Why is it said that deep lakes are more terrifying than the deep sea?
First, let's state the conclusion:
Because no matter how deep the sea is, it will have a not-so-deep continental shelf, the coast usually has a gentle slope. However, freshwater lakes do not have a similar "shelf," so the shores of deep lakes are extremely steep.
What is the continental shelf?
Schematic diagram of the continental shelf
The continental shelf in the figure is the extension of the continent into the ocean. The slope of the sea floor on the continental shelf is very gentle, and the water depth is relatively shallow, usually not exceeding 200 meters.
On the side of a passive continental margin, where there is no active tectonic movement along the coast, the width of the continental shelf can be hundreds of kilometers. For example, in the East China Sea, the water depth is still less than 200 meters 400 kilometers away from the coast, which is within the continental shelf. The width of the continental shelf in the South China Sea is also 200 kilometers, while the entire Bohai Sea and Yellow Sea are part of the continental shelf.
Outside the continental shelf (with land on the inside) is the steeper continental slope, also known as the continental slope in the diagram, where the water depth increases over a relatively short distance. For example, on the continental slope of the South China Sea, about 200 kilometers from the coastline, the water depth increases from 200 meters to over 2000 meters within a distance of 30 kilometers.
On the side of an active continental margin, where there is active tectonic movement along the coast, the continental shelf can be very narrow, but there will still be a continental shelf. The western coastline of California in the United States is a typical example of an active continental margin. Geological activity is prominent here, and the famous California State Route 1 runs along the sea on the edge of steep cliffs. Even so, the narrowest part of the continental shelf here is still about 2 kilometers wide. Beyond 2 kilometers, the water depth is no more than 100 meters, after which the slope increases and enters the continental slope area.
It can be easily calculated that the average slope of an active continental margin is only 5%. Walking forward one meter in the water, the depth increases by 5 cm. At an average person's height, when the water depth reaches 1.5 meters, the feet can still touch the bottom. At this point, the distance from the shore is already 30 meters. In contrast, the average slope of a passive continental margin is only 1/1000 of this. In most coastal areas of our country, the water remains shallow even at a considerable distance from the shore. The coast of northern Jiangsu is so shallow that, except for Lianyungang, it is impossible to build a port.
No matter how deep the center of the sea is, people always enter the sea from the shore. Whether you are swimming or boating, when you are a considerable distance from the coast, the depth of the sea is still quite limited, so there is no reason to feel afraid.
Why is there a continental shelf?
The reason for the formation of the continental shelf is the result of sedimentation after the silt carried by rivers to the sea drifts along the coast. Sound a bit confusing? Let me explain it slowly.
All rivers that flow into the sea carry sediment. Where does the sediment go after it enters the sea? Does it sink to the bottom or float on the surface?
Xiaoyangshan Port seawater
The water of Yangshan Port, located in Shengsi County, Zhejiang Province, can be described in one word: turbid.
The service object of Yangshan Port is Shanghai, but its location is in the Zhengsui Archipelago. It is already a distance from the mouth of the Yangtze River, but the water is still turbid.
Change the satellite map to see more clearly.
Satellite photos of the southeastern coastal region of China
The sediment carried by the Yangtze River not only turns the entire Hangzhou Bay yellow but also travels southward all the way to Ningde, Fujian. The entire coastal area of Zhejiang is covered.
This is not a phenomenon unique to China; similar situations occur in rivers around the world. Below are the river mouths of the Mississippi River in the United States and the Gironde River in France, which also form plumes of turbid water that drift with the ocean currents.
Mississippi River Delta
Gironde Estuary
Why does the sediment carried by rivers float on the surface of the sea? Because seawater is salty, even river water carrying a large amount of sediment is generally lighter than seawater. Therefore, when river water flows into the sea, it will float on the surface of the seawater for a considerable distance.
Regarding coastal currents, I won't go into detail here. Just remember: the net direction of seawater flow in coastal areas is usually parallel to the coastline. This is quite easy to understand. From the principle of continuity of matter, it is clear that seawater cannot flow into the land, nor can seawater spontaneously arise from the land and flow into the sea (except at river mouths).
Under the influence of coastal currents, river water carrying sediment will drift along the shore to coastlines where no rivers flow into the sea. During this process, the sediment in the river water will gradually settle and precipitate to the seabed. Over time, this forms the continental shelf.
Due to the shallow depth of the continental shelf (not exceeding 200 meters), which is less than the maximum depth that waves can affect, waves can resuspend sediments from the bottom of the continental shelf, transport them over some distance, and then deposit them, thus making the bottom of the continental shelf relatively flat.
