Notes on Earth's Interior (Layers, Seismic Evidence, and Boundaries)

Understanding the structure and composition of Earth's crust and interior (layers) from seismic studies; focus on their properties and boundaries.

Outermost layer of the Earth; thickness ranges from about 0 \text{ to } 100 \text{ km} (Page 5).
Two types:
- Continental crust: thickness 25-90\text{ km}; divided into tectonic plates; plates move slowly (a few cm/year).
- Oceanic crust: thickness 6-11\text{ km}; new crust forms at mid-ocean ridges as magma from the asthenosphere rises and cools.
The lithosphere = crust + upper mantle (not the entire upper mantle); both crust types float on the denser mantle (asthenosphere).

Located beneath the crust; the thickest layer, extending to about 2{,}900\text{ km} depth (Page 13).
Divided into upper mantle (includes the asthenosphere) and lower mantle.
Mohorovičić discontinuity (Moho): boundary between crust and mantle; identified by a sudden increase in seismic wave speed; discovered in 1909 by Andrija Mohorovičić (Page 14).
Asthenosphere: more fluid/plastic than the lithosphere above; rocks are partially molten due to temperature/pressure; enables plate tectonics (Pages 9–12).
Mantle convection currents drive movement of tectonic plates (Page 32).

Core is the Earth's innermost layer, beneath the mantle; divided into:
- Outer core: liquid layer of iron and nickel; thickness about 2{,}300\text{ km}; generates Earth’s magnetic field (geodynamo) (Pages 23–25).
- Inner core: solid sphere of iron and nickel; radius about 1{,}220\text{ km}; temperatures up to 5{,}700^{\circ}\text{C}; remains solid under high pressure (Pages 23–27).
Outer core is the only completely liquid layer; inner core is solid (Pages 23–24).

Seismic waves reveal Earth’s interior structure; two main types:
- P-waves (Primary): compressional; travel through solids and liquids (Pages 38–39).
- S-waves (Secondary): shear; travel only through solids (Pages 38–39).
Waves bend (refract) or reflect when crossing boundaries; speed changes help locate boundaries (Moho, core–mantle boundary) (Pages 39–42).
P-waves slow and bend when entering a liquid outer core, creating a shadow zone where no direct P-waves are detected (between 104^{\circ} and 140^{\circ}) (Pages 41–42).
S-waves disappear beyond 104^{\circ} from the epicenter, confirming the outer core is liquid (Pages 42–43).
Seismic studies show: outer core is liquid; inner core is solid; clear boundaries between layers (Pages 43–44).

Moho discontinuity: crust–mantle boundary.
Conrad discontinuity: boundary within the mantle (upper mantle transition zone).
Repetti discontinuity: boundary within the lower mantle.
Gutenberg discontinuity: mantle–outer core boundary.
Lehmann discontinuity: outer core–inner core boundary.
Diagram depths (approximate, from the schematic):
- Moho: near the crust–mantle boundary (5–70 km for crust; varies regionally).
- Conrad: around 670\text{ km}.
- Repetti: around 3150\text{ km}.
- Gutenberg: around 2900\text{ km}.
- Lehmann: around 6370\text{ km} (inner core boundary).

The crust is the outermost and thinnest layer; the mantle is the thickest; the core is the innermost layer.
Plate tectonics = movement of lithospheric plates on the asthenosphere; driven by mantle convection.
The geodynamo in the outer core generates Earth’s magnetic field; rotation + flowing molten metals create electric currents and magnetic fields.
Seismic waves are essential tools to map internal structure and boundaries.

Knowledge of layers and boundaries explains seismic wave behavior, volcanic activity, mountain formation, and the magnetic field that protects life.