What rivers can tell us about the earth's history | Liz Hajek

76,653 views ・ 2017-07-28

TED


Please double-click on the English subtitles below to play the video.

00:12
All right, let's get up our picture of the earth.
0
12804
3256
00:16
The earth is pretty awesome.
1
16380
1416
00:17
I'm a geologist, so I get pretty psyched about this,
2
17820
2429
00:20
but the earth is great.
3
20273
1363
00:21
It's powerful, it's dynamic, it's constantly changing.
4
21660
3456
00:25
It's a pretty exciting place to live.
5
25140
1800
00:28
But I want to share with you guys today my perspective as a geologist
6
28100
3696
00:31
in how understanding earth's past
7
31820
2216
00:34
can help inform and guide decisions that we make today
8
34060
3496
00:37
about how to sustainably live on earth's surface.
9
37580
3080
00:41
So there's a lot of exciting things that go on on the surface of the earth.
10
41340
3736
00:45
If we zoom in here a little bit,
11
45100
1560
00:47
I want to talk to you guys a little bit about one of the things that happens.
12
47180
3656
00:50
Material get shuffled around earth's surface all the time,
13
50860
2746
00:53
and one of the big thing that happens is material from high mountains
14
53620
3236
00:56
gets eroded and transported and deposited in the sea.
15
56880
2476
00:59
And this process is ongoing all the time,
16
59380
1956
01:01
and it has huge effects on how the landscape works.
17
61370
2386
01:03
So this example here in south India --
18
63780
1896
01:05
we have some of the biggest mountains in the world,
19
65700
2496
01:08
and you can see in this satellite photo
20
68220
1976
01:10
rivers transporting material from those mountains out to the sea.
21
70220
4176
01:14
You can think of these rivers like bulldozers.
22
74420
2175
01:16
They're basically taking these mountains and pushing them down towards the sea.
23
76620
3720
01:21
We'll give you guys an example here.
24
81260
1816
01:23
So we zoom in a little bit.
25
83100
1336
01:24
I want to talk to you guys specifically about a river.
26
84460
2536
01:27
We can see these beautiful patterns that the rivers make
27
87020
2656
01:29
as they're pushing material down to the sea,
28
89700
2096
01:31
but these patterns aren't static.
29
91820
1616
01:33
These rivers are wiggling and jumping around quite a bit,
30
93460
2686
01:36
and it can have big impacts on our lives.
31
96170
1966
01:38
So an example of this is this is the Kosi River.
32
98160
2396
01:40
So the Kosi River has this nice c-shaped pathway,
33
100580
2656
01:43
and it exits the big mountains of Nepal
34
103260
2456
01:45
carrying with it a ton of material,
35
105740
2136
01:47
a lot of sediments that's being eroded from the high mountains,
36
107900
2966
01:50
and it spreads out across India
37
110890
1856
01:52
and moves this material.
38
112770
1320
01:54
So we're going to zoom in to this area
39
114380
2296
01:56
and I'm going to tell you a little bit about what happened with the Kosi.
40
116700
3429
02:00
It's an example of how dynamic these systems can be.
41
120153
2563
02:02
So this is a satellite image from August of 2008,
42
122740
3576
02:06
and this satellite image is colored
43
126340
1696
02:08
so that vegetations or plants show up as green
44
128060
2176
02:10
and water shows up as blue.
45
130260
1656
02:11
So here again you can see that c-shaped pathway
46
131940
3536
02:15
that this river takes as it exits Nepal.
47
135500
1920
02:17
And now this is monsoon season.
48
137940
2016
02:19
August is monsoon season in this region of the world,
49
139980
2496
02:22
and anyone that lives near a river is no stranger to flooding
50
142500
2896
02:25
and the hazards and inconveniences at minimum that are associated with that.
51
145420
3572
02:29
But something interesting happened in 2008,
52
149260
2256
02:31
and this river moved in a way that's very different.
53
151540
2896
02:34
It flooded in a way that's very different than it normally does.
54
154460
3000
02:37
So the Kosi River is flowing down here,
55
157484
2752
02:40
but sometimes as these rivers are bulldozing sediment,
56
160260
2576
02:42
they kind of get clogged,
57
162860
1256
02:44
and these clogs can actually cause the rivers
58
164140
2136
02:46
to shift their course dramatically.
