Grotzinger Jordan Press Siever Understanding Earth Fifth Edition

Grotzinger • Jordan • Press • Siever Understanding Earth Fifth Edition Chapter 14: EXPLORING EARTH’S INTERIOR Copyright © 2007 by W. H. Freeman and Company

Concepts we will use in this class In bold are the ones I consider most important Compressional wave Conduction Convection Core-mantle boundary Depositional remanent magnetism Dipole Geotherm Lower mantle Low-velocity zone Mohorovičić discontinuity (MOHO) Paleomagnetism Phase change Principle of isostasy Seismic tomography Shadow zone Shear wave Thermo-remanent magnetization

Lecture Outline 1. Exploring the interior with seismic waves 2. Layering and composition of the interior 3. Earth’s internal heat and temperature 4. 3 -Dimensional structure of the mantle 5. Earth’s magnetic field

Chapter 14 Exploring Earth’s Interior

Background facts about Earth’s Interior • The center of the Earth is about 6400 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.

1. Exploring the interior with seismic waves ● Basic types of waves ● compressional (P waves) ● shear (S waves) ● Reflection and refraction

1. Exploring the interior with seismic waves ● Paths of seismic waves ● wave bending (refraction) ● shadow zones (P and S) ● reflection at internal boundaries

P-wave paths

S-wave paths

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 (a. k. a. 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

3. Earth’s internal heat and temperature ● Heat flow through Earth’s interior ● conduction (lithosphere) ● convection (mantle and core)

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

3. Earth’s internal heat and temperature ● Temperatures in the Earth ● geothermal gradient ● normally 20 to 30º C / km ● 1300 -1400º C at base of lithosphere ● 3000 -6000º C + in core

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

4. 3 -Dimensional structure of the mantle ● Seismic tomography ● Earth’s gravity field ● The geoid

Mantle structure

Mantle structure

Mantle structure

The geoid: shape of planet Earth

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

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

Earth’s magnetic field and the geodynamo

Earth’s magnetic field and the geodynamo

Earth’s magnetic field and the 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

5. Earth’s magnetic field and the dynamo ● Paleomagnetism ● records of magnetization ● thermo-remanent ● depositional remanent ● magnetic stratigraphy

Thermoremanent

Depositional remanent

Depositional remanent

Magnetic stratigraphy

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