http://www.sciencehuman.com 科学人 网站 2016-02-13
自然界最强大的力量之一：火山
中国科技网1月27日报道（张微 编译）毫无疑问，火山是我们见证过的自然界中最强大的力量之一。简单地说，它们是发生在地壳的大规模断裂所引发的，并将炽热的熔岩、火山灰和有毒气体喷射到地表和空气中。火山活动起源于地壳深处，并在地表留下永久的印记。
火山的各具体部位都叫什么名字?除了“火山锥”(即锥形山)之外，火山有许多不同部分和层次，其中大多数是位于山区或地壳深处。因此，真正地了解火山的构成，需要我们做一些探究。
虽然火山的形状和大小不一，但是它们有某些共同的特征。接下来我们对火山的具体部分进行分类，让你了解是什么赋予了火山庞大且可怕的自然力量。
岩浆库：岩浆库是一个大型地下熔岩池，位于地壳之下。岩浆库中的熔岩处于极端压力下，能够导致围岩的破裂，为岩浆冲出地表创造出口。同时岩浆与周围的地幔密度小，这使得它能够通过地幔的裂缝渗透到地表。
当岩浆到达地表之后，它会导致火山爆发。这就是为什么许多火山位于岩浆库的上方。大多数岩浆库靠近地表，通常在1-10公里的深度。这使得它们成为地壳的一部分(地壳深度大概5-70公里)。
熔岩：熔岩是一种硅酸盐岩石，它以高温液体状态存在，在火山喷发过程中被喷射出来。熔化岩石的热量来源被称为地热能，这种能量来自地球内部，它起于地球的熔融岩浆和放射性物质的衰变。熔岩从火山口喷发时，它的温度在700到1200°C(1292～2192°F)之间。当它与空气接触以及向山下流动过程中，开始逐渐变冷变硬。
中央喷出口：火山的中央喷出口是地壳中最薄弱点，岩浆库中滚烫的岩浆从这里上升直达地表。许多火山呈现出常见的锥形就是中央喷出口的特征，在火山喷发过程中，火山灰，岩石和熔岩从中央喷出口喷射而出，然后落回火山口周围，形成一个凸起。
火山喉管：火山口最上层的部分被称为火山喉管。这里是进入火山的入口，熔岩和火山灰也是从这里喷出地面。
火山口：除了锥形构造，火山活动还可能导致地表形成圆形凹陷。火山口通常是一个圆圆的洼地，有的半径非常大，有时候非常深，形状如碗。这种类型的火山，熔岩喷口位于火山口的底部。火山口的形成是火山喷发造成的，火山岩浆在它们的喷出口周围堆积，在地面上形成的环形坑就成了所谓的火山口。
火山碎屑流：也称为火山碎屑密度流、是一种横扫而过的气体和碎屑的混合物。它的流速高达700公里/小时(450英里每小时)，气体温度约1000°C(1830°F)。火山碎屑流通常紧贴地面，从喷发地点向山下流动。
火山碎屑流的速度取决于流体的密度、火山喷发速度和火山坡度。由于其速度很快、温度很高而且向山下流动，因此它是主要的火山杀手之一，对于火山喷发地点的周围环境来说，它具有极大的破坏性和致命性。
火山灰云：火山灰由火山喷发过程中产生的微小碎片、矿物和火山玻璃组成。这些碎片通常很小，直径小于2毫米(0.079英寸)。这种类型的火山灰是由于火山爆发形成的，火山喷发时岩浆中的烟尘气体和和固体石块被岩浆分解成细微粒子而形成火山灰。岩浆碎片随后冷却，凝固成火山岩和火山玻璃的碎片。
由于它们产生时的规模和爆炸力不同，火山灰能够随风飘散到距离喷发地点几公里远的地方。火山灰云能够随着风向改变方向，因此它们对当地环境造成破坏性影响，包括对人类和动物健康造成不利影响，扰乱航空安排，破坏基础设施，影响农业和水利系统。如果岩浆接触到水，会导致水蒸发成水蒸气，从而将岩浆粉碎成细小微粒，这种情况也会产生火山灰。
火山弹：除了火山灰，火山喷发时也会有岩石团块被抛到空中。这些被称为火山弹的喷出物，它们的直径超过64毫米(2.5英寸)，是火山喷发时，火山熔岩的不同形态碎片形成的。这些岩石团块在撞击地面前，已经从喷发地点被抛射出了几公里，在快速旋转飞行过程中受到大气作用，获得气动外形(例如流线型)。
虽然这个术语是用于几厘米大的喷射物，但是火山弹有时候尺寸也很大。有记录显示，几米大的物体从火山中喷发出来，运行轨迹高达几百米。无论尺寸大小，火山弹都是重要的火山灾害，往往会造成严重的伤害和多人死亡，当然这取决于它们降落到哪里。幸运的是，这种规模的爆发并不多见。
侧火山口：在大型火山上，岩浆可以通过几个不同的出口达到地面，形成了侧火山口。在火山表面，它们被火山灰和凝固的熔岩分割开，形成了岩墙峰。在这里侵入岩体裂缝内的岩浆再结晶就形成了岩床。
次级锥：也被称为寄生火山锥，次级锥位于大型火山的次级喷口附近。是当某个火山锥形成后，在其新通道附近喷出并堆积而成新的火山锥，它们很像主火山锥的一个角。
的确，火山既强大又危险。但是，如果没有这些地质现象偶尔地冲出地表，让火、烟和火山灰云统治世界，那么我们所熟知的世界将会有所不同。更有可能的情况是，这里将是一个无任何地质活动的地方，没有地壳的变化和演变。我想我们会一致认为，这样的世界可能更安全，但是也太死气沉沉了!
英文原文：
What are the different parts of A volcano?（一座火山各部分什么样？）
Without a doubt, volcanoes are one of the most powerful forces of nature a person can bear witness to. Put simply, they are what results when a massive rupture takes place in the Earth's crust (or any planetary-mass object), spewing hot lava, volcanic ash, and toxic fumes onto the surface and air. Originating from deep within the Earth's crust, volcanoes leave a lasting mark on the landscape.
