Earth's layers have distinct compositions that shape our planet's behavior. The crust, our rocky home, is made of silicate minerals. It's thinner and denser under oceans, thicker and lighter on continents. These differences drive plate tectonics and Earth's ever-changing surface.

Beneath the crust, the mantle and core have their own unique makeups. The mantle, mostly peridotite, flows slowly to move plates. The iron-nickel core generates Earth's magnetic field. Together, these layers create a dynamic, living planet.

Crust Composition

Silicate Minerals and Rocks

• Crust primarily composed of silicate minerals and rocks formed from silicon and oxygen atoms bonded with various metal ions (aluminum, iron, magnesium, calcium, sodium, potassium)
• Silica ($SiO_2$) content varies between oceanic and continental crust
• Silicate minerals include quartz, feldspar, mica, amphibole, pyroxene, and olivine
• Common silicate rocks include granite, basalt, gneiss, and schist

Oceanic Crust Characteristics

• Covers ~60% of Earth's surface beneath the oceans
• Relatively thin layer averaging 6-7 km in thickness
• Primarily composed of dense, dark-colored mafic igneous rocks like basalt and gabbro
• Higher density due to greater abundance of heavier elements (iron, magnesium)
• Younger geologic age (< 200 million years old) due to continuous recycling at subduction zones and mid-ocean ridges

Continental Crust Properties

• Covers ~40% of Earth's surface forming the continents and shallow seabed close to shores (continental shelves)
• Thicker layer averaging 30-50 km in thickness with roots extending up to 100 km beneath mountain ranges
• Primarily composed of lighter-colored felsic igneous rocks like granite and diorite, along with metamorphic and sedimentary rocks
• Lower density due to greater abundance of lighter elements (silicon, aluminum, oxygen)
• Older geologic age (up to 4 billion years old in cratonic cores) as it is not subducted and recycled like oceanic crust

Mantle Layers

Upper Mantle Characteristics

• Extends from the base of the crust to a depth of ~660 km
• Primarily composed of peridotite, an ultramafic rock rich in olivine and pyroxene minerals
• Uppermost part (lithospheric mantle) is rigid and combined with the crust forms the tectonic plates
• Asthenosphere layer beneath the lithosphere is partially molten and facilitates plate motion due to reduced viscosity
• Transition zone at the base of the upper mantle marked by mineral phase changes (olivine to wadsleyite and ringwoodite) due to increasing pressure

Lower Mantle Properties

• Extends from ~660 km depth to the core-mantle boundary at ~2900 km
• Primarily composed of high-pressure mineral phases of perovskite (bridgmanite) and ferropericlase
• Increased density and viscosity compared to the upper mantle
• Slow convection currents over millions of years contribute to heat transfer and plate tectonics
• Contains regions of anomalous seismic wave speeds (large low-shear-velocity provinces, ultra-low velocity zones) that may represent hot spots or partial melting

Density Stratification in the Mantle

• Mantle density increases with depth due to compression under the weight of overlying material
• Density stratification creates distinct layers with different physical properties and dynamics
• Transition zone separates upper and lower mantle with sharp increases in seismic wave speeds at 410 km and 660 km depths
• Differences in chemical composition between upper and lower mantle remain uncertain, but may involve changes in iron, aluminum, and silicon content
• Layered convection with limited material exchange between upper and lower mantle is supported by geochemical differences in magmas sourced from different depths

Core Materials

Iron-Nickel Alloy Composition

• Core primarily composed of an iron-nickel alloy with smaller amounts of lighter elements (sulfur, oxygen, silicon, carbon, hydrogen)
• High density (~10-13 g/cm³) due to the abundance of iron, the heaviest element that can be produced in significant quantities by nuclear fusion in stars
• Outer core is liquid with convection currents that generate Earth's magnetic field through the geodynamo process
• Inner core is solid due to the extreme pressure despite high temperatures exceeding 5000°C
• Solidification of the inner core releases latent heat and light elements that drive convection in the outer core

Chemical Differentiation and Core Formation

• Earth's layered structure is a result of chemical differentiation during its early history
• Differentiation involved the separation of immiscible components (metals, silicates) based on density differences
• Dense metallic iron along with siderophile elements (nickel, gold, platinum) sank to the center to form the core
• Lighter silicates and lithophile elements (silicon, oxygen, aluminum) remained in the outer layers to form the mantle and crust
• Core formation was largely complete within the first 50-100 million years of Earth's history based on geochemical evidence and modeling
• The concentration of heat-producing radioactive elements (potassium, uranium, thorium) in the silicate mantle and crust contributes to Earth's long-term thermal evolution and plate tectonics