Why don't freshwater lakes have "jia"?
Understanding why the sea has a "rack" also helps to understand why freshwater lakes do not have a "rack." Freshwater lakes are fresh, and any river water carrying sediment that flows into a freshwater lake is heavier than the lake's water. The sediment carried by the river will not float on the surface but will sink directly to the bottom.
The mouth of the Goloustnaya River on the north shore of Lake BaikalGoloustnaya River
The image above shows a river flowing into Lake Baikal. Lake Baikal is the world's deepest lake, with a maximum depth of 1,637 meters. It can be seen that the river carries a considerable amount of sediment, forming a delta. However, it is clear that the sediment from the river does not float on the surface but sinks directly to the bottom upon entering the lake.
This means that apart from the river mouths where rivers flow into Lake Baikal, there is no littoral current carrying drift sediment along the other shores of the lake, hence no "shelf." The lake bottom slopes directly from the shore to the deep parts, making it very steep.
Baikal Lake shore
Baikal Lake shore
On one hand, the water of Lake Baikal is indeed very clear. On the other hand, it can be seen that the lake shore is entirely composed of gravel produced locally, with no silt transported from the water. Due to the refraction of light in the water, it is difficult to tell how steep the lake bottom is from the shore. After entering the water, one would find that the lake is almost bottomless. Swimming four or five meters from the shore, you can no longer touch the bottom. Swimming tens of meters into the lake, what you see through your diving mask is a deep, dark blue world. In contrast, whether at Beidaihe, Sanya, or Shenzhen Dapeng Peninsula beaches, anyone can walk into the water for twenty to thirty meters without fear, and the seawater would only just reach their chest.
About the east coast of Yizhou
First, regarding the issue of the eastern coast of Yizhou. The eastern coast of Yizhou is located at a tectonic plate boundary and is known for its steepness, with cliffs extending directly to the sea. However, it is precisely because of this that the rivers here have a very high sediment content, delivering large amounts of silt to the sea. Therefore, the continental shelf here remains very narrow.
The above image shows the seabed topography of the east coast of Yizhou provided by nautical charts online. The depth contours are at 200 meters, 500 meters, 1000 meters, 2000 meters, and 3000 meters. The horizontal scale can be referenced by the latitude on the right. One minute of latitude is equivalent to 1.85 kilometers.
It can be seen that even on the east coast of Yizhou, the continental shelf, which is mostly less than 200 meters deep, is still at least 2 kilometers wide, with an average width of 4 kilometers. This means that near the coast, the seabed slope is 5%-10%. The steepest part of the seabed is from the 200-meter depth contour to the 1000-meter depth contour. In many places, the distance between these two lines is only 3-4 kilometers. This means that the seabed slope here reaches 20%-27%.
About Hangzhou Bay
The upstream of Hangzhou Bay is of course the Qiantang River, not the Yangtze River, but the Qiantang River carries very little sediment. The sediment in Hangzhou Bay is almost entirely brought by the Yangtze River, which bypasses the Nanhui Promontory to enter Hangzhou Bay.
About Fjord
Coasts are generally elevated. However, there are exceptions, and one of these is a fjord. What is the essence of a fjord? It is a glacial U-shaped valley that, after the sea level rises and the glacier melts, is filled with seawater. The appearance of glaciers at such low altitudes must occur in high-latitude regions. Therefore, fjords are found along the coasts of high-latitude areas such as Norway, Iceland, Alaska, Greenland, Svalbard, Novaya Zemlya, southern Chile, northern Alaska and Canada, and the South Island of New Zealand.
What does a glacier look like when it hasn't melted? We can see them today in high latitude and high altitude areas. This is the Baltoro Glacier in the Karakoram Mountains:
The glacier itself is hundreds of meters thick, and the U-shaped valleys it has carved can be thousands of meters deep.
The image shows a melted glacier in Bomi, Tibet, China. The U-shaped characteristic of the valley formed by the glacier can be clearly seen: the bottom is relatively flat while the sides are very steep.
Sognefjorden is a famous fjord in Norway, over 200 kilometers long. The underwater contour lines in the image are at 100 meters, 200 meters, 400 meters, 600 meters, 800 meters, and 1000 meters. It can be seen that the underwater topography of the fjord still retains the characteristic of a U-shaped valley: steep slopes and a flat bottom. The width of the fjord is approximately 5 kilometers. It is evident that within 1 kilometer from the shore, the water depth reaches 1000 meters, with a slope of 45 degrees, and the maximum slope approaches 60 degrees, which can basically be described as vertical.
