Earth’s interior

• Impossible to sample
• Require surface measurements
• One of major measurements is seismic wave velocity
• Others

– Heat flow
– Magnetism

Seismic waves

• Speed of seismic waves depend on material

– E.g. faster if more dense material
– Faster in mantle than crust

• Path waves take is complicated
• At boundaries between material they change directions:

– Sometimes they “bend”, e.g. refract like water waves
– Sometimes they reflect

Seismic wave path

• Assume earth material properties constant

– seismic waves would travel in straight line

• Earth made of layers

– Properties within layers vary
– Waves change direction at each layer

Waves traveling through earth

• Refraction
• P waves

– P waves not recorded between 105º and 142º from earthquake focus
– P waves pass through liquid, but slowly
– Result is refraction toward core

• S waves

– Cannot travel through liquid
– No S waves farther than 105º from focus

• Conclude – outer core is liquid
 

• Reflected waves:

– Can identify the reflected waves on arrival time and amplitude
– Imply boundary between two types of material
– Speed of waves known
– Possible to calculate distance to boundary

• Oil exploration

Interior Composition and Structure

• Analogy

– Car travels from LA to SF
– Know routes and speed limits
– Know how long it takes
– Possible to determine path it takes

• Similar problem for seismic waves in earth

• Possible to determine the speed of seismic waves with depth in earth
• The speed of seismic waves delineate the major layers

– Crust
– Mantle
– Core

Crust

• P-wave velocities 6 to 7 km/sec
• Seismic waves show:

– Continental crust averages 40 km thick

• Up to 65 km under mountains
• Made largely of granite (slower P-wave velocity)

– Oceanic crust averages 5 km thick

• Made entirely of basalt and gabbro

• Base of crust P-wave velocity increase to 8 km/sec

– Mohorovicic discontinuity (Moho)
– Indicate material below crust is denser
– Ultramafic rocks: Peridotite: Mantle

• Consistent with idea of isostacy
 

• Isostacy:

– The mass of material displaced equals the mass of material doing displacing
– Material floats if less dense than material it floats in
– Examples:

• Continents
• Mountains
• Icebergs

Mantle

• S-wave velocities used to determine structure of mantle
• Upper 0 to 200 km: Lithosphere

– 200 km thick under continents
– Fast S-wave velocities
– Indicate strong, solid material

• Directly below Lithosphere

– S-wave velocities decrease and are attenuated
– Indicate partial melt of material (few %)
– This zone called Asthenosphere

• From base of Asthenosphere to ~400 km depth

– Gradual increase in S wave velocity
– Reflects more dense material from high P

• At ~400 km, S-waves jump (discontinuity)

– Indicates change in mineral structure from high P
– More dense packing

• Similar increase at ~670 km depth

– Indicates a second change in mineral composition

• Changes in composition observed in laboratory studies
• No other discontinuities until core/mantle boundary
 

Core-mantle boundary

• Layer ~200 km thick
• Variable properties and thickness (?)
• Partially molten (?)
• Derived from heat from core (?)
• Source of hot spots (?)
 

Core

• P-wave velocities reflect liquid-solid nature
• Find S-waves transmitted through inner core
• Cosmic abundances of material indicate Fe major component
• 1/3 of earth mass in core – indicates Fe major component

Internal Heat

• Source of heat

– Radioactive decay
– Collision of planetesimals

• Continuously cooling – Earth’s heat flow

– Convection
– Conduction

• Conduction

– Transfer of heat energy without transfer of mass
– E.g. heating frying pan on stove

• Different materials have different ability to conduct heat

– Plastic v. metal
– Rocks very poor conductors

• Convection

– Rising of material when heated because of lower density
– Very efficient: all heat carried with material

• Convection occurs in earth

– Manifested as seafloor spreading

• Unclear what part of mantle convects

– Only asthenosphere (?)
– Whole mantle (?)

• Seismic tomography (like CAT scan using seismic waves

– Find fast P-wave velocities (cool rocks) extend to core-mantle boundary

Magnetic Field

• Earth has a magnetic field

– Most other planets don’t
– Strongest of all planets

• Field acts like bar magnet at center of earth
 
 

• Origin of magnetic field

– Heat destroys magnetism

• Generate magnetic field with electrical currents

– believe motion of liquid iron core creates magnetic field

Paleomagnetism

• Some minerals on surface are magnetic
• Magnetic direction aligns with earths magnetic field

– ONLY at time rocks gained their magnetic properties

• Remanent magnetization: record of magnetic orientation

– Thermal: cooling past curie point
– Deposition: sediments align with magnetic field

 

• Paleomagnetism

– Study of orientation of magnetic field through time
– Find that magnetic field has reversed in past
– Possible to use this information to date rocks

• Magnetic stratigraphy