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地質學中,峽灣(英語:fjord 或 fiord(i/ˈfjɔːrd/、i/fiˈɔːrd/))[1]指的是由冰川侵蝕、帶有狹長的入口和陡峭的山壁或懸崖的、與峽谷相似的地理特徵。[2]峽灣通常分布於中高緯度的海岸線,如北半球的阿拉斯加、不列顛哥倫比亞、格陵蘭、法羅群島、冰島、堪察加、紐芬蘭與拉不拉多、新地島、挪威、努納武特地區、魁北克、蘇格蘭和華盛頓;南半球則分布在南極洲、智利、凱爾蓋朗群島、紐西蘭和南喬治亞等地。[3] 其中,擁有世界第九長海岸線的挪威,在扣除掉峽灣的海岸線後,海岸線長度從原先的25,148公里大幅縮短為2,500餘公里。[4][5]
一般情況下,峽灣是因冰川流經底部略為傾斜的谷地,經冰川侵蝕產生過量下蝕的U型深谷,待冰川消融後,低於海平面、受海水入侵的谷地即為峽灣。具體來說,U型谷是冰磨損作用和冰分凝作用的產物,[6]而底部同樣是U型谷,但高於海平面的封閉水體則稱為冰蝕湖。[7] 大多數的峽灣皆較周圍的海域深,如挪威的松恩峽灣,灣內最深處達到海平面下1,300米(4,300英尺),而其相鄰的海域挪威海沿岸區域的平均深度僅達700米(2,300英尺);峽灣的入口處有時會有由基岩構成的海欄或淺灘,為冰川消融成冰水時淘選度上升,結合攜帶的沉積物及終磧產生的岩質或砂質堆積。[8] 這個構造有可能使鄰近水域漲退潮時產生急潮,如挪威的史克雅斯塔德峽灣(Skjerstad Fjord)與薩爾特海峽(Saltstraumen)的匯流處,每當海水從海峽灌入峽灣,便會產生一股被稱為薩爾特漩渦的強勁海流。另外,挪威德拉門峽灣的砂質終磧因為後冰期回彈的因素,在斯韋爾維克產生了一座高60米(200英尺)的突出物。[9] 前述特徵也成為分辨峽灣和溺灣(谷灣)的方法:溺灣是由河川侵蝕形成,被海水淹沒的海灣,其出口處通常沒有淺灘或沙洲。
19世紀時延斯·埃斯馬克提出峽灣由冰川形成及現今的北歐地區大部分被冰蓋覆蓋的理論:[10]他認為入口的淺灘及過度下蝕的谷底是支持其理論的強力證據,[11]特別是因為這些淺灘是岩質的,而非砂質的。約翰·華特·古格里則認為峽灣的成因與地質構造運動相關,並否定冰川與其的關聯性。他以哈當厄峽灣為例,指出其侵蝕方向由加里東造山運動造成的破裂缺口引導。但古格里的觀點無法解釋松恩峽灣的形成(因為無法找出地質褶皺與峽灣的關聯),因此其論點後來不被研究及出版品接受,並轉向支持埃斯馬克的理論。[10] 但是在靈恩地區仍有發現峽灣與裂口走勢相符,[12]推測是由第三紀時處於先冰期的河流侵蝕地表產生溝槽,引導冰川的侵蝕方向;此現象在挪威西岸很常見,由於伴隨著地層抬升,河川侵蝕力更強。[10]
多條支流冰川河流處將侵蝕出相對周遭區域更深的底部。典型挪威西海岸地區的冰川在離開山麓帶後,會藉由港灣(英語:sound)和低谷往低處擴散,且在過程中損失下蝕力,從而導致冰川無法突破入口淺灘(即入口淺灘會相對更淺。)波斯塔峽灣平均深度為160米(520英尺),而其淺灘只有 1.5米(4英尺11英寸)深;[7][10]平均1,300米(4,300英尺)深的松恩峽灣的入口淺灘也僅有100至200米(330至660英尺)深。[13][14] Hardangerfjord is made up of several basins separated by thresholds: The deepest basin Samlafjorden between Jonaneset (Jondal) og Ålvik with a distinct threshold at Vikingneset in Kvam.[10]
Hanging valleys are common along glaciated fjords and U-shaped valleys. A hanging valley is a tributary valley that is higher than the main valley and were created by tributary glacier flows into a glacier of larger volume. The shallower valley appears to be 'hanging' above the main valley or a fjord. Often, waterfalls form at or near the outlet of the upper valley.[15] Hanging valleys also occur under water in fjord systems. The branches of Sognefjord are for instance much shallower than the main fjord. The mouth of Fjærlandsfjord is about 400米(1,300英尺) deep while the main fjord is 1,200米(3,900英尺) nearby. The mouth of Ikjefjord is only 50 meters deep while the main fjord is around 1,300米(4,300英尺) at the same point.[11]
During the winter season there is usually little inflow of freshwater. Surface water and deeper water (down to 100米或330英尺 or more) are mixed during winter because of the steady cooling of the surface and wind. In the deep fjords there is still fresh water from the summer with less density than the saltier water along the coast. Offshore wind, common in the fjord areas during winter, sets up a current on the surface from the inner to the outer parts. This current on the surface in turn pulls dense salt water from the coast across the fjord threshold and into the deepest parts of the fjord.[16] Bolstadfjorden has a threshold of only 1.5米(4英尺11英寸) and strong inflow of freshwater from Vosso river creates a brackish surface that blocks circulation of the deep fjord. The deeper, salt layers of Bolstadfjorden are deprived of oxygen and the seabed is covered with organic material. The shallow threshold also creates a strong tidal current.[7]