The reason is that the fjords are very young. The last ice age ended only about 10,000 years ago, which is not long enough for enough sediment to accumulate and flatten the slopes.
Is the continental shelf a plain from the Ice Age?
Some people say that the continental shelf is not formed by sedimentation of silt in the sea, but rather by plains from the Ice Age. During the last Ice Age, the sea level was about 120 meters lower than it is now. Some believe that during the interglacial period, the sea level rose to its current position, causing these plains to be submerged and become the continental shelf.
This argument seems reasonable, but it fails to explain one issue: coastal areas are not all plains, and the continental shelf is not only found off plain coasts, but is continuously distributed off almost all continental coasts and large island coasts.
Taking the coast of our country as an example, the Liao River Plain, North China Plain, Yangtze River Delta Plain, Hangzhou-Shaoxing-Ningbo Plain, Pearl River Delta Plain, Zhangzhou-Xiamen-Quanzhou Plain, western coast of Taiwan, Chao-Shan Plain, Leizhou Peninsula, and most of the coastal areas of Hainan are indeed plains. However, our country also has many coastlines that are mountainous or even cliffs, such as the famous Laoshan Mountain in Qingdao:
For example, the Sai Kung Volcanic Rock Area of the Hong Kong Geopark:
If the continental shelf is formed by the coastal plain being submerged by seawater when the sea level rises, then only areas such as the Lower Liao River Plain, North China Plain, Yangtze River Delta, and Pearl River Delta would form continental shelves. Places like Laoshan or Hong Kong's Sai Kung Volcanic Rock Region, even if the sea level rises by another 100 meters, would still be submerged slopes and would not form flat, gently sloping continental shelves.
However, the fact is that, except for the east coast of Taiwan, all of our country's coastal areas have a relatively wide continental shelf. The peak of Victoria Hill in Hong Kong is 554 meters above sea level, and it is only 2 kilometers from the peak to the coast. However, it takes 60 kilometers from the coast to reach a water depth of 50 meters. The peak of Mount Laoshan in Qingdao is 1132.7 meters above sea level, and it is only 6 kilometers from the peak to the coast. It takes 200 kilometers from the coast of Qingdao to reach a water depth of 50 meters.
In other words, whether it is the coastal plain or the mountainous coast, once you enter the sea, the slope of the terrain suddenly becomes gentle, with the steep cliffs above sea level instantly turning into a gentle continental shelf. This is clearly difficult to explain with the theory that "the continental shelf is a submerged plain from the Ice Age." The most reasonable explanation remains the one I previously proposed: the continental shelf is formed by the deposition of sediments carried by rivers and drifted along the coast.
Edited on 2024-09-05 11:45・IP Location: Beijing
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Update front row warning, the following content may make you feel uncomfortable or shocked. Please read at your own discretion. I apologize to everyone for the discomfort caused to some people due to my negligence in not giving a prior warning. I am sorry for the unpleasant reading experience caused by my foolish oversight.
17 years old personal experience.
There is an artificial lake near my house. At that time, a closed residential community was planned to be built around the lake, so the developer completely renovated the lakeside infrastructure, laying marble brick paths, building statues, and constructing pavilions. They also placed a lot of floating boards with unknown plants in the water to purify the water quality. Combined with the many fish and turtles that were already in the lake, it became very beautiful.
