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Lethal Lahars
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When Mount St Helens erupted in May 1980 the world watched the largest landslide ever witnessed. It crashed down the mountainside, melting glaciers and boiling streams. It mixed with volcanic rock creating mudflows of exceptional speed and ferocity. American volcanologists had never seen anything like it. It was a rampaging monster laying waste the Washington countryside and the only word they knew for it was an Indonesian one—"lahar".
Three quarters of a cubic mile of Mt St Helens vanished in seconds and poured down the river valleys. It created a volcanic hazard whose lethal reach extended many miles from its source. Thirty miles below the volcano, Greg Drew was to witness its full force.
It was just incredible to watch the full-grown trees, the old growth trees, just be knocked down like little sticks. And it just kept rising and rising. It was the most incredible thing I think I've ever seen.
Lahars can travel at more than fifty miles an hour, and incorporate more debris on their relentless path.
There was just a tremendous crack, and the whole bridge just lifted and the whole flat surface of the bridge floated down the river on top of the mudflow. It was just amazing.
Lahars can contain up to ninety percent of solid debris—making them a thick and heavy mass that can suspend boulders, houses and forests in its flow. Many miles downstream they simply spread out and entomb everything. Gary Roggenback was to lose his first home in the mudflow.
I never dreamed it would be this bad. I thought you know, well, there'd be a mudflow and know the yard might get dirty and we'd have a little cleaning up to do. But I didn't dream that it would be what it was. Nobody did.
More than 200 houses were destroyed, leaving many people homeless. Tens of thousands of acres of prime forest, as well as, bridges, roads, and trails, were swept away.
As far as my own personal feelings, I was pretty emotional. And I still get that way, you know, it was really something. I mean, you read about this stuff and see it on the news. You know you feel for the people but unless you've actually been through it yourself, erm, it's a different feeling.
For much of its history the United States has largely been spared a major volcanic eruption. This is a wake-up call to a new and ominous threat.
In North America, Mount Rainier is especially dangerous. It carries twenty five times more ice on its slopes than Mt St Helens. The melting of a fraction of this could generate a Lahar that would threaten hundreds of square miles of Washington State. Orting, is one of the nearest towns to the mountain, with a population of over three and a half thousand.
I am aware of all the potential hazards and I still decided to move here anyway because those are the kinds of things we could work around. There's an illusion of safety. You'd never think that, you know, this is a dangerous place.
In 1990 excavators digging foundations for a new housing development made a disturbing discovery. Across the valley a local farmer also uncovered a mystery.
We've drilled three wells here and every time they've hit big cedar trees laying on their side and some of them may be eight, ten feet in diameter from the cores that come up.
The buried trees are the remains of an ancient forest, torn down by a wall of mud and rock. The whole town of Orting is a built on a Lahar flow from Mount Rainier. It has even left evidence of its power littering the surface. The boulders that we see in the Orting area are all pieces of Mount Rainier, and they've just essentially been carried to their present positions by the Lahar. This is where the boulders came from. Sunset Ampitheatre—it is a huge gash high on western flank of Mount Rainier. Five hundred years ago, a wall of broken rock broke away and tore down the river valleys forty miles away, it spread out where Orting is today. Geologists like Kevin Scott have uncovered evidence of more than fifty-five massive Lahar flows from Mount Rainier.
We know that there has been a really extensive record of lahars from Mount Rainier based on their deposits that have extended well over a hundred kilometers from the volcano.
The largest flowed as far as to Puget Sound as far as the cities of Tacoma and Seattle. Scientists are working high on Mount Rainier to try and discover if such a Lahar could happen again. They are uncovering the mountain's terrible secrets.
It's big mountain so it gives the impression initially that it's very strong and solid but in fact, it's a very weak volcano and it's collapsed many times because it's weak.
Its weakness lies in its lethal chemistry. Deep beneath its snow it is still an active volcano, and subterranean heat is constantly melting its glaciers from below.
As you have these hot fluids, migrating through the volcano, it changes the rock, from a strong, volcanic rock into a very weak rock that doesn't have much strength so it can't support the outside-unaltered parts of the volcano. And that is where the volcano can collapse.
And unleash a Lahar. An eruption can do it .
But the lahar Orting is built on was not caused by an eruption. Perhaps it was an earthquake. Perhaps the volcano rotted from within and simply gave way. No one knows for sure. They only know that if it happens again, there could be only one-hour warning for the towns below. This is Orings's early warning system. It waits patiently near the volcano's base. One day it will pick up the vibrations of a landslide and transmit a final vital warning signal, before it is itself destroyed by the Lahar.
We hope that we can provide at least an hour's warning to the residents who live in the populated valleys downstream.
Today thousands of families live in the pathways of the next big Lahar, and new homes are springing up all the time.
It's basically coming down as a wall, which just totally wipes everything clean.
Orting has an evacuation plan. The fire service will have one hour to warn and clear a valley of 5,000 people, before the river of rock hits the town.
We haven't practiced it exactly but we plan to grab our backpack, meet at the front door and we're gonna hike out of this area to the nearest ridge which is less than approximately a mile away. No one knows how smoothly the urgent evacuation will go.
Our first measure is to get everybody together, get them in the truck and get out the best we can. I don't know how successful we are going to be, because there's only one way out.
Some may not even hear the warning at all. I might hear the lunch siren but the other sirens I've never heard except for when they tested it one time at the end of my road. I just don't think it's going to happen in my lifetime. Maybe my grandchildren might worry about it, but I'm not.
Oring officials worry about it enough to test its alarm system on the first Monday of every month. It's like a wake-up call. We would never want to think that we are putting our family in jeopardy by living here.
It seems so hard to believe that something that we love so much could eventually be so threatening.
