G2 考试难点突破（自然部分）
题图说明了新的洋中脊是如何形成
（This graph shows how the new oceanic crust is created at a constructive boundary)
今天的内容集中分布在“自然环境”部分，分火山、海岸和热带雨林三个部分。
2.1 Earthquakes and Volcanoes
首先我们要明确的一点是地震和火山都是和地球内部结构中的——crust（地壳）和mantle（地幔）相关的。
The mantle is the widest section of the Earth. It has a thickness of approximately 2,900 km. The mantle is made up of semi-molten rock called magma. In the upper parts of the mantle the rock is hard, but lower down the rock is soft and beginning to melt.
The crust is the outer layer of the earth. It is a thin layer between 0-60 km thick. The crust is the solid rock layer upon which we live.
那么了解了我们所生活的地球表面并不是一整块，所以这些小块的地壳就有了一个新的名字：plate. (The Earth's crust is broken up into pieces called plates.)
当地壳不再是铁板一块的时候，这就意味着位居其下的地幔物质，熔融态的岩石在地幔中的上下流动，就可以有机会成为板块相离和相对运动的动力。
(Heat rising and falling inside the mantle creates convection currents generated by radioactive decay in the core. The convection currents move the plates. Where convection currents diverge near the Earth's crust, plates move apart. Where convection currents converge, plates move towards each other. The movement of the plates, and the activity inside the Earth, is called plate tectonics.)
在一次考试中曾经问过这样一个问题：
Explain why volcanoes erupt at the destructive plate boundaries.
首先要明确火山喷发，喷发的是什么，是magma（岩浆），就是来源于mantle（地幔）的熔融岩石。
尤其关注第三点
再来一图胜过千言万语。
这张图表示了由于位于两个不同性质板块相遇时会发生什么情况？
ocean crust/plate move towards the continental crust/plate and sink beneath it. This subduction destroys the older crust which makes the continental crust has weakness or cracks for magma to rise.
如果换一个问题，问为何多发地震呢？
The subduction also release the previous pressure balance here, then pressure imbalance cause earthquakes.
还可以在变一个角色，因为在destructive boundaries/margin 还有一种地形出现——fold mountains (但往往没有大陆板块相互碰撞产生的高）
这张图描述了具体的南美洲安第斯山脉形成的原因（板块的俯冲不仅会导致形成火山，也会形成海岸山脉）：
The continental crust is folded into fold mountains.
参考网站：
http://www.bbc.co.uk/schools/gcsebitesize/geography/natural_hazards/tectonic_plates_rev1.shtml
2.3 Coast
这部分有三个内容今天有同学提出疑问：一个关于海蚀地貌(erosion)，一个关于海积地貌(deposition)，还有一个就是海边的沙丘 (sand dunes)。
先看海蚀地貌的演变史：
其次是如何鉴别spit，解释其成因。
再接着是沙丘：sand dunes
首先沙丘应该可以说是自然地理环境中变化最快的一个，许多很大的改变可能就是一阵风的事儿。沙滩广布的海岸很容易形成沙丘因为强烈的海风会把这些沙子往陆地上一阵猛刮，而沙子一旦遇到地形的阻碍，就会形成dunes.
(a dune is a hill of loose sand built by wind or the flow of water)
而海边有这样一系列的沙丘平行排列，越靠近海的越新，越靠近陆地的越老，而生长着固沙植被的沙丘是保护海岸线重要的一部分。因为很少有植被能够在如此干旱（都是沙子）、又如此咸（盐碱地）、海风又呼呼吹着的这里生存下来。
而对于G2阶段的学生而言，第一步要先了解的是什么情况下会产生dune?
Dunes form where the beach is wide enough to allow for the accumulation of wind-blown sand, and where prevailing onshore winds tend to blow sand inland.
而与沙丘相伴而生的就是一些拓荒植物，它们进化成了适合这种气候条件和沙土的物种。
As a dune forms, plant succession occurs.
这些先锋植物“占领”了沙丘之后，就起到很好的让沙子不再继续往内陆跑的作用。随后，先锋植物让位给新的“守成”植物，相比之下此时的植物更加能够固定沙丘，而且此时的沙土也逐渐变为有一点肥沃的土壤。
这样逐步的植物战场，如果我们去到一些沙滩广布的海岸边，可能走上几百米就会看到四五种生态系统的更替。
参考资料：1.https://en.wikipedia.org/wiki/Dune#Coastal_dunes
2.http://www.geographyinaction.co.uk/Text_based_site/Magilligan/Mag_intro.htm
2.5 Climate and Natural Vegetation
第一步：要明确emergent和 canopy的界限（书本解释的不明确）
第二步：如果选择了热带雨林，出现了分层，以此为准。
第三步：说明热带雨林里为什么有那么多的生物？
具体有多夸张呢？
These incredible places cover only 6% of the Earth's surface but yet they contain MORE THAN 1/2 of the world's plant and animal species!
Rainforests have an abundance of plants and animals for the following reasons:
Climate: because rainforests are located in tropical regions, they receive a lot of sunlight. The sunlight is converted to energy by plants through the process of photosynthesis. Since there is a lot of sunlight, there is a lot of energy in the rainforest. This energy is stored in plant vegetation, which is eaten by animals. The abundance of energy supports an abundance of plant and animal species.
Canopy: the canopy structure of the rainforest provides an abundance of places for plants to grow and animals to live. The canopy offers sources of food, shelter, and hiding places, providing for interaction between different species. For example, there are plants in the canopy called bromeliads that store water in their leaves. Frogs and other animals use these pockets of water for hunting and laying their eggs.
那既然说到气候了，就不得不提热带雨林气候特征与赤道之间的关系。
比如这道收录在昨天发的材料中的C问：
(c)举例说明热带雨林的主要特征是如何受到赤道气候的影响的。7
Amazon Rainforest[area] in South America [Continent]
(注意只有60%在Brazil巴西，13%在Peru，10%在Colombia...)
首先就要表明热带雨林的主要特征是：全年高温多雨，这一点对应的正是赤道的情况。
然后解释这两个特征形成的原因。
The high rainfall (annual between 2500-4500mm) and average high temperature around 25-30C in the tropical rainforest areas is influenced by the equatorial climate from at least three aspects- hot conditions throughout the year which means a lack of seasons, high levels of rainfall mainly caused by convection.
The tropical rainforests are generally near the equator. The overhead sun is high in the sky, which means they receive high-intensity insolation. Because of this location, the equatorial climates have high temperatures all year around. The warm and wet air rises high enough, cool, condense and form clouds and rain. This daily convection leads to high level of rainfalls.
但这个示例还只是一个L2的答案，你觉得是为什么呢？
（还要结合你举出的例子给出一些细节）