湖的面积不大,估摸着也就长宽70米x40米,一般的市区人工湖都是面积规模和这个差不多,深度也就三四米差不多了,但这个湖不一样,最早它是一个大坑,后来变成重工厂的排水池,最后才变成现在的人工湖,它的深度中心最深处三十米深,而且从四周向中心深度变化并不完全是有弧度递减,而是不规则的,有的地方是断崖,可能离岸边没两三米,就有10米深了,不同于天然湖还有地下水,这完完全全就是一摊死水。
我经常在湖边遛弯,没事就爱走到湖边蹲着看水底,也就不到一米深,有的地方可能一米多,但是从小到大,我都觉得就算湖边浅也是看不见水底,湖水特别是这种死水没有风的时候特别静,水面一动不动,就算那么浅的水,它也是黑色,那种黑并不是物体颜色的黑,而是像把所有的光线都吸进去,什么也逃不走的暗,用暗来形容最恰当,小时候好多大孩子在岸边抓小虾蝌蚪,我就不明白都是怎么看见的,我只能在岸边抓几只倒霉王八,然后用力往湖里扔看能扔多远,有时候还用王八打水漂。
当天下午三点多,我在湖边溜达看改造好的湖景,没事又接着蹲岸边看湖水,一样纹丝不动的水面,一样暗的没边的水底,我只能费劲的看见有一两条小虾游动,视线稍微一挪动,我看见水底有泥鳅在动,接着我看见了水底不一样的东西,像是别人放的鸟笼一样的网,离岸边一米左右 ,湖面反光加上湖水暗 ,我也看不清楚, 找了一个开放商广告旗的木杆子,给网的一头勾过来,想看看里面有没有鱼,拽起来以后发现好大的网,并不像渔网不是用在水里的那种,更像是拦什么东西的网,特别沉。
紧接拽出来三四米的时候我发现网从远处带出来东西了,根本看不清是什么,拽到岸边,漏出来一部分在水面我都看不出来,直到一条胳膊,加上漏出的上半身和脖子,我才发现是个人,我当时害怕,但是第一反应先弄上来,拽到岸边时候我发现拽不上来,后面的网应该拖住了,我那时还没有实感,做梦一样,想给他赶紧弄上来,我拽着网往侧面抻给他翻过来,脑袋漏出水面给岸边带起波纹的一瞬间我才反应过来我在捞一具尸体,我看见了他正面身体,被水泡坏了,不是胀,是浮肿一样,就像变质老面包一样,皮肤像脚皮一样的白,体型看着就是小学五六年级的孩子,男孩,有一侧腮帮子和嘴唇都没了,嘴张着舌头只剩下一小半连着随着水面晃动,脸肿的快能遮住眼睛了,眼皮也没了,只能看见里面是白色的,小胳膊也少了不少肉,被水泡的发白,伤口周围像长了毛一样的东西在飘,从嘴里,脖子,还有身上钻出来好多泥鳅还有不知名的水虫。
我怕直接拽身体强拖上来把他身体破坏了,给网缠到岸边的树干上,掏出手机报警,没几分钟警察就来了,到场先拍照,然后仔细的问我,让我指了一遍经过的所有位置,又来了一波警察以后才给他从水里带上来,佝偻着,两条胳膊抬到胸前的高度弯曲着,整个身体还有更多的伤口,法医说全是被虾和王八还有虫子咬的吃掉了,湖里的鲤鱼没法咬成这样,应该落水没几天,网也被捞上来了,特别大的网,应该是很早很早以前扔在水里的垃圾。
到了五点多,距离我发现网的位置往里面几米的位置,打捞的小船又发现了一个女人,样子更惨,上岸了以后脖子伤口里还有泥鳅。 后来警察告诉我法医鉴定应该是孩子不知什么原因落水,妈妈下水救人,也不怎么会水,被惊慌失措的孩子拖到水底,先没意识的被后没意识的混乱中挣脱开,两个人相隔一米多,孩子沉到水底斜坡以后挂到网上,要不还能往更深的地方沉,就在距离岸边几米,两个人就这这么溺毙了。
还告诉我之前一个人冬天掉进冰窟窿里,春天开化了才被发现,所以我觉得湖水是诡异的,暗到什么都看不见,那种暗能略过你所有感官探知,就像扔进水里的石头,除了落进去的水声,再也接收不到一丁点回应,就连生命消失的过程都悄无声息,就像另外一个我们无法进入的世界,我回忆我当时经历,大晴天,外面刺眼的光线,只有我一个人,周围连水面都静的没有一点波纹,特别安静,水面就像分界线,我在水面上努力想看清里面,水面下咫尺距离却有两个再也没法回来的人。
小时候去过一个堰塞湖,水很清,但也很深,表层的水是介于蓝绿之间,可见度还是很深,但因为光线不足,垂直看下去是黑色。两边是山,湖是由于山谷汇水形成,给人一种沉重的压迫感,另外湖岸很陡峭,可以直接看到崖壁从水面深入到水中深处,直到光线不足,水成黑色看不见。我在岸边走,心里特别害怕,让我喘不过气来,生怕自己掉进这深不见底的湖里,消失在黑色的深不见底的深处。
湖的可怕就在于它的静、深。虽然海要比湖深得多,但它充满生机,是运动的,让人将注意力聚焦在它的广阔和一望无际,或者是汹涌的波涛。而湖,看上去像一个无底洞,带给人一种未知的恐惧,和幽闭的压迫感。