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泥石流
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1980年5月圣海伦斯火山爆发时，世界目睹了历史上最大的山崩。山体崩塌，冰川被熔化，溪流沸腾。
崩塌的山体裹挟火山岩块，形成泥石流，速度极快，来势汹汹。美国的火山学家从未见过这种情形，它犹如任意肆虐的怪兽，把华盛顿周围的乡村夷为废墟，它的名字来源于印尼语，"Lahar" 火山泥流。
3/4立方英里的山体瞬间崩塌，冲向河谷下游。致命的泥流带来极大的威胁，甚至影响着离源头很远的地区。
在火山下游30英里的地方，格雷.迪恩目睹了泥流的威力。
“这真是难以置信，长成多年的大树像嫩树枝一样被冲倒，泥流不断地上涨，这是我最难以置信的经历。”
泥流以超过50英里/小时的速度在流动中不断裹挟更多的碎屑。
“我听见巨大的撞击声，整座桥被举起，整个桥面浮在泥流上向下游移动，真是让人吃惊。”
泥流中，大约90%是固体碎屑，所以又稠又重，足以让巨石、房屋和森林悬浮其中。泥流扩展到下游很远的地方，吞噬一切。盖瑞.罗杰贝克在这次火山泥流中失去了他的第一处住所。
“我做梦也没想到会这么糟糕，我以为不过是一次泥石流，院子会被弄脏，我们要打扫打扫。想不到是这样的火山泥流，所有人都没想到。”
200多栋房屋被摧毁，许多人无家可归。方圆数十万英亩的原始森林以及桥梁、道路全被冲垮。
“那时候我觉得很悲伤，现在仍然是这样。那是一场灾难，你可能看到了有关的新闻，并且有所触动。但除非是亲身经历，那种感觉是不同的。”
在美国的历史上本来没有大的火山爆发，圣海伦斯火山的爆发让人们警醒，并意识到这种可怕的威胁。
在北美，瑞尼尔火山也极为危险。它山体上覆盖的冰川是圣海伦斯火山的25倍，一小部分冰川的融化就能引发泥流，危及华盛顿州上千平方英里的地区。
Orting镇是离瑞尼尔山最近的小镇，有3500多人住在这里。
“我知道可能发生的危险，但我仍决定搬到这里。因为我们能对付那些危险。”
“这里似乎很安宁，真想不到这是个危险的地方。”
1990年，人们在挖掘地基时发现了令人不安的东西。山谷那边，当地的一个农民也发现了秘密。
“我们在这边挖了三口井，每一次都在井边挖到横躺的雪松，有几棵的直径有8至10英尺。这些埋于地下的树很久以前是古森林，一次火山泥流吞没了那片森林。”
而整个Orting镇就是建在瑞尼尔山的泥流上，现在还能看到灾难的证据，它们零乱地分布在地面上。Orting地区的巨石都是那时从瑞尼尔山上崩落的，它们被泥流冲到了现在的位置。
巨石就是从这里被冲走的。Sunset峡谷是瑞尼尔山西侧一道巨大的沟壑，五百年前一大片岩石碎裂崩落，引发的泥流沿河谷一直蔓延到40英里外Orting镇的位置。地质学家凯文.斯科特已经找到证据证实，瑞尼尔火山曾引发超过55次的泥流。
“这里泥流的痕迹延伸极广，因为泥流沉积物甚至延展到距火山100公里处，最大的那次一直流到Puget Sound、Tacoma城和西雅图。”
科学家们来到瑞尼尔火山的高处，确定火山泥流再次发生的可能。他们在揭开可怕的秘密。
“这座山很大，所以，最初它给人的印象是强大而坚固，但事实上它非常脆弱，曾经多次坍塌。”
它内部的化学反应是脆弱的根源。它是被深层冰雪覆盖的活火山，地热不断溶化底层的冰川，这些炽热的液体在火山内部流动，把坚硬的火山岩变得脆弱。火山内部的岩石无力支撑外层的坚硬部分，火山就在这些地方坍塌，火山泥流就这样发生了。一次火山爆发就能引发这个过程。
但形成Orting镇地面的泥流不是火山爆发造成的，或许是因为一场地震，或许火山从内部变得脆弱而坍塌，没有人知道确切的原因。唯一可知的是如果泥流再次发生，山下的居民只有一小时的预警时间。
这是Orting镇的居民预警系统，它在火山脚下耐心等待。有一天它将感应到山崩的振动，将生死攸关的信号传送出去，然后自身被泥流吞没。
“我们希望，能够提前一小时把警报传给下游山谷里的居民。”
数千户家庭住在泥流必经的地方，而且不断有更多的家庭迁来。
“泥流会像一面巨墙移过来，毁掉一切。”
Orting镇有一个撤离计划，消防队只有一小时发出警报，并把5000名居民从河谷撤走，接着泥流就会到达这里。
“我们不曾精确地演习过，但根据计划我们会迅速拿起背包，在门口集合，然后撤到最近的山脊上，估计有一英里远。”
没有人知道紧急撤离能否顺利进行。
“第一步是集合，然后坐上卡车、尽快离开。我不知道有多大可能成功，可这是唯一的办法。”
有人甚至根本没听到警报。
“我也许能听到午餐铃，而不是别的什么铃声。除了有一次，铃声就在路那头响起。我想在我的有生之年这不会发生，也许我的孙辈会为此担心，但是我不会。”
但Orting镇的官员们却是忧心忡忡，他们在每个月的第一个星期一测试预警系统。
“那听起来就像是闹钟，我们不愿意去想，住在这里是将家人置于险境。这真是难以置信。我们深爱的地方居然如此危险。”
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