During the summer season there is usually a large inflow of river water in the inner areas. This freshwater gets mixed with saltwater creating a layer of brackish water with a slightly higher surface than the ocean which in turn sets up a current from the river mouths towards the ocean. This current is gradually more salty towards the coast and right under the surface current there is a reverse current of saltier water from the coast. In the deeper parts of the fjord the cold water remaining from winter is still and separated from the atmosphere by the brackish top layer. In fjords with a shallow threshold this deep water is not replaced every year and low oxygen concentration makes the deep water unsuitable for fish and animals. In the most extreme cases there is a constant barrier of freshwater on the surface and the fjord freezes over such that there is no oxygen below the surface. Drammensfjorden is one example.[16]
The Gaupnefjorden branch of Sognefjorden is strongly affected by freshwater as a glacial river flows in. Velfjorden has little inflow of freshwater.[17]
As late as 2000, some coral reefs were discovered along the bottoms of the Norwegian fjords.[18] These reefs were found in fjords from the north of Norway to the south. The marine life on the reefs is believed to be one of the most important reasons why the Norwegian coastline is such a generous fishing ground. Since this discovery is fairly new, little research has been done. The reefs are host to thousands of lifeforms such as plankton, coral, anemones, fish, several species of shark, and many more. Most are specially adapted to life under the greater pressure of the water column above it, and the total darkness of the deep sea.[19]
New Zealand's fjords are also host to deep-water corals, but a surface layer of dark fresh water allows these corals to grow in much shallower water than usual. An underwater observatory in Milford Sound allows tourists to view them without diving.[20]
In some places near the seaward margins of areas with fjords, the ice-scoured channels are so numerous and varied in direction that the rocky coast is divided into thousands of island blocks, some large and mountainous while others are merely rocky points or rock reefs, menacing navigation. These are called skerries.[19] The term skerry is derived from the Old Norse sker, which means a rock in the sea.[21]
Skerries most commonly formed at the outlet of fjords where submerged glacially formed valleys perpendicular to the coast join with other cross valleys in a complex array. The island fringe of Norway is such a group of skerries (called a skjærgård); many of the cross fjords are so arranged that they parallel the coast and provide a protected channel behind an almost unbroken succession of mountainous islands and skerries. By this channel one can travel through a protected passage almost the entire 1,601 km(995 mi) route from Stavanger to North Cape, Norway. The Blindleia is a skerry-protected waterway that starts near Kristiansand in southern Norway, and continues past Lillesand. The Swedish coast along Bohuslän is likewise skerry guarded. The Inside Passage provides a similar route from Seattle, Washington, and Vancouver, British Columbia, to Skagway, Alaska. Yet another such skerry protected passage extends from the Straits of Magellan north for 800 km(500 mi).
