Unveiling the Volcanic Mystery: Exploring the Absence of Ultra-Acidic Igneous Rocks

Unveiling the Volcanic Mystery: Where Are All the Ultra-Acidic Rocks?

Ever wondered about the stuff volcanoes spit out? We call it igneous rock, and it’s a wild mix of different ingredients, mainly silica. Think of silica as the rock’s backbone. Now, rocks are usually sorted by how much silica they’ve got – from the low-silica ultramafic rocks to the high-silica felsic ones, also known as acidic rocks. But here’s the kicker: you almost never find rocks that are super high in silica, what I’d call “ultra-acidic,” packing, say, 85% to 90% silica. It’s like nature’s decided to stop at a certain point. Why is that? It’s a head-scratcher that keeps geologists like me up at night.

So, what do I mean by “acidic” anyway? It’s a bit of a historical quirk, really. Back in the day, scientists used “acidic” and “basic” to describe rocks before they really understood what was going on chemically. It’s not about pH levels, I promise! These days, we prefer terms like “felsic” for silica-rich rocks and “mafic” for those heavy in magnesium and iron. But “acidic” stuck around, and it basically means a rock loaded with silica, usually with minerals like quartz, feldspar, and mica thrown in. Granite and rhyolite? Classic examples.

Now, how do these rocks even form? It all starts with magma, that molten rock bubbling beneath the Earth’s surface. Most of this magma is mafic, straight from the mantle. To get those silica-rich rocks, the magma has to change, to evolve. And that happens through a process called fractional crystallization.

Imagine a pot of soup simmering on the stove. As it cools, some ingredients start to solidify and sink to the bottom. That’s kind of what happens with magma. Minerals with less silica crystallize first, leaving the remaining melt richer in silica. It’s like slowly concentrating the silica in the soup. But here’s the thing – you can only concentrate it so much.

Eventually, you hit a point where quartz, which is pure silica, starts to crystallize. And once that happens, you can’t really enrich the magma in silica any further. Any extra silica just turns into more quartz. So, you’re stuck – no ultra-acidic magma for you!

And it’s not just about the process. What the magma starts with matters too. Since most magma comes from the mantle, it’s already mafic. Sure, it can pick up some extra silica from the Earth’s crust as it rises, but even the crust isn’t pure silica. It’s more like an andesite, somewhere in the middle. So, even with the best efforts, you can’t turn a naturally mafic magma into something that’s almost pure silica. It’s like trying to bake a cake with mostly water – it’s just not going to work.

There’s also the issue of how goopy the magma is. Silica makes magma thick and sticky. Think of it like honey versus water. The more silica, the thicker it gets. And super thick magma has a hard time erupting. It might just get stuck underground, cooling down before it ever sees the light of day. And even if it does erupt, it might cool so fast that it turns into a glassy rock like obsidian, instead of a nice, crystalline one.

Now, some folks might argue that certain rhyolites, those rocks with a ton of silica, are almost ultra-acidic. And they’ve got a point. These rocks can have over 76% silica, which is pretty darn close. Maybe our current way of classifying rocks just isn’t precise enough.

That’s where experimental petrology comes in. These are the folks who try to recreate the conditions inside the Earth in a lab. They can cook up their own magmas and watch how they crystallize. It’s like playing volcano in a test tube! And it helps us understand what’s really going on deep down.

So, where does that leave us? Well, the lack of ultra-acidic rocks is probably a mix of a few things: the limits of how magma changes, what it starts with, and how thick it gets. Sure, maybe there’s some super high-silica magma out there, but it might not be able to form a rock we’d recognize. And who knows, maybe future research will change our minds. That’s the fun of geology – there’s always something new to discover!