59
166300
1696
02:48
So this satellite image is from just two weeks later.
60
168020
2856
02:50
Here's the previous pathway,
61
170900
1936
02:52
that c-shaped pathway,
62
172860
1616
02:54
and you notice it's not blue anymore.
63
174500
2096
02:56
But now what we have is this blue pathway
64
176620
1976
02:58
that cuts down the middle of the field of view here.
65
178620
2456
03:01
What happened is the Kosi River jumped its banks,
66
181100
2336
03:03
and for reference, the scale bar here is 40 miles.
67
183460
2696
03:06
This river moved over 30 miles very abruptly.
68
186180
3520
03:10
So this river got clogged and it jumped its banks.
69
190380
3176
03:13
Here's an image from about a week later,
70
193580
2096
03:15
and you can see these are the previous pathways,
71
195700
2256
03:17
and you can see this process of river-jumping continues
72
197980
2656
03:20
as this river moves farther away from its major course.
73
200660
2572
03:23
So you can imagine in landscapes like this,
74
203740
2056
03:25
where rivers move around frequently,
75
205820
2616
03:28
it's really important to understand when, where and how they're going to jump.
76
208460
4496
03:32
But these kinds of processes also happen a lot closer to home as well.
77
212980
3880
03:37
So in the United States,
78
217460
1816
03:39
we have the Mississippi River that drains most of the continental US.
79
219300
4056
03:43
It pushes material from the Rocky Mountains
80
223380
2376
03:45
and from the Great Plains.
81
225780
1536
03:47
It drains it and moves it all the way across America
82
227340
3216
03:50
and dumps it out in the Gulf of Mexico.
83
230580
1920
03:53
So this is the course of the Mississippi that we're familiar with today,
84
233140
3456
03:56
but it didn't always flow in this direction.
85
236620
2096
03:58
If we use the geologic record,
86
238740
1656
04:00
we can reconstruct where it went in the past.
87
240420
2720
04:03
So for example, this red area here
88
243940
2296
04:06
is where we know the Mississippi River flowed and deposited material
89
246260
3536
04:09
about 4,600 years ago.
90
249820
1640
04:12
Then about 3,500 years ago it moved
91
252060
2096
04:14
to follow the course outlined here in orange.
92
254180
2376
04:16
And it kept moving and it keeps moving.
93
256580
2016
04:18
So here's about 2,000 years ago,
94
258620
1896
04:20
a thousand years ago,
95
260540
1696
04:22
700 years ago.
96
262260
1216
04:23
And it was only as recently as 500 years ago
97
263500
2416
04:25
that it occupied the pathway that we're familiar with today.
98
265940
2839
04:29
So these processes are really important,
99
269899
1937
04:31
and especially here, this delta area,
100
271860
2856
04:34
where these river-jumping events in the Mississippi
101
274740
3816
04:38
are building land at the interface of the land and the sea.
102
278580
2816
04:41
This is really valuable real estate,
103
281420
1736
04:43
and deltas like this are some of the most densely populated areas on our planet.
104
283180
5256
04:48
So understanding the dynamics of these landscapes,
105
288460
2456
04:50
how they formed and how they will continue to change in the future
106
290940
3136
04:54
is really important for the people that live there.
107
294100
2400
04:57
So rivers also wiggle.
108
297300
1536
04:58
These are sort of bigger jumps that we've been talking about.
109
298860
2896
05:01
I want to show you guys some river wiggles here.
110
301780
2256
05:04
So we're going to fly down to the Amazon River basin,
111
304060
2536
05:06
and here again we have a big river system
112
306620
2016
05:08
that is draining and moving and plowing material from the Andean Mountains,
113
308660
4376
05:13
transporting it across South America
114
313060
1816
05:14
and dumping it out into the Atlantic Ocean.
115
314900
2680
05:18
So if we zoom in here, you guys can see these nice, curvy river pathways.
116
318380
4696
05:23
Again, they're really beautiful, but again, they're not static.
117
323100
2976
05:26
These rivers wiggle around.
118
326100
1736
05:27
We can use satellite imagery over the last 30 or so years
119
327860
3616
05:31
to actually monitor how these change.