An epishelf lake forms when meltwater is trapped behind a floating ice shelf and the freshwater floats on the denser saltwater below. Its surface may freeze forming an isolated ecosystem.
The word fjord comes from Norwegian (pronounced [ˈfjuːr], [ˈfjøːr], [ˈfjuːɽ] or [ˈfjøːɽ] in various dialects), where it can have a more general meaning: in many cases to refer to any long narrow body of water, inlet or channel (for example, see Oslofjord).
The Norse verb ferd (travelling/ferrying), the Norse noun substantive fjǫrðr means a "lake-like" waterbody used for passage and ferrying, which is of Indo-European origin (*prtús from *por- or *per).[22]
The Scandinavian fjord, Proto-Scandinavian *ferþuz, is the origin for similar Germanic words: Icelandic fjörður, Swedish fjärd (for Baltic waterbodies), Scots firth.[22] The Norse noun fjǫrðr was adopted in German as Förde, used for the narrow long bays of Schleswig-Holstein, and in English as firth "fjord, river mouth". The English word ford (compare German Furt, Low German Ford or Vörde, in Dutch names voorde such as Vilvoorde, Ancient Greek πόρος, poros, and Latin portus) is assumed to originate from Germanic ferþu- and Indo-European root *pertu- meaning "crossing point". Fjord/firth/Förde as well as ford/Furt/Vörde/voorde refer to a Germanic noun for a travel: North Germanic ferd or färd and of the verb to travel, Dutch varen, German fahren; English to fare.[23]
As a loanword from Norwegian, it is one of the few words in the English language to start with the sequence fj.[24] The word was for a long time normally rendered fiord,[25] a spelling preserved in place names such as Grise Fiord, but now generally current only in New Zealand English.
The use of the word fjord in Norwegian, Danish and Swedish is more general than in English and in international scientific terminology. In Scandinavia, fjord is used for a narrow inlet of the sea in Norway, Denmark and western Sweden, but this is not its only application. In Norway and Iceland, the usage is closest to the Old Norse, with fjord used for both a firth and for a long, narrow inlet. In eastern Norway, the term is also applied to long narrow freshwater lakes (for instance Mjøsa [commonly referred to as fjorden], Randsfjorden and Tyrifjorden) and sometimes even to rivers (in local usage, for instance in Flå in Hallingdal, the Hallingdal river is referred to as fjorden). In southeast Sweden, the name fjard fjärd is a subdivision of the term 'fjord' used for bays, bights and narrow inlets on the Swedish Baltic Sea coast, and in most Swedish lakes. This latter term is also used for bodies of water off the coast of Finland where Finland Swedish is spoken. In Danish, the word may even apply to shallow lagoons. In modern Icelandic, fjörður is still used with the broader meaning of firth or inlet. In Faroese fjørður is used both about inlets and about broader sounds, whereas a narrower sound is called sund. In the Finnish language, a word vuono is used although there is only one fjord in Finland. Small waterfalls within these fjords are also used as freshwater resources for the people of Scandinavia and, in particular, Norway.
In old Norse genitive was fjarðar whereas dative was firði. The dative form has become common place names like Førde (for instance Førde), Fyrde or Førre (for instance Førre).[26]
The German use of the word Föhrde for long narrow bays on their Baltic Sea coastline, indicates a common Germanic origin of the word. The landscape consists mainly of moraine heaps. The Föhrden and some "fjords" on the east side of Jutland, Denmark are also of glacial origin. But while the glaciers digging "real" fjords moved from the mountains to the sea, in Denmark and Germany they were tongues of a huge glacier covering the basin of which is now the Baltic Sea. See Förden and East Jutland Fjorde.
