控制山脈可以長到多高的因素是什麼？
By Josef Zens
是什麼力量與機制決定了山脈的高度？德國明斯特和波茨坦的研究人員發現了的最新答案相當驚人：決定山岳高度上限的因素不是岩石受到的侵蝕與風化作用，而是在地殼當中的作用力如何達到平衡。對於地球科學界來說這是全新且重要的基礎發現。研究論文發表於學術期刊《自然》(Nature)。
從太空中看到的安地斯山脈。安地斯山就和地球其他山脈一樣，高度皆是由構造作用力所決定。圖片來源：NASA;
Astronaut photograph ISS059-E-157
地球上最高的幾座山脈，像是喜馬拉雅山和安地斯山皆矗立於聚合型板塊邊界。在這類板塊邊界兩座板塊會朝彼此移動，其中一方會被拉到另一方之下而進到地函內部。這種隱沒作用持續進行時會讓板塊交界一再發生強烈地震，並在歷經數百萬年之後於大陸邊緣逐漸把山脈推擠出來。
山脈的高度主要是由地球內部的構造作用決定，還是由刻劃地表的侵蝕作用決定？這是地球科學當中爭議許久的問題。
德國地質科學研究中心的Armin
Dielforder領導的研究團隊最近證明河流與冰河造成的侵蝕作用對於山脈的高度沒有顯著影響。分析不同板塊邊界的強度並計算作用在板塊交界的各個力量之後，他和德國地質科學研究中心與明斯特大學的科學家解決了這項歷時已久的爭議。
研究人員得出這項驚人結論的方法是計算作用在地球不同板塊邊界的力量。他們用的數據可以提供板塊邊界強度方面的資訊。這些數據的來源像是測量地表下方的熱能如何流動。而聚合型板塊邊界的熱流又受到大陸板塊邊界的摩擦能量所影響。
我們可以運用桌巾來想像山脈如何形成。如果你把雙手放在桌巾下面然後往彼此靠攏，桌巾就會產生褶皺，但同時也會稍微往外滑過你的手背。這些隆起的褶皺就像是安地斯山脈，而桌巾往外滑過手背的作用則形同於地底的摩擦力。此外，依據岩石的特型不同也會有張力累積在地底深處，當它們釋放出來的時候就會造成大地震，在隱沒帶尤其容易發生。
研究人員從文獻中蒐集世界各地不同高度山脈之下(喜馬拉雅山、安地斯山、蘇門答臘、日本)和摩擦力有關的數據，並且計算其造成的應力以及對應到各山脈的抬升力道有多強。他們透過這種方法證明出在活躍的山脈下方，作用在板塊邊界的力量會和山脈本身重量與高度造成的力量達到平衡。
他們研究的山脈雖然位處不同的氣候帶，因此侵蝕速率也差異甚大，但每座山脈皆達到了上述的力平衡。這項結果代表山脈可以隨著地表作用而做出反應，並在侵蝕快速的時候隨之成長，藉此來維持山脈的高度與力平衡。這項在基礎層面的新發現開啟了許多機會，來讓我們更加詳細的研究山脈的長期發育與成長過程。
Which factors control the
height of mountains?
Which forces and mechanisms determine the
height of mountains? A group of researchers from Münster and Potsdam has now
found a surprising answer: It is not erosion and weathering of rocks that
determine the upper limit of mountain massifs, but rather an equilibrium of forces
in the Earth's crust. This is a fundamentally new and important finding for the
earth sciences. The researchers report on it in the scientific journal Nature.
The highest mountain ranges on Earth – such as the
Himalayas or the Andes – arise along convergent plate boundaries. At such plate
boundaries two tectonic plates move toward each other, and one of the plates is
forced beneath the other into the Earth's mantle. During this process of
subduction, strong earthquakes repeatedly occur on the plate interface, and
over millions of years mountain ranges are built at the edges of the
continents.
Whether the height of mountain ranges is mainly
determined by tectonic processes in the Earth's interior or by erosional
processes sculpturing the Earth's surface has long been debated in geosciences.
A new study led by Armin Dielforder of GFZ German
Research Centre for Geoscience now shows that erosion by rivers and glaciers
has no significant influence on the height of mountain ranges. Together with
scientists from the GFZ and the University of Münster (Germany), he resolved
the longstanding debate by analysing the strength of various plate boundaries
and calculating the forces acting along the plate interfaces.
The researchers arrived at this surprising result by
calculating the forces along different plate boundaries on the Earth. They used
data that provide information about the strength of plate boundaries. These
data are derived, for example, from heat flow measurements in the subsurface.
The heat flow at convergent plate boundaries is in turn influenced by the
frictional energy at the interfaces of the continental plates.
One can imagine the formation of mountains using a
tablecloth. If you place both hands under the cloth on the table top and push
it, the cloth folds and at the same time it slides a little over the back of
your hands. The emerging folds would correspond, for instance, to the Andes,
the sliding over the back of the hands to the friction in the underground.
Depending on the characteristics of the rock, tensions also build up in the
deep underground which are discharged in severe earthquakes, especially in
subduction zones.
The researchers collected worldwide data from the
literature on friction in the subsurface of mountain ranges of different
heights (Himalayas, Andes, Sumatra, Japan) and calculated the resulting stress
and thus the forces that lead to the uplift of the respective mountains. In
this way they showed that in active mountains the force on the plate boundary
and the forces resulting from the weight and height of the mountains are in
balance.
Such a balance of forces exists in all the mountain
ranges studied, although they are located in different climatic zones with
widely varying erosion rates. This result shows that mountain ranges are able
to react to processes on the Earth's surface and to grow with rapid erosion in
such a way that the balance of forces and the height of the mountain range are
maintained. This fundamentally new finding opens up numerous opportunities to
study the long-term development and growth of mountains in greater detail.
原始文獻：Armin Dielforder, Ralf Hetzel, Onno Oncken. Megathrust shear force
controls mountain height at convergent plate margins. Nature, 2020; 582 (7811):
225 DOI: 10.1038/s41586-020-2340-7
引用自：GFZ GeoForschungsZentrum Potsdam, Helmholtz
Centre. "What control the height of mountains? “