120
331500
2136
05:33
So take a minute and just watch any bend or curve in this river,
121
333660
4176
05:37
and you'll see it doesn't stay in the same place for very long.
122
337860
2976
05:40
It changes and evolves and warps its pattern.
123
340860
2280
05:44
If you look in this area in particular,
124
344660
2376
05:47
I want you guys to notice there's a sort of a loop in the river
125
347060
2976
05:50
that gets completely cut off.
126
350060
1416
05:51
It's almost like a whip cracking
127
351500
1576
05:53
and snaps off the pathway of the river at a certain spot.
128
353100
2680
05:56
So just for reference, again,
129
356180
1616
05:57
in this location, that river changed its course over four miles
130
357820
4936
06:02
over the course of a season or two.
131
362780
1680
06:04
So the landscapes that we live in on earth,
132
364900
2896
06:07
as this material is being eroded from the mountains
133
367820
2416
06:10
and transported to the sea,
134
370260
1336
06:11
are wiggling around all the time.
135
371620
1616
06:13
They're changing all the time,
136
373260
1456
06:14
and we need to be able to understand these processes
137
374740
2456
06:17
so we can manage and live sustainably on these landscapes.
138
377220
2736
06:19
But it's hard to do if the only information we have
139
379980
3056
06:23
is what's going on today at earth's surface.
140
383060
2336
06:25
Right? We don't have a lot of observations.
141
385420
2056
06:27
We only have 30 years' worth of satellite photos, for example.
142
387500
4816
06:32
We need more observations to understand these processes more.
143
392340
2896
06:35
And additionally, we need to know
144
395260
1616
06:36
how these landscapes are going to respond to changing climate
145
396900
3216
06:40
and to changing land use
146
400140
1256
06:41
as we continue to occupy and modify earth's surface.
147
401420
2840
06:44
So this is where the rocks come in.
148
404980
2536
06:47
So as rivers flow,
149
407540
2496
06:50
as they're bulldozing material from the mountains to the sea,
150
410060
2896
06:52
sometimes bits of sand and clay and rock get stuck in the ground.
151
412980
3456
06:56
And that stuff that gets stuck in the ground gets buried,
152
416460
2696
06:59
and through time, we get big, thick accumulations of sediments
153
419180
3736
07:02
that eventually turn into rocks.
154
422940
2016
07:04
What this means is that we can go to places like this,
155
424980
2576
07:07
where we see big, thick stacks of sedimentary rocks,
156
427580
2696
07:10
and go back in time
157
430300
1496
07:11
and see what the landscapes looked like in the past.
158
431820
2736
07:14
We can do this to help reconstruct
159
434580
2136
07:16
and understand how earth landscapes evolve.
160
436740
4320
07:21
This is pretty convenient, too,
161
441980
1616
07:23
because the earth has had sort of an epic history. Right?
162
443620
2976
07:26
So this video here is a reconstruction of paleogeography
163
446620
4816
07:31
for just the first 600 million years of earth's history.
164
451460
3616
07:35
So just a little bit of time here.
165
455100
1976
07:37
So as the plates move around,
166
457100
2736
07:39
we know climate has changed, sea level has changed,
167
459860
3136
07:43
we have a lot of different types of landscapes
168
463020
3656
07:46
and different types of environments that we can go back --
169
466700
2736
07:49
if we have a time machine --
170
469460
1376
07:50
we can go back and look at,
171
470860
1336
07:52
and we do indeed have a time machine
172
472220
1736
07:53
because we can look at the rocks that were deposited at these times.
173
473980
3336
07:57
So I'm going to give you an example of this
174
477340
2016
07:59
and take you to a special time in earth's past.
175
479380
2216
08:01
About 55 million years ago, there was a really abrupt warming event,
176
481620
3216
08:04
and what happened was a whole bunch of carbon dioxide
177
484860
2496
08:07
was released into earth's atmosphere,
178
487380
1816
08:09
and it caused a rapid and pretty extreme global warming event.
179
489220
3856
08:13
And when I say warm, I mean pretty warm,
180
493100
2656
08:15
that there were things like crocodiles and palm trees
181
495780
2976
08:18
as far north as Canada and as far south as Patagonia.