Whereas fjord names mostly describe bays (though not always geological fjords), straits in the same regions typically are named Sund, in Scandinavian languages as well as in German. The word is related to "to sunder" in the meaning of "to separate". So the use of Sound to name fjords in North America and New Zealand differs from the European meaning of that word.
The name of Wexford in Ireland is originally derived from Veisafjǫrðr ("inlet of the mud flats") in Old Norse, as used by the Viking settlers—though the inlet at that place in modern terms is an estuary, not a fjord.
Before or in the early phase of Old Norse angr was another common noun for fjords and other inlets of the ocean. This word has survived only as a suffix in names of some Scandinavian fjords and has in same cases also been transferred to adjacent settlements or surrounding areas for instance Hardanger, Stavanger and Geiranger.[27][28]
The differences in usage between the English and the Scandinavian languages have contributed to confusion in the use of the term fjord. Bodies of water that are clearly fjords in Scandinavian languages are not considered fjords in English; similarly bodies of water that would clearly not be fjords in the Scandinavian sense have been named or suggested to be fjords. Examples of this confused usage follow.
The Bay of Kotor in Montenegro has been suggested by some to be a fjord, but is in fact a drowned river canyon or ria. Similarly the Lim bay in Istria, Croatia, is sometimes called "Lim fjord" although it was not carved by glacial erosion but instead is a ria dug by the river Pazinčica. The Croats call it Limski kanal, which does not translate precisely to the English equivalent either.
In the Danish language any inlet is called a fjord, but none of the fjords of Denmark may be considered a fjord in the geological sense. Limfjord in English terminology is a sound, since it separates the North Jutlandic Island (Vendsyssel-Thy) from the rest of Jutland. Ringkøbing Fjord on the western coast of Jutland is a lagoon. The long narrow fjords of Denmark's Baltic Sea coast like the German Förden were dug by ice moving from the sea upon land, while fjords in the geological sense were dug by ice moving from the mountains down to the sea.
The fjords in Finnmark (Norway), which are fjords in the Scandinavian sense of the term, are not universally considered to be fjords by the scientific community.[29] Although glacially formed, most Finnmark fjords lack the steep-sided valleys of the more southerly Norwegian fjords since the glacial pack was deep enough to cover even the high grounds when they were formed. The Oslofjord on the other hand is a rift valley, and not glacially formed.
In Acapulco, Mexico, the calanques—narrow, rocky inlets—on the western side of the city, where the famous cliff-divers perform daily, are described in the city's tourist literature as being fjords.
Some Norwegian freshwater lakes that have formed in long glacially carved valleys with sill thresholds, ice front deltas or terminal moraines blocking the outlet follow the Norwegian naming convention; they are frequently named fjords. Ice front deltas developed when the ice front was relatively stable for long time during the melting of the ice shield. The resulting landform is an isthmus between the lake and the saltwater fjord, in Norwegian called "eid" as in placename Eidfjord or Nordfjordeid. The post-glacial rebound changed these deltas into terraces up to the level of the original sea level. In Eidfjord, Eio has dug through the original delta and left a 110米(360英尺) terrace while lake is only 19米(62英尺) above sea level.[30][31] Such deposits are valuable sources of high quality building materials (sand and gravel) for houses and infrastructure.[32] Eidfjord village sits on the eid or isthmus between Eidfjordvatnet lake and Eidfjorden branch of Hardangerfjord.[33] Nordfjordeid is the isthmus with a village between Hornindalsvatnet lake and Nordfjord.[34][35] Such lakes are also denoted fjord valley lakes by geologists.[36]
