EXPLORING EARTH’S INTERIOR Grotzinger • Jordan Understanding Earth Sixth Edition Chapter 14: EXPLORING EARTH’S INTERIOR © 2011 by W. H. Freeman and Company

Exploring Earth’s Iinterior Chapter 14 Exploring Earth’s Iinterior

About Earth’s Interior The center of the Earth is about 6,400 km below us … the deepest well is 10 km. Heat inside the Earth drives the core’s geodynamo and the mantle’s convection. Earth’s interior is explored by using information from seismic waves and their passage through the body of the Earth.

Lecture Outline 1. Exploring the interior with seismic waves 2. Layering and composition of the Earth’s interior 3. Earth’s internal temperature 4. Visualizing Earth’s 3-D structure

Lecture Outline 5. Earth’s magnetic field and the geodynamo

Exploring the Interior with Seismic Waves ● Basic types of waves ● compressional (P waves) ● shear (S waves) ● Reflection and refraction

Exploring the Interior with Seismic Waves ● Paths of seismic waves ● wave bending ● shadow zones (P and S) ● reflection at internal boundaries

P- and S-wave paths from an earthquake

P-, S-, and surface-wave paths: from focus to seismograph

P, S, and surface waves recorded on a seismograph

2. Layering and Composition of the Interior ● Seismology model of Earth’s layers ● crust and lithosphere ● asthenosphere ● mantle with internal phase change at 400 km

2. Layering and Composition of the Interior ● Seismology model of Earth’s layers (continued) ● core-mantle boundary ● core (inner and outer core)

Earth’s layering revealed by seismology

Earth’s mantle structure beneath an old ocean basin: S-wave velocity to depth of 900 km

Thought questions for this chapter The Moon shows no evidence of tectonic plates or their motions, nor has it been volcanically active for billions of years. What does this observation imply about the state and temperature of the interior of this planetary body? How do the existence of Earth’s magnetic field, iron meteorites, and the abundance of iron in the cosmos support the idea that Earth’s core is mostly iron and the outer core is liquid? How would you use seismic waves to find a chamber of molten magma in the crust?

● Heat flow through Earth’s interior ● conduction (lithosphere) 3. Earth’s Internal Temperature ● Heat flow through Earth’s interior ● conduction (lithosphere) ● convection (mantle and core)

Topography of mid-ocean ridges in the Atlantic and Pacific oceans

● Temperatures in the Earth ● geothermal gradient 3. Earth’s Internal Temperature ● Temperatures in the Earth ● geothermal gradient ● normally 20 to 30ºC / km ● 13001400ºC at base of lithosphere ● 30006000ºC + in core

An estimate of the geotherm: temperature increase with depth in the Earth

● Earth’s gravity field ● The geoid 4. Visualizing Earth’s 3-D Structure ● Seismic tomography ● Earth’s gravity field ● The geoid

Mantle structure

The geoid: shape of planet Earth

Thought questions for this chapter How would seismic tomography answer the question, “How deep do the subducted slabs go before they are assimilated?” Where in the mantle might you look to find regions of anomalously low S-wave speeds?

5. Earth’s Magnetic Field and Geodynamo ● Earth’s dipole field ● complexity of the magnetic field ● non-dipole field component ● secular variations in the ● magnetic reversals

Change in the location of the North magnetic pole from 1600 to 2000

5. Earth’s Magnetic Field and Geodynamo

5. Earth’s Magnetic Field and Geodynamo

5. Earth’s Magnetic Field and Geodynamo

Earth’s magnetic field lines

Earth’s magnetic field reversals: Step 1

Earth’s magnetic field reversals: Step 2

Earth’s magnetic field reversals: Step 3

Earth’s magnetic field reversals: Step 4

● records of magnetization ● thermoremanent ● depositional remanent 5. Earth’s Magnetic Field and Geodynamo ● Paleomagnetism ● records of magnetization ● thermoremanent ● depositional remanent ● magnetic stratigraphy

Thermoremanent

Depositional remanent

Paleomagnetic time scale

● Magnetic field and the biosphere ● magnetic orientation 5. Earth’s Magnetic Field and Geodynamo ● Magnetic field and the biosphere ● magnetic orientation ● magnetic frame of reference ● field offers protection from the solar wind

Key terms and concepts Conduction Convection Core-mantle boundary Compression wave Conduction Convection Core-mantle boundary Depositional remanent magnetism Dipole Geotherm Isostasy Low-velocity zone Lower mantle Mohorovičić discontinuity Paleomagnetism Phase change Seismic tomography Shadow zone

Key terms and concepts Thermoremanent magnetization Transition zone Shear wave Thermoremanent magnetization Transition zone Upper mantle