What is the continental crust made of?

Decoding Earth’s Landmasses: What’s Continental Crust Made Of?

Ever wonder what exactly you’re standing on? I mean, really standing on? It’s not just “dirt,” that’s for sure. We’re talking about continental crust, the very foundation of our continents, a wild and fascinating mix of geological ingredients forged over billions of years. Unlike the ocean floor, which is mostly basalt, continental crust is a diverse cocktail of rocks, a composition that has shaped our planet and continues to do so. So, let’s dive in and see what this stuff is really made of.

Layers Upon Layers

Think of continental crust not as a single layer cake, but more like a multi-tiered masterpiece, with each layer having its own distinct personality. We generally talk about the upper, middle, and lower crust, though the lines between them aren’t exactly sharp.

• Upper Crust: This is where things get really interesting. It’s a mishmash of everything from fresh volcanic rocks and layered sedimentary deposits to metamorphic rocks that have been squeezed and cooked over time. Imagine a geological scrapbook, filled with all sorts of memories!
• Middle Crust: Things get a bit more uniform here. The middle crust is estimated to be about 62-69% SiO2.
• Lower Crust: Ah, the mysterious lower crust. We don’t know as much about this zone, but studies suggest it’s a mix of rock types, including some surprising felsic (silica-rich) rocks.

Rock Stars of the Crust

Continental crust is built from a rock concert of igneous, sedimentary, and metamorphic types.

• Igneous Rocks: These are the “born of fire” rocks, formed from the cooling of molten magma or lava. Granite, that classic countertop material, is a prime example. Andesite is another common type of igneous rock found in the continental crust.
• Sedimentary Rocks: Think of these as the “layered history” rocks. They’re made from compressed sediments, like sandstone (think beaches), shale (think mud), and limestone (think ancient reefs).
• Metamorphic Rocks: These are the “rock transformers,” rocks that have been changed by intense pressure and heat. Gneiss and schist are two common examples, often displaying beautiful banded patterns.

The Mineral Makeup

If rocks are the ingredients, minerals are the spices. Feldspars are the most abundant, making up about 41% of the crust by weight. Quartz comes in second at 12%, followed by pyroxenes at 11%. The main mineral constituents of the continental mass are silica and alumina. These compounds form the bulk of minerals such as feldspar and quartz.

Chemical Signatures

Compared to the ocean crust and the mantle below, continental crust is enriched in certain elements. These incompatible elements are enriched by a factor of 50 to 100 in the continental crust relative to primitive mantle rock, while oceanic crust is enriched with incompatible elements by a factor of about 10. The average density of the continental crust is about 2.83 g/cm3.

Sial vs. Sima: A Quick Chemistry Lesson

You might hear the terms “sial” and “sima” thrown around. “Sial” refers to the continental crust’s richness in aluminum silicates (Al-Si) and its lower density. “Sima,” on the other hand, describes the oceanic crust, which is richer in magnesium silicate (Mg-Si) minerals.

How It All Came to Be

Continental crust is primarily formed at magmatic arcs associated with subduction. All continental crust is ultimately derived from mantle-derived melts (mainly basalt) through fractional differentiation of basaltic melt and the assimilation of pre-existing continental crust. Approximately 41% of Earth’s surface area and about 70% of the volume of Earth’s crust are continental crust.

Size Matters: Thickness and Density

Continental crust is significantly thicker than oceanic crust, ranging from 25 to 70 km. Oceanic crust, in contrast, averages only 7 to 10 km thick. The continental crust has an average density of 2.83 g/cm3, less dense than the ultramafic material that makes up the mantle (around 3.3 g/cm3) and also less dense than oceanic crust (around 2.9 g/cm3).

A Work in Progress

The story of continental crust is far from over. Plate tectonics are constantly reshaping it, creating new crust at convergent boundaries and sometimes destroying it. Continental material also gets recycled into the ocean through sedimentation. It’s a dynamic, ever-evolving system, and understanding its composition is key to understanding our planet’s past, present, and future.