Gaining a Better Understanding on how Sand Powers Work

Question

Awhile ago, I posted a question on this site asking how a group of air-controlling mages could use their powers to create and project lightning/electricity, sorta like an Avatar/LoK sub-bending art and while I've expanded on some of the abilities of each of my four classes since then, I have a decent idea how each of them work, scientifically speaking. There is however, one exception to this. Sand. I want to give my earth mages the ability to change rocks and stones into sand and vice-versa, but unlike water’s transformation into ice, I have no idea what happens to rocks and sand when they undergo this kind of transformation. I’m also unsure how potentially damaging to the environment this could since in real life, a rock turns into sand via erosion, and sand turns into rock via compaction, and my mages are using magic to simulate these effects via molecular modification (the basic idea here is that my characters syphon magical energy from a separate D&D-inspired plane of existence, and use it to influence the world around them). Because of this, I need to know the answers to the following questions:

• What’s the difference between sand and the grains produced from grinding two rocks against each other?
• When a rock undergoes its transformation into sand via erosion, what actually happens to it on a molecular level (same goes for grain’s/sand’s transformation into rock/sandstone via compaction)?
• Since my mages have the ability to do this in a matter of seconds, will I have the same problem as a water mage quickly freezing a large body of water into ice where the amount of energy released from these actions will cause huge amounts of damage to the environment? Like, erosion and compaction both involve physically affecting the object via force and pressure, but if I’m applying THAT force and pressure to the object in such a short amount of time, will the output energy manifest as heat and spread outwards burning everything around the characters (including the characters themselves)? To make this question easier to answer, I’ll write down the largest amount of stone and sand/grain my characters can erode/compact, and the amount of time they do it in:

For erosion, an earth mage can turn a stretch of solid land as long and as wide as 40 inches and as deep as 60 inches into a bunch grains in roughly 4 seconds. They spend 3 seconds to weaken the rocks, and then use the last second to rip the solid bonds between the now weaker rocks apart from each other, creating a bunch of grains

For compaction, an earth mage can compact and harden a large amount of sand/grain into a giant boulder roughly half as tall, wide, long and heavy as this boulder https://en.wikipedia.org/wiki/Giant_Rock in roughly 15 seconds. They spend around half of that time moving and shaping the grains/sand into the shape of a boulder, and then use the other half to press all those grains/that sand together into a huge boulder that’s about as sturdy as your average metamorphic rock

I apologize if this question is a bit specific and/or wordy. I originally tried to post it on Quora, but after spending all afternoon yesterday turning word documents into PNGs and JPGs only for a stupid bot to keep making each and every comment I made invisible to everyone but my account, I just gave up and decided to come here instead. Besides, this is a really reliable fantasy/sci-fi world building Q&A, makes sense that you guys and girls would probably be able to help me out more on something like this.

Answer 1

What’s the difference between sand and the grains produced from grinding two rocks against each other?

Basically, particle size. You can actually produce sand by grinding two rocks against each other until you obtain the correct particle size. There might also be some differences in chemical composition due to some elements from the original rock being washed away by erosion, or because things usually not contained in the "original rock" (e.g shell fragments) are incorporated to the sand through the flow of the elements

When a rock undergoes its transformation into sand via erosion, what actually happens to it on a molecular level (same goes for grain’s/sand’s transformation into rock/sandstone via compaction)?

For erosion or weathering, I think I partially addressed this in the answer to the previous question - the rock is broken down progressively to smaller and smaller pieces until it reaches the size of sand. The rest depends on what kind of exposure to the elements happen - water can wash away chemicals from the surface of the particle (e.g. aluminium-silicates wash away faster and silicon-dioxide remains in the rock) and can also incorporate other things to the sand, such as shell fragments or carbonate particles

For rock formation, it largely depends on what kind of rock formation you're looking at. Since you only mention sedimentation and not other processes like metamorphism or solidification of magma, compaction is not the only thing required to turn sand into stone, you also need cementation, which is the accumulation of minerals that are dissolved in water between the grains of sand, gluing them together. In a very simplified manner, this is microscopic-level brick-layering

Note that you'll not be able to form all types of rock that way, only sandstone. Your mages will have to actually MELT the sand / ground into magma and then cool it down to rock to produce igneous rocks, or use a 2-step process (make sandstone -> apply temperature and pressure) to make metamorphous rocks

Edit to add after comments in question: since your mages cannot transmute chemical elements, they'll also be limited by the materials they have at hand. They might be able to convert, say, granite (igneous silica-rich rock) into sandstone (sedimentary silica-rich rock) but not marble (metamorphic calcium-rich rock) into sandstone (or vice-versa)

Since my mages have the ability to do this in a matter of seconds, will I have the same problem as a water mage quickly freezing a large body of water into ice where the amount of energy released from these actions will cause huge amounts of damage to the environment?

You'll have the OPPOSITE problem, unless they're cooling down magma. For applying temperature and pressure to make rock, they'll need massive amounts of energy from a source (I'm assuming that will come from the other plane / dimension you mentioned)