Lava spattering from the Halema`uma`u lava lake at Kilauea on May 12, 2012. Image: HVO/USGS.
I get a lot of questions here at Eruptions, but one of the more common themes is the properties of rocks – and specifically why they melt where they melt to produce magma? There are a lot of misconceptions out there about the interior of the Earth, namely that the tectonic plates that we make our home (both the continental and oceanic kinds) are sitting on a “sea of magma” that makes up the mantle. As I’ve said before, the mantle of the Earth, that layer of silicate rocks that starts at ~10-70 km depth and goes down to the outer core at ~2900 km depth that constitutes a large volume of the planet, is not molten, but rather a solid that can behave plastically. This means it can flow and convect, which is one of the ways that geologists have theorized that plate motion is started and sustained. However, as we know, rocks are found entirely molten within the Earth, so how can so much of the planet be solid but then some parts of it melt as well?
This sketch illustrates why rocks melt on Earth. The geotherm (solid line) would suggest that rock shouldn’t melt as it never cross the dry mantle solidus (the point where mantle rock would melt merely by heating it). Adding water moves the solidus to the wet mantle solidus (short dashed line). Decompressing mantle at constant temperature allows for the mantle to cross the solidus as the mantle rises (thick solid line). See text for more details. Image: Erik Klemetti
It starts with the question “how do you melt a rock”? The most straightforward way that might pop into you head is “raise the temperature!”. That is what happens with ice — it is solid water that melts when the temperature exceeds 0oC/32F. However, when it comes to rocks, we run into a problem. The Earth actually isn’t really hot enough to melt mantle rocks, which are the source of basalt at the mid-ocean ridges, hotspots and subduction zones. If we assume the mantle that melts is made of peridotite*, the solidus (the point where the rock starts to melt) is ~2000oC at 2o0 km depth (in the upper mantle). Now, models for the geothermal gradient (how hot it gets with depth; see above) on Earth as you go down through the crust into the upper mantle pegs the temperature at 200 km at somewhere between 1300-1800oC, well below the melting point of peridotite. So, if it is cooler as you head up, why does this peridotite melt to form basalt?
Well, that is where you need to stop thinking about how to heat the rock to melting but rather how to change the rock’s melting point (solidus). Think about our ice analogy. During the winter, there are a lot of times where you’d like to get rid of that
Sketch illustrating melting at a subduction zone. Water from the downgoing slab is released at depth as it heats, causing the mantle above the slab to partially melt, forming basalt. Image: Erik Klemetti
ice but the ambient temperature is below the air temperature. So, what do you do? One solution is to get that ice to melt at a lower temperature by disrupting the bonding between the H2O molecules — thus, halting the formation of rigid ice. Salts are a great way to disrupt this, so throw some NaCl or KCl on ice and it will melt at a lower temperature than 0oC. For a rock, water behaves as its salt. Add water into a mantle peridotite and it will melt at a lower temperature because the bonds in the minerals that make up the rock will be disrupted by the water molecule (we call it a “network modifier”). In a subduction zone (like the Cascades or the Andes), where an oceanic plate slides down under another plate, that downgoing slab releases its water as it heats up. That water then rises up into the mantle above it, causing it to melt at a lower temperature and, bam! Basalt is produced in the process called flux melting.
Sketch illustrating decompression melting at the mid-ocean ridge. Warm, fertile mantle rises, partially melts to form basalt, then moves laterally away from the ridge as it cools. Image: Erik Klemetti
Wait! The largest volcanic system on Earth is the mid-ocean ridge system, where you don’t have any subduction to bring water down into the mantle to help melting along. Now, why do you get basalt there? This time we have to use another method to melt that peridotite – we need to decompress it at constant temperature. This is called adiabatic ascent. The mantle is convecting, bringing hot mantle from depth up towards the surface and as it does so, the mantle material stays hot, hotter than the surrounding rocks. The melting point (solidus) of peridotite changes with pressure, so the 2000oC melting point at 200 km is only ~1400oC at 50 km. So, keep that mantle material hot and decompress it and you get melting to form basalt! So, underneath mid-ocean ridges (and at hotspots like Hawaii), the mantle is upwelling, causing decompression melting to occur.
Let’s review: Under normal conditions, mantle rock like peridotite shouldn’t melt in the Earth’s upper mantle — it is just too cool. However, by adding water you can lower the melting point of the rock. Alternatively, by decompressing the rock, you can bring it to a pressure where the melting point is lower. In both cases, basalt magma will form and considering it is hotter and less dense than the surrounding rock, it will percolate towards the surface … and some of that erupts!
