Undersea topography - Kaiteikei

Japanese: 海底地形 - かいていちけい
Undersea topography - Kaiteikei

The undulating form of the ocean floor. Generally speaking, a topography is a structural topography formed by internal forces (tectonic movement) originating from inside the Earth, which is then transformed by external forces (erosion and deposition) caused by solar energy and gravity. On land, erosion is severe due to rivers, glaciers, waves, wind, etc., but on the ocean floor, structural topography once formed is preserved without erosion and is filled in by slow sedimentation.

The tectonic movements of the Earth are caused by plate movement deep in the crust. When a plate resting on the mantle layer moves, the relative movement that occurs at the boundary between the plate and another plate can be classified as collision, separation, or sliding past each other. At the plate boundary where the plates are moving apart, mantle material rises up near the ocean floor where the crust is thin, and the heat causes the ocean floor to bulge, forming a mid-ocean ridge.

In the Arctic, Atlantic, Indian, and Southern Oceans, the mid-ocean ridge forms a steep underwater mountain range that runs almost through the center of the ocean, and in the Pacific Ocean, it forms a wide, bulging underwater mountain range in the southeast. At the mid-ocean ridge, magma erupts or intrudes underground and cools, forming new plates. As the new plates move away from the mid-ocean ridge, they cool, contract, and sink, forming a vast, low, flat ocean floor, which is eventually covered by sediments and flattened.

At the plate boundary where the plates collide, one plate sinks under the other. This creates a trench, a long, narrow depression with steep slopes on both sides. The subducted plate melts at its depth and becomes mantle material again. At the boundary where the plates rub against each other, there is no production or consumption of plates, but they simply rub against each other, creating an earthquake zone. This is a transform fault, and both ends of the fault are connected to mid-ocean ridges or trenches. At transform faults connecting ridges, as the plates move apart, the traces of the fault extend onto the ocean floor, forming the topography of the fracture zone. Fracture zones are extremely long, narrow, linear zones accompanied by seamounts (undersea mountains), troughs (depressions that are not as steep as trenches), and undersea cliffs. The Arctic Ocean and the Atlantic Ocean do not have trenches, and the speed of plate movement is slow. The Indian Ocean has trenches in some parts.

The mid-ocean ridges of these oceans have a slow rate of spreading, and have graben-like depressions called axial valleys at their summits. On the other hand, in the Southern Ocean and Pacific Ocean, plates subduct in the trenches, and the spreading rate is fast, so the mid-ocean ridges have a bulging topography and no axial valleys are seen. Trenches are accompanied by island arcs, volcanic activity, and deep earthquakes, but there are differences in the topography and other geological phenomena of each trench. There are two theories: one that sees this as the evolution of plate subduction over time, and another that explains it by differences in the relative movement of the land and sea plates.

[Tsutoshi Sato]

[Reference] | Seafloor

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

海底の起伏などの形態。一般的にいえば、地球内部に起因する内力(構造運動)によって形成された構造地形が、太陽エネルギーと重力に起因する外力(侵食と堆積(たいせき)の作用)によって変形されたものが地形である。陸上では河川、氷河、波、風などで著しく侵食されるが、海底ではいったん生じた構造地形は侵食されずに保たれ、緩慢な堆積作用によって埋積されてゆく。

 地球の構造運動は地殻深部のプレート運動に起因する。マントル層の上にのっているプレートが移動するとき、プレートと他のプレートの境界におこる相対運動は、互いにぶつかるか、離れるか、擦れ違うかに分かれる。離れていくプレート境界ではマントル物質が地殻が薄くなった海底近くまで上昇し、その熱のために海底が膨らんで中央海嶺(かいれい)を形成する。

 中央海嶺は、北極海、大西洋、インド洋、南大洋(南極海)では大洋のほぼ中央を走る急峻(きゅうしゅん)な海底山脈をなし、太平洋では南東部に幅広く膨らんだ海底山脈となっている。中央海嶺ではマグマが噴出したり地下に貫入し冷却して新しいプレートがつくられる。新しいプレートは中央海嶺から遠ざかるにつれ冷却し収縮して沈下し、広大で低平な大洋底を形成するが、やがて堆積物で覆われ平坦(へいたん)化されていく。

 ぶつかるプレート境界では、一方のプレートは他のプレートの下へ沈み込んでいく。ここでは深くて両側に急斜面をもつ細長いへこみである海溝(かいこう)が生じる。沈み込んだプレートは深部で溶融し、ふたたびマントル物質になる。擦れ違う境界ではプレートの生産も消費もおこらず、ただこすれ合って地震帯を生じる。これがトランスフォーム断層で、その両端は中央海嶺か海溝かにつながっている。海嶺と海嶺をつなぐトランスフォーム断層では、プレートが離れていくにしたがって断層の痕跡(こんせき)が大洋底に伸び出して、断裂帯の地形を形成していく。断裂帯は海山(海底の山)、トラフtrough(海溝ほど急峻でないへこみ)、海底崖(がい)などを伴うきわめて細長い直線的な地帯である。北極海、大西洋は海溝がなく、プレート運動の速度は遅い。インド洋は一部に海溝を有する。

 これらの大洋の中央海嶺は拡大速度が遅く、山頂には中軸谷という地溝状凹地がある。一方、南大洋、太平洋では海溝でプレートの沈み込みがあり、拡大速度が速く、中央海嶺は膨らんだ海膨の地形を呈し、中軸谷もみられない。海溝では島弧、火山活動、深発地震などが伴うが、個々の海溝の地形などもろもろの地学現象には差異がある。これをプレート沈み込みの時間的進化とみる説、陸側と海側のプレートの相対的な動きの違いによって説明する説とがある。

[佐藤任弘]

[参照項目] | 海底

出典 小学館 日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について 情報 | 凡例

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