need to worry about that moving things around and changing the dynamics of these?
32:57
SPEAKER_00: It's just a super complicated problem, and and I don't know how best to approach it.
33:02
SPEAKER_01: So Yeah, I I don't either.
33:05
SPEAKER_01: Um, I guess something that you and I get a little irritated about, I think, is you know, when you think about barrier islands, they are so often, you know, they're in the news a lot, especially during storm season, hurricane season.
33:17
SPEAKER_01: They're often described as these very fragile systems.
33:20
SPEAKER_01: And and geologically we look at this and we think, well, not really.
33:23
SPEAKER_01: They've been around for thousands of years, and they're really not that fragile.
33:27
SPEAKER_01: So where does that come from?
33:30
SPEAKER_01: Because geologically, you and I agree that well, they're not really fragile ecosystems.
33:35
SPEAKER_00: They're constantly moving for sure, and they're maybe fragile when we think about climate change, like huge global scale changes.
33:42
SPEAKER_00: If sea level rises, human-induced sea level rise, then they become fragile ecosystems.
33:47
SPEAKER_00: But they're not naturally fragile places.
33:50
SPEAKER_00: They're moving and they're sort of ephemeral, but they're not fragile.
33:52
SPEAKER_00: And so this was summarized really well with a quote.
33:55
SPEAKER_00: So I'll just read a direct quote.
33:56
SPEAKER_00: This is from Patrick Lynch here, and I kind of paraphrase a little bit, but he writes sand islands are almost as fluidly changeable as the seas around them.
34:04
SPEAKER_00: Only when we build fixed structures on these dynamic variable strands do the islands begin to seem frail.
34:11
SPEAKER_00: In reality, it is our buildings, roads, and bridges that are fragile when we construct them on shifting sand.
34:17
SPEAKER_00: And Chris, I just kind of love that quote.
34:19
SPEAKER_00: Like I think that frames at least my attitude, and I think kind of yours as well, uh, about this.
34:26
SPEAKER_01: I could not think of a better way to end this episode with that quote.
34:32
SPEAKER_01: I think it stands alone.
34:33
SPEAKER_01: I'm serious.
34:34
SPEAKER_01: It it it sums it up about as well as I've ever seen it.
34:38
SPEAKER_00: And I think you know, it can it frames the problem.
34:40
SPEAKER_00: Like it is a complicated problem here to consider what rules should we impose on buildings on barrier islands?
34:47
SPEAKER_00: Should we just you know outlaw it or or how should we think about this, right?
34:52
SPEAKER_00: Like there's a huge range of scenarios to this.
34:54
SPEAKER_00: So anyway.
34:56
SPEAKER_01: Hey, real quick, I got one thing that I want to talk about.
34:59
SPEAKER_01: Jenny and I went kayaking in the bayou on Friday, and we pulled off along a um, it was the inside of a bend in the in the river, and so there's this nice sandy beach there, right?
35:11
SPEAKER_01: So I pulled the kayak up onto the beach, and I'm standing, you know, maybe three or four feet away from the water's edge.
35:19
SPEAKER_01: And I'm on solid, moist sand, right?
35:23
SPEAKER_01: And then I shifted my feet a little bit, and I noticed that my left foot started to sink into the sand and was soon covered in this really, really wet mud.
35:35
SPEAKER_01: And I'm like, hey, Jenny, get over here.
35:38
SPEAKER_01: You gotta check this out.
35:39
SPEAKER_01: So I moved to a different part of the beach of the shoreline, and I'm like, this is what happens with liquefaction when you take this place that has a very high water table and it's built on loose, unconsolidated sand, and you put weight on it, like buildings and structures and things like this, and then an earthquake comes along right in a Jenny bull ice, and you put weight on it and you jiggle the sand with an earthquake.
36:07
SPEAKER_01: An earthquake happens, right?
36:09
SPEAKER_01: And the buildings sink down into it because the water table rises up, and it was like so.
36:16
SPEAKER_01: We actually got our phone out, and I did a little five-minute um lesson on liquefaction on a beach in Louisiana.
36:24
SPEAKER_01: Very, very cool.
36:25
SPEAKER_00: That's great.
36:26
SPEAKER_00: Yeah, that's a great example.
36:27
SPEAKER_00: Just for another term, unrelated to Barry Islands, but we'll just throw another geological term that we've talked about into the mix here.
36:34
SPEAKER_00: Totally unrelated.
36:35
SPEAKER_01: But I but my point is is look, the geology is everywhere.
36:39
SPEAKER_01: Geology makes the world go round.
36:42
SPEAKER_00: Yes, it is everywhere.
36:43
SPEAKER_00: And you know, pay attention and you'll see geology around you.
36:46
SPEAKER_00: It's totally cool.
36:47
SPEAKER_00: Even on vacation.
36:48
SPEAKER_00: So good.
36:50
SPEAKER_00: All right, Chris.
36:50
SPEAKER_00: Hey, that's a wrap.
36:51
SPEAKER_00: You can follow us on all our social medias.
36:52
SPEAKER_00: We're at Planet Geocast.
36:54
SPEAKER_00: You can like, subscribe, support us.
36:57
SPEAKER_00: You can go to our website, planetgeocast.com, shoot us an email, planetgeocast at gmail.com.
37:02
SPEAKER_00: And uh last thing, if you like learning about geology, learning the basics, this is kind of a basics thing or kind of a deep dive into barrier islands.
37:11
SPEAKER_00: But if you want to learn the basics of geoscience, just Chris, as you teach it in your hippie geology class, as I teach it, the first class you'd get in a college-level geology course, you can go to geo.campcourses.com.
37:23
SPEAKER_00: That is our audio textbook for the geosciences.
37:26
SPEAKER_00: Check it out, it's free, and uh, we would love to have some feedback.
37:30
SPEAKER_01: I have one question for you before we sign off, Jesse.
37:33
SPEAKER_01: One very important question.
37:34
SPEAKER_01: Are you ready?
37:35
SPEAKER_00: Boy, yes, I'm nervous.
37:37
SPEAKER_01: So, who has seen the most rocks in their lifetime?
37:43
SPEAKER_01: Me or you?
37:45
SPEAKER_02: Loaded question.
37:47
SPEAKER_02: Uh we've seen a lot of rocks together.
37:51
SPEAKER_02: Jesse, this is not very difficult question.
37:55
SPEAKER_02: I think it might be.
37:56
SPEAKER_02: I think it might be me.
37:58
SPEAKER_02: Please.
38:01
SPEAKER_01: Not true.
38:02
SPEAKER_01: Okay, I've seen more rocks, and you know, as the saying goes, the best geologist is the is the person that's seen the most rocks.
38:09
SPEAKER_02: So there we go.
38:11
SPEAKER_02: That's a good one.
38:12
SPEAKER_02: That actually is interesting.
38:13
SPEAKER_00: That is a famous line.
38:14
SPEAKER_00: Uh, and uh we it's used a lot, especially in the geological survey circle.
38:19
SPEAKER_00: So, like up in Canada.
38:20
SPEAKER_00: Actually, and it's very true.
38:21
SPEAKER_00: The best geologist I ever met, a woman named Val Jackson, she mapped for like 45 years in the Canadian North every summer, looking at rocks all summer long, and she knew the most about rocks of any.
38:34
SPEAKER_00: I mean, it's absolutely fit the bill for that one.
38:36
SPEAKER_00: So I don't know if I'm willing to give up that crown to you, Chris, but it's a great and very reliable quote.
38:44
SPEAKER_02: All right.
38:45
SPEAKER_02: All right, that's a wrap.
38:46
SPEAKER_00: Peace.