地球上的岩石为什么会熔化？
2012年5月12日，熔岩从基拉厄维火山 Halema`uma`u 火山湖中飞溅而出。图片来源：HVO/USGS。
我被问及许多关于火山爆发的问题，但是更普遍的问题是石头的性质——尤其是为什么它们会熔化，以及它们在哪里熔化并产生岩浆？对于地球的内部人们有很多误解，我们生活所在的地址板块（大陆和大洋板块）都是坐落于形成地幔的“岩浆海”之上的。我之前曾经说过，地球的地幔是始于10~70千米，一直延伸到约2900千米处外部核心的硅酸盐岩石层，这层岩石组成了地球大部分的体积，但它没有熔化，而只是发生塑性变化的固体。这意味着它可以流动，也可以对流，这也是地质学家们认为的板块运动开始以及维持的方式之一。然而，正如我们所知的那样，在地球核心岩石是完全熔化的，那么地球如何能有这么多固体岩层，而其他地方确是熔化的呢？
这幅简图揭示了地球上的岩石为什么会熔化。从图上的地热等温线（图中实线）来看，岩石不应该熔化，因为它与干地幔固相线（仅靠加热使地幔岩石熔化的点）从不相交。水的加入使得固相线移到了湿地幔固相线（图中短虚线）处。地幔的恒温减压使得地幔在上升过程中（图中粗实线）能够与固相线相交。图片来源：Erik Klemetti
首先我们问个问题：“你要如何熔化一块岩石？”你脑海中跳出来的最直接的办法就是“升高温度！”。对于冰来说是这样的——固态水在0oC（或32F）时就会融化。但对于岩石来说，我们就遇上了麻烦。事实上地球还没有热到能熔化地幔岩石的程度，这些岩石是洋中脊、热点以及俯冲带玄武岩的来源。如果我们假设熔化的地幔由橄榄岩*组成，那么在200千米深处（地幔上层）固相线约为2000oC。然而地热梯度（随着深度增加温度如何变化）模型却显示，如果你沿着地壳深入到200千米深的地幔楔上层，温度会在约1300~1800oC，远低于橄榄岩的熔点。那么，如果说越往上走温度越低，那么这里的橄榄岩为什么会熔化形成玄武岩呢？
这幅简图揭示了俯冲带发生的熔化现象。向下俯冲板块中的水在深层温度升高时被释放出来，造成板块上方的地幔部分熔融，并形成玄武岩。图片来源：Erik Klemetti
好吧，现在你要想的不是如何加热一块岩石以使其熔化，而是想想如何改变岩石的熔点（固相线）。让我们来想象一块冰。在冬天，有许多时候你不想看到冰，但是环境温度却在冰点以下。那么你该怎么办？一种方法是通过阻碍水分子间的化学键——从而阻碍固体冰的形成——以让冰在更低的温度下熔化。盐类物质很容易做到这一点，只要在冰上撒一些氯化钠或者氯化钾，冰就会在0oC以下开始融化。对于岩石来说，水的作用就像盐对冰的作用一样。把水加入地幔橄榄岩会使其在更低的温度下熔融，因为矿物之间的化学键被水分子破坏（我们称之为“变网”）。在俯冲带（比如喀斯特山脉和安第斯山脉）里，当海洋板块俯冲到另一板块之下时，俯冲板块中的水随着温度升高就会被释放出来，这些水上升到它上方的地幔处，使得它在较低的温度下熔融，然后“砰”一声，玄武岩就在一种叫做“注流熔融”的过程中形成了。
这幅图片显示了洋中脊地区减压熔融过程。温度较高的地幔上升，部分熔化形成玄武岩，然后在移向洋中脊侧面的过程中逐渐冷却。图片来源：Erik Klemetti
等等！地球上最大的火山系统是洋中脊火山系统，哪里没有任何的板块俯冲，地幔也得不到水来发生熔融，那么那里的玄武岩是从哪里来的呢？这次我们要换种方法来使橄榄岩熔融——我们将它恒温减压，这一过程叫做绝热提升。地幔发生对流，将深处炽热的地幔物质带到表层冷却，在冷却过程中，地幔物质比周围的岩石温度要高。由于熔点（固相线）随着压强的变化而变化，因而在200千米深处2000oC的熔点在50千米深处只有约1400oC。因而，保持地幔物质炽热的同时减压，你就可以让熔融的橄榄岩变成玄武岩！因而，在洋中脊下面（以及在像夏威夷这样的热点），地幔一直在上涌，使得减压熔融过程一直在发生。
让我们回顾一下：在通常情况下，像橄榄岩这样的地幔岩石不该在地球上地幔熔融——那里还不够热。但是，水的加入降低了岩石的熔点。或者通过给岩石减压，在某一个压强下岩石的熔点会降低。在上述两种情况中，玄武岩岩浆就会形成，由于它比周围岩石更热以及密度更小，它会向水面渗出……有些就直接喷发了出来！
*地幔绝不是均匀的，但我们只关心所谓的“富集地幔”——这些地幔从未发生过熔融，并能产生玄武岩液体。