One of Norway's largest is Tyrifjorden at 63米(207英尺) above sea level and an average depth at 97米(318英尺) most of the lake is under sea level. Norway's largest lake, Mjøsa, is also referred to as "the fjord" by locals.[31] Another example is the freshwater fjord Movatnet (Mo lake) that until 1743 was separated from Romarheimsfjorden by an isthmus and connected by a short river. During a flood in November 1743 the river bed eroded and sea water could flow into the lake at high tide. Eventually Movatnet became a saltwater fjord and renamed Mofjorden (Mofjorden[挪威语]).[37] Like fjords, freshwater lakes are often deep. For instance Hornindalsvatnet is at least 500米(1,600英尺) deep and water takes an average of 16 years to flow through the lake.[38] Such lakes created by glacial action are also called fjord lakes or moraine-dammed lakes.[39]
Some of these lakes were salt after the ice age but later cut off from the ocean during the post-glacial rebound.[17] At the end of the ice age Eastern Norway was about 200米(660英尺) lower (the marine limit). When the ice cap receded and allowed the ocean to fill valleys and lowlands, and lakes like Mjøsa and Tyrifjorden were part of the ocean while Drammen valley was a narrow fjord. At the time of the Vikings Drammensfjord was still four或five米(13或16英尺) higher than today and reached the town of Hokksund, while parts of what is now the city of Drammen was under water.[40] After the ice age the ocean was about 150米(490英尺) at Notodden. The ocean stretched like a fjord through Heddalsvatnet all the way to Hjartdal. Post-glacial rebound eventually separated Heddalsvatnet from the ocean and turned it into a freshwater lake.[41][42] In neolithic times Heddalsvatnet was still a saltwater fjord connected to the ocean, and was cut off from the ocean around 1500 BC.[43]
Some salt water fish got trapped in lakes that originally were part of the salt fjord and gradually became freshwater fish such as the arctic char.[44] Some freshwater fjords such as Slidrefjord are above the marine limit.
Like freshwater fjords, the continuation of fjords on land are in the same way denoted as fjord-valleys. For instance Flåmsdal (Flåm valley) and Måbødalen.[10][45][46]
Outside of Norway, the three western arms of New Zealand's Lake Te Anau are named North Fiord, Middle Fiord and South Fiord. Another freshwater "fjord" in a larger lake is Western Brook Pond, in Newfoundland's Gros Morne National Park; it is also often described as a fjord, but is actually a freshwater lake cut off from the sea, so is not a fjord in the English sense of the term. Locally they refer to it as a "landlocked fjord". Such lakes are sometimes called "fjord lakes". Okanagan Lake was the first North American lake to be so described, in 1962.[47] The bedrock there has been eroded up to 650米(2,133英尺) below sea level, which is 2,000米(6,562英尺) below the surrounding regional topography.[48] Fjord lakes are common on the inland lea of the Coast Mountains and Cascade Range; notable ones include Lake Chelan, Seton Lake, Chilko Lake, and Atlin Lake. Kootenay Lake, Slocan Lake and others in the basin of the Columbia River are also fjord-like in nature, and created by glaciation in the same way. Along the British Columbia Coast, a notable fjord-lake is Owikeno Lake, which is a freshwater extension of Rivers Inlet. Quesnel Lake, located in central British Columbia, is claimed to be the deepest fjord formed lake on Earth.
A unique family of freshwater fjords are the embayments of the North American Great Lakes. Baie Fine is located on the northwestern coast of Georgian Bay of Lake Huron in Ontario, and Huron Bay is located on the southern shore of Lake Superior in Michigan.
峽灣主要形成於冰河期曾有冰川流經的中高緯度的山區,緯度最高可抵北緯80度(斯瓦爾巴和格陵蘭)。峽灣地形通常於擁有西風吹拂的山嶽區域會較為發達,因水氣受山脈阻擋而地形舉升造成大量降雪,有助於冰川的發展;因此符合此條件的地區如挪威西岸、普吉特海灣、紐西蘭西岸、南美洲西南海岸(智利)等地大都有發現峽灣。
其餘區域亦有峽灣的存在,但可能因西風的水氣未受山岳太多阻攔等因素造成峽灣並不發達或不明顯。這些區域包括:
The longest fjords in the world are:
Deep fjords include:
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