海洋科普 | 大洋中脊

大洋中脊又称为中央海岭，是指贯穿世界四大洋、成因相同，特征相似的海底山脉系列。中洋脊为地球上最长、最宽的环球性洋中山系。在太平洋，其位置偏东，称东太平洋海隆（海岭）。大西洋中脊呈“S”形，与两岸近于平行，向北可延伸至北冰洋。印度洋中脊分 3 支， 呈“入”字形。
% G+ H2 H$ S! K( v, v1 ZMid-ocean ridges, also known as central ridge, are a series of submarine mountains that run through the world’s four oceans and have the same genesis and similar characteristics. The mid-ocean ridge is the longest and widest global mid-ocean mountain system on Earth. In the Pacific Ocean, its location is eastward, so it is called the East Pacific Rise (Ridge). The mid-Atlantic ridge is “S” shaped, nearly parallel to both coasts, and can extend northward to the Arctic Ocean. The Indian mid-ocean ridge is divided into three branches which are in the shape of “入”.
大洋中脊轴部地震和火山活动频繁，故又称活动海岭，以别于不活动的无震海岭。地震分布在中脊轴部或中央裂谷，也分布在脊轴之间的断裂带活动段落，它们构成大洋中脊地震带。震级一般不大，为浅源地震，震源机制显示为垂直于中脊走向的引张作用。
The mid-ocean ridge axis has frequent seismic and volcanic activities, so it is also known as the active ridge in order to be distinguished from the inactive non-seismic ridge. The earthquakes, constituting the mid-ocean ridge seismic zone, are distributed in the mid-ocean ridge axis or central rift valley, and also in the active section of the fracture zone between the ridge axis. The magnitude of the earthquakes is generally small and shallow, and the seismic mechanism is the induced extensional effect which is perpendicular to the strike of the mid-ocean ridge.
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图 1-1  中洋脊剖面

Fig. 1-1 Mid-ocean ridge profile

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关于大洋中脊的成因，大多采用海底扩张说和板块构造说来解释。此说认为：中脊轴部是海底扩张的中心，热地幔物质沿脊轴不断上升形成新洋壳，故中脊顶部的热流值甚 高，火山活动频繁。中脊的隆起地形实际上是脊下物质热膨胀的结果。在地幔对流带动 下，新洋壳自脊轴向两侧扩张推移。在扩张和冷却的过程中，软流圈顶部物质逐渐冷凝， 转化为岩石圈，致使岩石圈随远离脊顶而增厚。冷却凝固伴随着密度增大、体积缩小，洋底岩石圈在扩张增厚的过程中逐渐下沉，于是形成轴部高两翼低的巨大海底山系。长时间后，再变成另外的样子。
The causes of mid-ocean ridges are mostly explained by the seafloor spreading hypothesis and the theory of plate tectonics. According to this theory, the axis of the mid-ocean ridge is the center of seafloor spreading, and the hot mantle material keeps rising along the ridge axis to form new oceanic crust, so the heat flow value at the top of the mid-ocean ridge is very high and the volcanic activity is frequent. The uplifted topography of the ridge is actually the result of thermal expansion of the material under the ridge. Driven by the mantle convection, the new oceanic crust expands and pushes from both sides of the ridge axis. In the process of expansion and cooling, the material at the top of the soft current circle gradually condenses and transforms into lithosphere, resulting in the thickening of the lithosphere as it moves away from the ridge top. Cooling and solidification are accompanied by an increase in density and a decrease in volume, and the lithosphere on the ocean floor gradually sinks during the process of expansion and thickening, thus forming a huge submarine mountain system with high axes and low flanks. After a long period of time, it then changes in shape.

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图 1-2 大洋中脊的地理形成

Fig. 1-2 Geographic formation of mid-ocean ridges

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大洋中脊的热流值大于周围洋盆，由脊顶向两侧热流值逐渐降低。高热流值见于脊顶或中央裂谷带，热流平均值可达 2～3HFU 以上。翼部区的热流值接近大洋平均值。在脊顶或裂谷带附近实测的热流值高低变化很大，甚至在短距离内便相差几个数量级。这可能与热水对流作用有关。冷海水顺脊顶张裂隙渗入洋壳内部，逐渐受热，然后以热泉形式从海底排出。由于在冷、热两种海水对流的过程中要释放出大量的热，故在某些地点测得的热流值明显偏低（海底热流计测得的热流值只是总热流量中的传导部分）。在东太平洋海隆北纬 21°和加拉帕戈斯海岭顶部，由深潜器直接观察到的海底热泉，其温度高达 380°C,这进一步说明脊顶是热地幔物质上涌之处，其下可能有炽热的岩浆房。深潜器还发现热泉周围栖息着各种奇异的动物，包括长达 3 米的巨大管栖蠕虫。
/ i9 H, e  t# xThe heat flow values at mid-ocean ridges are greater than those of the surrounding ocean basins, and the heat flow values decrease gradually from the ridge top to the sides. High heat flow values are found at the ridge top or in the central rift zone, where the average heat flow can reach more than 2 to 3 HFU. The heat flow values in the flanking areas are close to the oceanic average. The measured heat flow values near ridge tops or rift zones vary greatly, even by several orders of magnitude over short distances, which may be related to the convective effect of hot water. Cold seawater seeps into the interior of the oceanic crust along the top of the ridge and is gradually heated and then discharged from the sea floor in the form of hot springs. Because of the large amount of heat released during the convection of cold and hot seawater, the measured heat flow values at some locations are significantly lower (the heat flow values measured by the seafloor heat flow meter are only the conductive part of the total heat flow). In the East Pacific Rise at 21°N and the top of the Galapagos Ridge, the seafloor hot springs directly observed by deep submersibles have temperatures as high as 380°C, which further indicates that the ridge top is where the hot mantle material is upwelling and there may be a hot magma chamber underneath. The deep submersible also found a variety of exotic animals inhabiting the hot springs, including giant tube-dwelling worms up to 3 meters long.