Is pumice felsic intermediate or mafic?

Pumice: Felsic, Intermediate, or Mafic? Let’s Get to the Bottom of This Volcanic Mystery

Pumice. You’ve probably seen it – that lightweight, bubbly rock often hanging around volcanic areas. Maybe you even used it to smooth your skin! Its knack for floating on water is just one of its cool quirks. But have you ever wondered what it’s actually made of? Is it felsic, intermediate, or mafic? Well, buckle up, because like many things in geology, the answer is… it depends!

Felsic, Intermediate, Mafic: Decoding the Rock Rainbow

Think of igneous rocks – those born from fiery magma or lava – as falling into different families based on their silica content and mineral makeup. It’s like a geological rainbow!

• Felsic rocks are the silica queens and kings, boasting more than 63% silica. They’re packed with minerals like quartz and feldspar, giving them a light complexion. Think granite countertops or the pale, crumbly rhyolite you might find on a hike.
• Intermediate rocks are the mixed bags of the rock world. They clock in with 55% to 63% silica and sport a blend of light and dark minerals, like plagioclase feldspar, hornblende, and pyroxene. Diorite and andesite are your typical “in-betweeners.”
• Mafic rocks are the dark and mysterious ones. They’re lower in silica (45-52%) but loaded with magnesium and iron, thanks to minerals like pyroxene and olivine. Basalt, the stuff of Hawaiian lava flows, is a classic mafic rock.

Pumice’s Personality: It’s Complicated!

So, where does pumice fit into all this? Here’s the thing: pumice is usually – but not always – felsic or intermediate. In other words, it leans towards the silica-rich side of things, but there are exceptions to the rule.

• The Usual Suspect: Most pumice you’ll find is loaded with silicon dioxide (SiO2), a dash of aluminum oxide (Al2O3), and a sprinkle of other elements. I remember analyzing a pumice sample from Egypt once, and it was almost 71% silicon dioxide! That’s a pretty felsic rock.
• Silica’s the Star: Generally, pumice contains between 55% and 75% silica (SiO2). But keep your eyes peeled, because you might stumble upon a rare specimen with a more basic composition – we’re talking 45-52% silica.
• Mineral Guests: Pumice is mostly volcanic glass, a result of super-fast cooling. But if you look closely, you might spot tiny crystals of feldspar, augite, hornblende, or even zircon hanging out in there.
• Color Clues: The color of pumice can be a handy guide. Felsic pumice tends to be light – think white, light gray, or cream. Intermediate pumice often rocks a darker vibe, like medium gray, dark gray, or even a greenish hue. And mafic pumice? It’s usually rocking the black or dark gray look.

Why the Mix-Up?

The secret lies in the magma. Pumice is born from silica-rich to intermediate magmas – the kind that gives you rhyolite, dacite, or even trachyte. Pumice formed from low-silica magmas like basalt? Now that’s a rare find.

The Bottom Line

So, is pumice felsic, intermediate, or mafic? Most of the time, it’s felsic to intermediate. But remember, mafic pumice exists! The exact composition depends on the magma it came from. It’s just another reminder that geology is full of surprises, and that’s what makes it so darn interesting.