182
498780
3016
08:21
So this was a pretty warm time and it happened really abruptly.
183
501820
3176
08:25
So what we can do
184
505020
1216
08:26
is we can go back and find rocks that were deposited at this time
185
506260
3056
08:29
and reconstruct how the landscape changed in response to this warming event.
186
509340
3600
08:33
So here, yay, rocks.
187
513380
1896
08:35
(Laughter)
188
515300
2336
08:37
Here's a pile of rocks.
189
517660
1736
08:39
This yellow blob here,
190
519420
1616
08:41
this is actually a fossil river,
191
521060
1736
08:42
so just like this cartoon I showed,
192
522820
1667
08:44
these are deposits that were laid down 55 million years ago.
193
524511
3285
08:47
As geologists, we can go and look at these up close
194
527820
3096
08:50
and reconstruct the landscape.
195
530940
1429
08:53
So here's another example.
196
533060
1496
08:54
The yellow blob here is a fossil river.
197
534580
2456
08:57
Here's another one above it.
198
537060
1536
08:58
We can go and look in detail and make measurements and observations,
199
538620
3296
09:01
and we can measure features.
200
541940
1376
09:03
For example, the features I just highlighted there
201
543340
2376
09:05
tell us that this particular river was probably about three feet deep.
202
545740
3456
09:09
You could wade across this cute little stream
203
549220
2096
09:11
if you were walking around 55 million years ago.
204
551340
2240
09:14
The reddish stuff that's above and below those channels,
205
554300
2896
09:17
those are ancient soil deposits.
206
557220
1776
09:19
So we can look at those to tell us what lived and grew on the landscape
207
559020
3856
09:22
and to understand how these rivers were interacting with their floodplains.
208
562900
3560
09:27
So we can look in detail and reconstruct with some specificity
209
567500
4376
09:31
how these rivers flowed and what the landscapes looked like.
210
571900
2856
09:34
So when we do this for this particular place
211
574780
2480
09:37
at this time,
212
577980
1216
09:39
if we look what happened before this abrupt warming event,
213
579220
2736
09:41
the rivers kind of carved their way down from the mountains to the sea,
214
581980
3616
09:45
and they looked maybe similar to what I showed you in the Amazon River basin.
215
585620
5136
09:50
But right at the onset of this climate change event,
216
590780
2456
09:53
the rivers change dramatically.
217
593260
1480
09:55
All of a sudden they got much broader,
218
595300
1856
09:57
and they started to slide back and forth across the landscape more readily.
219
597180
3560
10:01
Eventually, the rivers reverted back to a state that was more similar
220
601700
3896
10:05
to what they would have looked like before this climate event,
221
605620
3816
10:09
but it took a long, long time.
222
609460
1640
10:12
So we can go back in earth's time and do these kinds of reconstructions
223
612060
3856
10:15
and understand how earth's landscape has changed
224
615940
2616
10:18
in response to a climate event like this or a land use event.
225
618580
3776
10:22
So some of the ways that rivers change
226
622380
2096
10:24
or the reasons that rivers change their pattern and their movements
227
624500
4776
10:29
is because of things like with extra water falling on the land's surface
228
629300
3936
10:33
when climate is hotter,
229
633260
1936
10:35
we can move more sediment and erode more sediment,
230
635220
2536
10:37
and that changes how rivers behave.
231
637780
1840
10:40
So ultimately,
232
640500
2096
10:42
as long as earth's surface is our home,
233
642620
2616
10:45
we need to carefully manage the resources and risks
234
645260
3376
10:48
associated with living in dynamic environments.
235
648660
2976
10:51
And I think the only way we can really do that sustainably
236
651660
3936
10:55
is if we include information
237
655620
2216
10:57
about how landscapes evolved and behaved in earth's past.
238
657860
4096
11:01
Thank you.
239
661980
1216
11:03
(Applause)
240
663220
4440
About this website

This site will introduce you to YouTube videos that are useful for learning English. You will see English lessons taught by top-notch teachers from around the world. Double-click on the English subtitles displayed on each video page to play the video from there. The subtitles scroll in sync with the video playback. If you have any comments or requests, please contact us using this contact form.

https://forms.gle/WvT1wiN1qDtmnspy7