[小知识] 尼亚加拉大瀑布的成因和历史演变
尼亚加拉河的源头有许多支流，最终像大多数河流一样流入大湖；它也没有一个典型的V形山谷，而是一个后冰川切割的山谷。这与其冰河时代的历史有关。
尼亚加拉河以及它所属的整个五大湖盆地都是上一个冰河时代的遗产。 18000年前，安大略省南部被两到三公里厚的冰盖覆盖。随着冰盖向南推进，它们挖出了五大湖的盆地。然后当它们最后一次向北融化时，它们将大量的融水释放到这些盆地中。我们的水是“化石水”。不到百分之一的水是每年可再生的，其余的来自冰盖。
尼亚加拉半岛大约在12500年前摆脱了冰的覆盖。随着冰向北退缩，其融水开始流经伊利湖，尼亚加拉河和安大略湖，直至圣劳伦斯河和大西洋。最初有五个从伊利湖到安大略湖的溢洪道。最终，在Queenston-Lewiston的悬崖上，这些原始的尼亚加拉瀑布被缩减为一个。从这里开始，瀑布开始通过基岩稳定侵蚀。
然而，大约在10500年前，通过地质效应的相互作用，包括交替的撤退和冰的重新进展，以及当从冰的强烈压力（等静压反弹）释放时土地的反弹，这个过程被中断。冰川融水通过安大略省北部，绕过南部路线。在接下来的5000年里，伊利湖的面积只有今天的一半，尼亚加拉河的面积减少到目前流量的10％左右，尼亚加拉峡谷地区（Niagara Glen）的瀑布大幅减少。
大约5500年前，融化的水再次穿过安大略省南部，恢复了河流并完全降落。然后瀑布到达了尼亚加拉漩涡（Whirlpool Rapids）。
这是一次短暂和暴力的相遇：一个持续数周，甚至只有几天的地质时刻。在这一刻，年轻的尼亚加拉河的瀑布与一条古老的河床相交，这条河床在上一个冰河时代被埋葬和封存。瀑布变成了这个被埋没的峡谷，撕裂了填满它的冰川碎片，并清理了旧河底。它现在可能不是一个瀑布，而是一个巨大的，翻腾的急流。当它全部结束时，它在我们今天称为漩涡的河流中留下了90度的转弯，以及我们今天称为漩涡急流的北美最大系列驻波。
然后瀑布在惠而浦急流桥的区域附近重新建立，并恢复穿过坚固的岩石到现在的位置。
空穴作用是一种特殊类型的侵蚀，发生在瀑布处，因为只有在瀑布的底部才有足够的速度产生足够的气泡足够接近岩石来影响它。这是最快的侵蚀类型。随着水越过瀑布，它会加速，失去内部压力，空气会像气泡或空洞一样逸出。当水停下来时，这些空洞会坍塌，向周围的岩石发出冲击波，使其瓦解。
How Old is the Falls?
The Niagara River does not start small, have many tributaries, and end big like most rivers; nor does it have a typical V-shaped valley, but rather a post-glacial incised valley; this has to do with its Ice Age history.
The Niagara River, and the entire Great Lakes Basin of which it is a part, is a legacy of the last Ice Age. 18,000 years ago, Southern Ontario was covered by ice sheets two to three kilometers thick. As the ice sheets advanced southward, they gouged out the basins of the Great Lakes. Then as they melted northward for the last time, they released vast quantities of melt water into these basins. Our water is “fossil water.” Less than one percent of it is renewable on an annual basis, the rest leftover from the ice sheets.
The Niagara Peninsula became free of the ice about 12,500 years ago. As the ice retreated northward, its melt waters began to flow down through what became Lake Erie, the Niagara River and Lake Ontario, down to the St. Lawrence River and on to the Atlantic Ocean. There were originally five spillways from Lake Erie to Lake Ontario. Eventually, these were reduced to one, the original Niagara Falls, at the escarpment at Queenston-Lewiston. From here, the falls began its steady erosion through the bedrock.
However, about 10,500 years ago, through an interplay of geological effects including alternating retreats and re-advances of the ice, and rebounding of the land when released from the intense pressure of the ice (isostatic rebound), this process was interrupted. The glacial melt waters were rerouted through Northern Ontario, bypassing the southern route. For the next 5,000 years, Lake Erie remained only half the size of today, the Niagara River was reduced to about 10 percent of its current flow, and a much-reduced falls stalled in the area of the Niagara Glen.
About 5,500 years ago, the melt waters were once again routed through Southern Ontario, restoring the river and falls to their full power. Then the falls reached the whirlpool.
It was a brief and violent encounter: a geological moment lasting only weeks, maybe even only days. In this moment, the falls of the youthful Niagara River intersected an old riverbed, one that had been buried and sealed during the last Ice Age. The falls turned into this buried gorge, tore out the glacial debris that filled it, and scoured the old river bottom clean. It was probably not a falls at all now but a huge, churning rapids. When it was all over, it left behind a 90-degree turn in the river we know today as the Whirlpool, and North America’s largest series of standing waves we know today as the Whirlpool Rapids.
The Falls then re-established at about the area of the Whirlpool Rapids Bridge and resumed carving its way through solid rock to its present location.
Cavitation is a special type of erosion that happens at waterfalls because only at the base of waterfalls is there enough speed to produce enough bubbles close enough to rock to affect it. This is the fastest type of erosion. As the water goes over the falls, it speeds up, loses internal pressure, air escapes as bubbles or cavities. These cavities collapse when the water comes to rest, sending out shock waves to the surrounding rock, disintegrating it.
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