But what are the specific parts of a volcano? Aside from the "volcanic cone" (i.e. the cone-shaped mountain), a volcano has many different parts and layers, most of which are located within the mountainous region or deep within the Earth. As such, any true understanding of their makeup requires that we do a little digging (so to speak!)
While volcanoes come in a number of shapes and sizes, certain common elements can be discerned. The following gives you a general breakdown of a volcanoes specific parts, and what goes into making them such a titanic and awesome natural force.
Magma Chamber: A magma chamber is a large underground pool of molten rock sitting underneath the Earth's crust. The molten rock in such a chamber is under extreme pressure, which in time can lead to the surrounding rock fracturing, creating outlets for the magma. This, combined with the fact that the magma is less dense than the surrounding mantle, allows it to seep up to the surface through the mantle's cracks.
When it reaches the surface, it results in a volcanic eruption. Hence why many volcanoes are located above a magma chamber. Most known magma chambers are located close to the Earth's surface, usually between 1 km and 10 km deep. In geological terms, this makes them part of the Earth's crust – which ranges from 5–70 km (~3–44 miles) deep.
Lava: Lava is the silicate rock that is hot enough to be in liquid form, and which is expelled from a volcano during an eruption. The source of the heat that melts the rock is known as geothermal energy – i.e. heat generated within the Earth that is leftover from its formation and the decay of radioactive elements. When lava first erupted from a volcanic vent (see below), it comes out with a temperature of anywhere between 700 to 1,200 °C (1,292 to 2,192 °F). As it makes contact with air and flows downhill, it eventually cools and hardens.
Main Vent: A volcano's main vent is the weak point in the Earth's crust where hot magma has been able to rise from the magma chamber and reach the surface. The familiar cone-shape of many volcanoes are an indication of this, the point at which ash, rock and lava ejected during an eruption fall back to Earth around the vent to form a protrusion.
