2.2 Composition of the Earth's layers

Earth's layers have distinct compositions that control how our planet behaves, from plate tectonics at the surface to the magnetic field generated deep in the core. Understanding what each layer is made of helps explain why these layers behave so differently. The crust is built from silicate minerals, the mantle from denser ultramafic rock, and the core from iron-nickel alloy. This compositional layering is the result of chemical differentiation early in Earth's history.

Crust Composition

The crust is Earth's outermost solid layer, and it comes in two fundamentally different varieties: oceanic and continental. Both are built from silicate minerals, meaning minerals based on silicon and oxygen bonded with various metal ions (aluminum, iron, magnesium, calcium, sodium, potassium). But the specific mix of those elements differs between the two crust types, and that difference has huge consequences for how they behave tectonically.

Silica content ($SiO_2$) is the key variable. Continental crust is silica-rich (felsic), while oceanic crust is silica-poor but iron- and magnesium-rich (mafic). Common silicate minerals include quartz, feldspar, mica, amphibole, pyroxene, and olivine. Common silicate rocks include granite, basalt, gneiss, and schist.

Oceanic Crust Characteristics

Oceanic crust covers about 60% of Earth's surface beneath the oceans. It's relatively thin, averaging just 6-7 km in thickness.

• Composed primarily of dense, dark-colored mafic igneous rocks like basalt (fine-grained, at the surface) and gabbro (coarse-grained, at depth)
• Higher density than continental crust because of the greater abundance of heavier elements like iron and magnesium
• Geologically young: no oceanic crust is older than about 200 million years, because it's continuously created at mid-ocean ridges and destroyed at subduction zones

Continental Crust Properties

Continental crust covers about 40% of Earth's surface, forming the continents and the shallow continental shelves near shores. It's much thicker than oceanic crust, averaging 30-50 km, with roots extending up to 100 km beneath major mountain ranges.

• Composed primarily of lighter-colored felsic igneous rocks like granite and diorite, along with metamorphic and sedimentary rocks
• Lower density than oceanic crust because it contains more of the lighter elements: silicon, aluminum, and oxygen
• Much older: cratonic cores (the ancient, stable hearts of continents) contain rocks up to 4 billion years old, because continental crust is too buoyant to be subducted and recycled

This density difference is why continental crust "floats" higher on the mantle than oceanic crust, and why oceanic plates dive beneath continental plates at subduction zones rather than the other way around.

Mantle Layers

The mantle makes up the bulk of Earth's volume, extending from the base of the crust down to about 2,900 km depth. Its primary rock type is peridotite, an ultramafic rock rich in olivine and pyroxene. But increasing pressure with depth transforms these minerals into different high-pressure phases, creating distinct sub-layers with different physical properties.

Upper Mantle Characteristics

The upper mantle extends from the base of the crust to about 660 km depth.

• Composed primarily of peridotite (rich in olivine and pyroxene minerals)
• The uppermost portion is rigid and, together with the crust, forms the lithosphere, which is broken into tectonic plates
• Below the lithosphere sits the asthenosphere, a zone where rock is partially molten and has reduced viscosity. This weak layer is what allows tectonic plates to move
• At the base of the upper mantle lies a transition zone, marked by mineral phase changes: olivine transforms to wadsleyite (at ~410 km) and then to ringwoodite (at ~520 km) as pressure increases. These transformations cause sharp increases in seismic wave speed

Lower Mantle Properties

The lower mantle extends from about 660 km depth to the core-mantle boundary at roughly 2,900 km.

• Composed primarily of high-pressure mineral phases, especially bridgmanite (a magnesium silicate perovskite) and ferropericlase
• Denser and more viscous than the upper mantle
• Slow convection currents operating over millions of years transfer heat from the core toward the surface and contribute to driving plate tectonics
• Contains seismically anomalous regions: large low-shear-velocity provinces (LLSVPs) and ultra-low velocity zones (ULVZs) near the core-mantle boundary, which may represent zones of elevated temperature or partial melting

Density Stratification in the Mantle

Mantle density increases with depth due to compression under the weight of overlying material. This stratification creates layers with distinct physical properties.

• The transition zone separates upper and lower mantle, with sharp seismic velocity increases at 410 km and 660 km depth
• Whether the upper and lower mantle differ in chemical composition (not just mineral phase) remains an open question. Possible differences involve iron, aluminum, and silicon content
• Geochemical evidence from magmas sourced at different depths suggests there may be layered convection with limited material exchange between the upper and lower mantle, rather than whole-mantle mixing

Core Materials

Iron-Nickel Alloy Composition

The core is composed primarily of an iron-nickel alloy, with smaller amounts of lighter elements such as sulfur, oxygen, silicon, carbon, and hydrogen. Its density ranges from about 10 g/cm³ at the top of the outer core to about 13 g/cm³ at the center. Iron dominates because it's the heaviest element produced in large quantities by stellar nucleosynthesis (fusion in stars).

The core has two distinct parts:

• Outer core (liquid): Convection currents in this molten iron alloy generate Earth's magnetic field through the geodynamo process
• Inner core (solid): Despite temperatures exceeding 5,000°C, the extreme pressure at Earth's center forces the iron alloy into a solid state

As the inner core slowly solidifies over time, it releases latent heat and expels lighter elements into the outer core. Both of these effects help drive the convection that sustains the geodynamo.

Chemical Differentiation and Core Formation

Earth's layered structure didn't exist from the start. It formed through chemical differentiation during the planet's early history, when Earth was hot enough for materials to separate by density.

1. In the early, largely molten Earth, dense metallic iron sank toward the center, carrying with it siderophile ("iron-loving") elements like nickel, gold, and platinum. This formed the core.
2. Lighter silicate materials and lithophile ("rock-loving") elements like silicon, oxygen, and aluminum remained in the outer layers, forming the mantle and eventually the crust.
3. This process was largely complete within the first 50-100 million years of Earth's history, based on geochemical evidence and computer modeling.

One important consequence of differentiation: heat-producing radioactive elements (potassium-40, uranium, thorium) are lithophile, so they concentrated in the silicate mantle and crust rather than the core. The decay of these elements provides a significant portion of Earth's internal heat, helping sustain mantle convection and plate tectonics billions of years after the planet formed.