What is the Mohorovicic discontinuity made of?

Cracking the Earth’s Code: What’s the Moho Really Made Of?

Ever wonder what’s going on way, way down beneath your feet? I’m talking about the Mohorovičić discontinuity, or the “Moho” as those in the know call it. It’s basically Earth’s ultimate layer cake line, the spot where the crust ends and the mantle begins. But it’s not just a clean break – it’s a messy, fascinating zone where things get seriously interesting. So, what exactly is this Moho made of? Let’s dig in.

The “Aha!” Moment: How We Found the Moho

Back in 1909, a Croatian seismologist named Andrija Mohorovičić had a hunch. After a local earthquake, he studied the seismic waves and noticed something weird: they sped up at a certain depth. Bingo! He figured out that there must be a boundary down there, separating layers with different stuff in them. That sudden change in speed? That’s the “discontinuity” we’re talking about.

Crust vs. Mantle: A Tale of Two Layers

To understand the Moho, you’ve got to know what’s on either side. Think of it like this:

• The Crust: This is Earth’s skin, the part we live on. It’s made of silicate rocks, but it’s not all the same. Oceanic crust, under the oceans, is thin and mostly basalt. Continental crust, making up the continents, is thicker and has all sorts of rocks, like granite. Basically, the crust is packed with silicon, aluminum, and other elements. It also has a bunch of radioactive elements that generate heat.
• The Mantle: Now, this is the big guy. The mantle is a massive layer that makes up most of Earth’s volume. It’s made of denser, heavier stuff than the crust – mostly iron- and magnesium-rich silicate rocks called peridotite. Peridotite is low on silica and loaded with minerals like olivine and pyroxene.

So, the Moho is where you go from the crust’s feldspar-filled rocks to the mantle’s feldspar-free zone. It’s a major change in the rock menu!

Not So Sharp: The Moho’s Transition Zone

Don’t picture the Moho as a perfectly crisp line. It’s more like a transition zone, a few kilometers thick, where the rock gradually changes from crustal to mantle stuff. Imagine a rocky road where you find a mix of different rocks all jumbled together.

Seismic Speed Bumps: How We “See” the Moho

The whole reason we know about the Moho is because of those seismic waves. P-waves, in particular, zip through denser stuff faster. They go from about 6.7-7.2 km/s in the lower crust to a blazing 7.6-8.6 km/s in the upper mantle. That speed boost tells us we’ve hit something denser and different.

Moho Complications: When Things Get Messy

Okay, it’s not always that simple. Sometimes, things can get confusing down there:

• Eclogitization: Basalt in the lower crust can turn into eclogite, a super-dense rock that acts like mantle rock. This can trick us into thinking we’ve hit the Moho when we haven’t.
• Serpentinization: Water can mess with mantle rocks, making them less dense and slower. This can make the Moho seem deeper than it really is.
• Magmatic Underplating: Sometimes, magma gets trapped under the Moho, hiding the real boundary.

Why Should You Care About the Moho?

So, why bother learning about this stuff? Because the Moho is a key to understanding how our planet works! It tells us about:

• Plate Tectonics: The Moho changes depending on where you are on Earth. It can tell us about plate boundaries, subduction zones, and how mountains are built.
• Earth’s Heat: The Moho helps us figure out how hot it is inside Earth and how heat flows around.
• Crust-Mantle Chat: The Moho is where the crust and mantle swap materials and influence each other.

We haven’t actually seen the Moho directly – it’s too deep to drill! But scientists are using seismic waves, studying rocks from the deep Earth that have been pushed to the surface (ophiolites), and building computer models to learn more. The Moho is a puzzle, but every piece we find helps us understand our planet a little better.