eli5: Why does sand which is a bunch of tiny rocks, turn into glass, but we can’t turn big rocks into glass?

Question

I’m very tired. Please help me understand this.

In: Other

Answer 1

Sand isn’t just tiny rocks; it is specifically silica. “Why would a bunch of silica remain as sand? Why isn’t it all in giant crystals of quartz (which is made of silica)?” is a good follow-up question. Here is a slightly over-simplified explanation, but it should suffice for ELI5.

Highly composite rocks usually have a huge mix of mineral types, and they all have different ways of weathering (breaking down chemically, especially with exposure to cycles of hot and cold, moist and dry). All the stuff that can dissolve away goes first; that includes sodium, potassium, lithium, etc. (That’s one of the reasons the sea is salty; all the salt dissolves out of land rocks and flows to the oceans where it has been accumulating since earth first formed oceans.) Phorphorous bearing minerals also gradually get taken away, whether by biological action of things like fungi, or by weathering. The rest of the stuff tends to decompose into clay particles, and washes away. Since silica is what glass is made of, and glass is essentially inert (with a few rare exceptions of potent acids that can react with silica), rocks that have a big mix of quartz and mica and other stuff (such as granite) end up as a porous bunch of crumbling rock that’s mostly silica. You may have seen crumbling weathered granite if you ever hike to any place with exposed granite in the mountains. Most of the crumbly stuff that remains is silica/quartz. As it actually crumbles into rubble, more of the stuff that can weather away does weather away. This crumbled mostly silica stuff, if it is near the ocean where physical bashing of the waves can really grind it up, crumbles further and breaks down into the smallest particles that waves will grind it into over long periods of time. The weathering of minerals when in contact with pounding waves of salt water is also accelerated, but since silica is essentially inert, only it remains. But sea water isn’t the only thing that will make sand. Any rock which has silica mixed in with other minerals that will eventually weather away will leave you with sand, especially if there is some kind of abrasion or physical impact to help it along.

That’s how silica sand is made.

BTW, Beach sand often has a lot of tiny mollusk shells and tiny bits of broken larger mollusk shells, so beach sand isn’t always pure silica.