Throat: The uppermost section of the main vent is known as the volcano's throat. As the entrance to the volcano, it is from here that lava and volcanic ash are ejected.
Crater: In addition to cone structures, volcanic activity can also lead to circular depressions (aka. craters) forming in the Earth. A volcanic crater is typically a basin, circular in form, which can be large in radius and sometimes great in depth. In these cases, the lava vent is located at the bottom of the crater. They are formed during certain types of climactic eruptions, where the volcano's magma chamber empties enough for the area above it to collapse, forming what is known as a caldera.
Pyroclastic Flow: Otherwise known as a pyroclastic density current, a pyroclastic flow refers to a fast-moving current of hot gas and rock that is moving away from a volcano. Such flows can reach speeds of up to 700 km/h (450 mph), with the gas reaching temperatures of about 1,000 °C (1,830 °F). Pyroclastic flows normally hug the ground and travel downhill from their eruption site.
Their speeds depend upon the density of the current, the volcanic output rate, and the gradient of the slope. Given their speed, temperature, and the way they flow downhill, they are one of the greatest dangers associated with volcanic eruptions and are one of the primary causes of damage to structures and the local environment around an eruption site.
Ash Cloud: Volcanic ash consists of small pieces of pulverized rock, minerals and volcanic glass created during a volcanic eruption. These fragments are generally very small, measuring less than 2 mm (0.079 inches) in diameter. This sort of ash forms as a result of volcanic explosions, where dissolved gases in magma expand to the point where the magma shatters and is propelled into the atmosphere. The bits of magma then cool, solidifying into fragments of volcanic rock and glass.
Because of their size and the explosive force with which they are generated, volcanic ash is picked up by winds and dispersed up to several kilometers away from the eruption site. Due to this dispersal, ash an also have a damaging effect on the local environment, which includes negatively affecting human and animal health, disrupting aviation, disrupting infrastructure, and damaging agriculture and water systems. Ash is also produced when magma comes into contact with water, which causes the water to explosively evaporate into steam and for the magma to shatter.
Volcanic Bombs: In addition to ash, volcanic eruptions have also been known to send larger projectiles flying through the air. Known as volcanic bombs, these ejecta are defined as those that measure more than 64mm (2.5 inches) in diameter, and which are formed when a volcano ejects viscous fragments of lava during an eruption. These cool before they hit the ground, are thrown many kilometers from the eruption site, and often acquire aerodynamic shapes (i.e. streamlined in form).
While the term applies to any ejecta larger than a few centimeters, volcanic bombs can sometimes be very large. There have been recorded instances where objects measuring several meters were retrieved hundreds of meters from an eruptions. Small or large, volcanic bombs are a significant volcanic hazard and can often cause serious damage and multiple fatalities, depending on where they land. Luckily, such explosions are rare.
Secondary Vent: On large volcanoes, magma can reach the surface through several different vents. Where they reach the surface of the volcano, they form what is referred to as a secondary vent. Where they are interrupted by accumulated ash and solidified lava, they become what is known as a Dike. And where these intrude between cracks, pool and then crystallize, they form what is called a Sill.
Secondary Cone: Also known as a Parasitic Cone, secondary cones build up around secondary vents that reach the surface on larger volcanoes. As they deposit lava and ash on the exterior, they form a smaller cone, one that resembles a horn on the main cone.
Yes indeed, volcanoes are as powerful as they are dangerous. And yet, without these geological phenomena occasionally breaking through the surface and reigning down fire, smoke, and clouds of ash, the world as we know it would be a very different place. More than likely, it would be a geologically dead one, with no change or evolution in its crust. I think we can all agree that while such a world would be much safer, it would also be painfully boring!
[中科网]
相关 链接
科学家证实中国学者火山喷发理论 活火山地层发现超400℃岩浆囊体[评论]
科学专题：地震活动规律兼论滇、川、甘板块断层地震有效预测方法简介