Due to rock moister, are dry rocks more brittle than wet rocks? Like, for instance, the rocks on Mars without much moisture there, would they fundamentally act differently that the rocks here on Earth, simply due to moisture content?
In: Earth Science
The effects of moisture on rocks very much depends on the type of rock. If it’s porous like sandstone or limestone then it will become denser from absorbing it and possibly explode if exposed to high heat.
In regards to brittleness, water has very little to do with it as it’s based on the strength of the bond between particles in the stone. If there’s water soluble minerals then it can definitely weaken it, but those are very rare since water likely had already been through it (at least on earth)
There is a slight effect that water can have on rocks that break concoidally (aka like glass). Water filling any voids in the stone can allow cracks to propagate more easily. This trick is used by flint knappers to make some rocks easier to work with when making arrow heads/knives
It’s really difficult to give a simplified answer that holds true in general, because it depends on so many other things like what type of rock is it? How much pore space does it have? Is that even throughout the rock? How much pressure is it under? What is the temperature of the rock? (Those last two basically amount to “how deep in the crust is it?”) What is the local stress regime and the rate of strain? Is the water supercritical? Are there gases dissolved in the water? Are there any pre-existing fractures in the rock? Etc. etc.
However, there are a few very broad things we can say. In many cases, the addition of water can assist in the development of new fractures and provide a medium for atomic scale diffusion to take place along grain boundaries of it is present in pore space, particularly at high temperatures. High pressures tend to mitigate any new fractures, but if there are stresses operating on the rock (ie. not just equal pressure in all directions which acts to confines the rock), then this doesn’t matter so much. The presence of water on pore spaces also allows cracks to grow without having to reach critical stress levels, which is what happens in completely dry rock. Lastly, when we have pre-existing faults in the rock, particularly large old ones that have been accumulating stress for a long time, the movement of water along the fault plane acts to lubricate the surfaces somewhat so that movement on the fault (ie. an earthquake) will occur sooner than it otherwise would have.
Conversely, if you just have unconsolidated sediment or even solid mudstones which have a lot of clay minerals (and can thus become further water saturated with the right conditions), then this causes them not to deform in a brittle manner, but to become less competent so that they bend and fold a lot more before breaking. Look up pictures of “soft sediment deformation” to see what I mean, that’s where a bunch of sediment was buried in the crust but not too deep and it hadn’t turned into solid rock yet. Then it got gradually squished by tectonic forces before it ever had a chance to become proper rock, it bent all twisty and that, *then* it eventually solidified into a sedimentary rock with interesting shapes in.
The rocks on Mars will behave differently to those on Earth due to oiusture content, but we don’t know enough about conditions there to say how just yet. Also, it’s clearly not as wet a place as Earth, but we *have* detected moisture in the Martian regolith, locked up in clay minerals. This is one of the many pieces of evidence that scientists use in order to point to the likelihood that Mars was probably a lot wetter in the distant last, with seas and rivers operating on large parts of the planet.
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