Ninety five per cent of active volcanoes occur close to the major tectonic plate margins, the rest occur in intraplate locations and are often associated with hotspot activity. Despite their apparent similarity, volcanoes differ in cause, structure and effect.
95％的活火山发生在主要的板块边缘附近，其余的发生在板块内的位置，往往与热点活动有关。尽管它们表面上很相似，但火山在原因、结构和效果上都有所不同。
The nature of the three main types of magma 三种主要岩浆的性质
Rhyolitic: Rhyolitic magma is viscous (‘sticky’). High viscosity is related to high silica content (65 to 75 per cent). Silica originates from the destruction and melting of plates. Rhyolitic magma traps gas and coagulates up in the vent of the volcano. Pressure builds up over time until it is suddenly released in a catastrophic eruption. Lavas have relatively low temperatures of between 650 and 900°C, flow slowly and can damage property. Large explosive eruptions can be highly dangerous.
流纹岩型：流纹岩岩浆是粘性的（"粘性"）。高粘性与高二氧化硅含量（65%至75%）有关。硅石源于板块的破坏和熔化。流纹岩岩浆捕获气体并在火山口凝结起来。压力随着时间的推移不断增加，直到它在灾难性的爆发中突然释放。岩浆的温度相对较低，在650至900摄氏度之间，流动缓慢，可以破坏财产。大规模的爆炸性喷发可能是非常危险的。
Andesitic: Andesitic magma has intermediate viscosity, with intermediate silica content (55 to 65 per cent). Lavas have temperatures between 800 and 1000°C. Eruptions can be very destructive, especially when the volcano has been dormant and hasn’t erupted recently.
安第斯岩浆：安第斯岩浆具有中等粘度，二氧化硅含量中等（55％至65％）。岩浆的温度在800至1000℃之间。爆发可能是非常具有破坏性的，特别是当火山一直处于休眠状态，最近没有爆发的时候。
Basaltic: Basaltic magma has a low-silica content (45 to 55 per cent) and is relatively fluid because of its low viscosity (low gas content). Eruptions are effusive (non-explosive) and regular. Lavas have temperatures between 1000 and 1200°C and can flow quickly over long distances
玄武岩：玄武岩浆具有低硅含量（45%至55%），并且由于其低粘度（气体含量低）而相对流动。爆发是喷发式的（非爆炸性的）和定期的。熔岩的温度在1000至1200°C之间，可以快速流动到很远的地方。
Volcanic hazards 火山的危害
There are a range of volcanic hazards, which include the following 有一系列的火山危害，其中包括以下内容:
- nuées ardentes/pyroclastic flows 炽热的火星/碎石流
- lava flows 熔岩流
- mudflows/lahars 泥石流/拉哈斯
- pyroclastic and ash fallout 火山碎屑和火山灰掉落物
- gases/acid rain 气体/酸雨
- tephra 头岩
The most dangerous and explosive eruptions are associated with destructive plate margins and the formation of composite volcanoes. Magma that builds these volcanoes is viscous, rich in silica with high gas content.
最危险和爆炸性的喷发与破坏性的板块边缘和复合火山的形成有关。构建这些火山的岩浆是粘稠的，富含硅石，气体含量高。
Large angular fragments (volcanic blocks) and aerodynamic volcanic bombs ejected during explosive eruptions are known as pyroclasts. These volcanic rocks can crush people, buildings and cars when they fall close to the volcano.
爆炸性喷发期间喷出的大角碎片（火山块）和空气动力火山弹被称为火岩。这些火山石落在火山附近时，可以压碎人、建筑物和汽车。
Clouds of tephra and volcanic ash ejected from the volcano can rise up and form an eruption column up to 45km into the atmosphere. It can then be transported on the wind over long distances. Tephra and ash falls will cover all surfaces including buildings, roads and farms over thousands of kilometres and cause major disruption to everyday life. The weight of ash falls can collapse buildings through roof loading; this is a fairly rare occurrence and tends to happen when over ten centimetres of ash accumulates.
从火山喷出的头盖骨和火山灰云可以上升，形成喷发柱，最高可达45公里的大气。然后，它可以随风传播到很远的地方。头孢和火山灰落将覆盖所有表面，包括数千公里内的建筑物、道路和农场，并对日常生活造成重大破坏。火山灰的重量可以通过屋顶负荷使建筑物倒塌；这是一个相当罕见的情况，往往发生在火山灰积累超过10厘米的时候。
If the eruption column collapses a dangerous fluid-like pyroclastic flow composed of hot rocks, ash and volcanic gases can travel down the flanks of the volcano. Pyroclastic flows are the most destructive and deadly volcanic hazards. They can move at hurricane-force speeds of up to 200 metres per second with temperatures up to 1000°C often incinerating everything in their path.
如果喷发柱倒塌，由热岩石、火山灰和火山气体组成的危险的流体状火碎流就会沿着火山的侧翼移动。火山碎屑流是最具破坏性和致命的火山危险。它们可以以飓风般的速度移动，速度高达200米/秒，温度高达1000°C，往往会焚烧其路径上的一切。
Lahars are dangerous mudflows and they represent the second greatest threat to humans after pyroclastic flows. When water from rivers, snow-capped volcanoes or crater lakes mixes with rock fragments and volcanic ash, it combines to form fast-flowing mudflows or lahars that rush down the slopes of a volcano.
拉哈斯是危险的泥流，它们是继火成岩流之后对人类的第二大威胁。当来自河流、白雪皑皑的火山或火山口湖的水与岩石碎片和火山灰混合时，就会形成快速流动的泥流或拉哈尔斯，从火山的斜坡上冲下来。
Spatial distribution of volcanic hazards 火山灾害的空间分布
The frequency of volcanic eruptions depends largely on their location. 火山喷发的频率主要取决于其位置。
- Composite volcanoes, located on destructive plate margins, tend to erupt infrequently due to the viscous nature of rhyolitic or andesitic magma. Plates descend deep into the mantle and melt due to the surrounding heat and friction from the subduction process. The less dense magma, rich in silica formed here, rises to the surface to create composite volcanoes with a gaseous, explosive nature. The accumulating layers of lava – ash – lava – ash … gives these volcanoes their name (‘composed’ of many layers).
位于破坏性板块边缘的复合火山，由于流纹岩或安山岩岩浆的粘性，往往不经常喷发。板块下降到地幔深处，由于周围的热量和俯冲过程的摩擦而融化。在这里形成的密度较小、富含二氧化硅的岩浆上升到地表，形成具有气态、爆炸性的复合火山。熔岩-火山灰-熔岩-火山灰的累积层......使这些火山得名（"由许多层组成"）。
- Shield volcanoes, located on constructive plate margins or hotspots, tend to erupt frequently due to the fluid nature of basaltic magma. Over a hotspot, hot and runny basaltic magma rises and breaks through at the surface. The lava travels long distances before it cools forming low, wide volcanoes (like a ‘shield’ lying on the ground).
位于建设性板块边缘或热点的盾状火山，由于玄武岩浆的流体性质，往往会经常爆发。在一个热点上，热的和流动的玄武岩浆上升并在表面突破。熔岩在冷却之前走了很远的距离，形成了低而宽的火山（像一个躺在地上的 "盾牌"）。
