9a_InternalStructure

Earth’s Changing Landscape Systems

Earth’s Internal Heat Engine
- Major processes contributing to Earth's internal heat:
  - Heat from colliding particles during Earth's formation.
  - Heat released from gravitational contraction of the early Earth.
  - Heat from radioactive decay of unstable isotopes within Earth.
  - Early impact with a Mars-sized object which caused significant heating.
  - Heat released as iron crystallized to form the solid inner core.

Planetesimals Aggregated to Form Larger Bodies
- Formed approximately 4.6 billion years ago, leading to planetary formation.
- Earth was struck by a large Mars-sized body around 4.5 billion years ago.
  - This collision resulted in extensive heating and melting of both the Earth and the impactor.
  - Formation of the Moon from ejected material into space.
Global Chemical Differentiation
- Began about 4.5 billion years ago, setting the stage for Earth’s internal structure.

Magma Ocean
- Initial magma ocean was hundreds of kilometers deep.
- Cooled and crystallized from the bottom-up to form a solid mantle capped by primitive crust composed of basalt and komatiite.
Radioactive Isotopes
- Short-lived radioactive isotopes with a few million years half-lives have long since decayed.
- Long-lived isotopes continue to warm the planet.
Internal Layers of Earth Today
- Iron sank during differentiation to form outer and inner core.
- The mantle and crust were formed after cooling of the magma ocean.
- Studying Earth’s interior is challenging due to:
  - Limited depth of drilling (few kilometers into the crust).
  - Rocks sampled may not represent deeper layers.

Magma brings up pieces of deeper rocks through:
- Mines, drilling, and deep, uplifted rock exposures.

Techniques for subsurface exploration include:
- Magnetic data.
- Gravity data and models.
- Electrical surveys.
- Seismic-reflection data.

Definition and Behavior
- Tectonic forces cause Earth’s crust to deform and break along faults, releasing energy as seismic waves.
- Seismic waves travel through Earth in all directions, contributing to our understanding of earthquakes.
- The point of initial fault slippage is known as the focus, with the point directly above it on the surface referred to as the epicenter.
Seismographs
- Instruments measuring ground vibrations, capturing three components of motion:
  - Vertical up-down motion.
  - Horizontal east-west motion.
  - Horizontal north-south motion.

P-waves (Primary Waves)
- Fastest seismic waves, arriving first at a station.
- Travel through solids, liquids, and gases at an average speed of ~5 km/sec.
- Push or pull particles of matter in the direction of travel.
S-waves (Secondary Waves)
- Arrive second; half the speed of P-waves.
- Can only travel through solid rock; shear waves that push material at right angles.
Surface Waves
- Slowest waves, confined to surface and outer layers of the Earth.
- Cause most earthquake destruction, similar in motion to ocean waves.

When seismic waves encounter boundaries (different materials), they split into reflected and refracted waves:
- If the waves speed up, they bend upward; if they slow down, they bend downward.

Oceanic Crust
- Ranges from 3 to 15 km thick, primarily composed of basalt and gabbro.
Continental Crust
- ~40 to 65 km thick, average composition of granite, more buoyant than oceanic crust.

Lithosphere
- Cool, rigid layer averages about 100 km thick but can be up to 250 km beneath older continental regions.
- Comprised of the crust and solid upper portion of the mantle.
Asthenosphere
- Lies just below the lithosphere, a weaker zone of partially melted peridotite.
- Seismic velocities slow down in the upper portion.

Upper Transition Zone
- Marks the change from olivine to spinel crystal structure (400 km depth).
Lower Transition Zone
- Marks the transition from spinel to perovskite crystal structure (670 km depth).
- Seismic waves bend outward in the deep mantle due to increasing velocities.

Outer Core
- Composed of liquid iron, where P-waves slow down and S-waves cannot pass through.
- P-wave shadow zone created due to bending of seismic waves.
Inner Core
- Consists of solid iron and nickel, where P-waves speed up again.

Generated by circulation of liquid iron in the outer core due to convection driven by internal heat.
Electric currents from this motion create Earth’s magnetic field.

Earth’s temperature increases with depth, known as the geothermal gradient:
- Averages between 20°C and 30°C per km in the crust, less in other layers.
- Variations can occur in heat flow across different regions.

Heat continues to escape from the Earth's interior:
- High heat flow in mid-ocean ridges and volcanically active areas.
- Lower heat flow within stable continental interiors.

Heat flow in the crust: conduction, generally inefficient at transferring heat.
Heat flow in the mantle: moderate temperature increase with depth, effective heat transfer likely through convection.

Two-Layer Mantle Convection
- Separate convection cells for upper and lower mantle, defined by the 670 km transition zone.
Whole-Mantle Convection
- Upper and lower mantle form one system with large convection cells originating at the core-mantle boundary.

Creates 3D models of Earth's interior based on seismic wave data.
Requires extensive seismic records from various earthquakes, revealing regions with varying wave speeds, attributed to temperature and material differences.

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