Lecture 3
Lecture Notes on Earth’s Interior
Office Hours
Monday: 11:30 AM – 1:00 PM on Zoom
Thursday: 12:00 PM – 1:00 PM in P-110
By appointment
Game Plan
Topics Covered:
Layer composition of the Earth's interior
Earth’s magnetic field and geodynamo
Exploring Earth’s interior through seismology
Announcements/Reminders:
Module 1 Quiz posted, due Wednesday 9/10 at midnight
Module 1 Activity: Submit if not already done
Remember to comment on 3 posts!
Lab 1: Scheduled for today (9/5) and Tuesday (9/9)
Learning Outcomes
After this lecture, students should be able to:
List the layers of the Earth by composition and behavior.
Describe the composition/characteristics and know the relative sizes of each layer.
Explain the role of outer core convection.
Describe the two types of seismic body waves (P-waves and S-waves).
Explain methods used to determine the composition and location of Earth's layers.
Utilize P and S waves in the study of the Earth’s interior.
Understand the significance of a shadow zone.
Composition of Earth’s Interior
Summary of Layers
Crust:
Composition:
Silicon (28%), Aluminum (8%), Iron (6%), Magnesium (4%), Calcium (2.4%), Other (5.6%), Oxygen (46%)
Average Density: ~2.7 g/cm³
Mantle:
Composition:
Silicon (21%), Aluminum (2.4%), Iron (6.3%), Magnesium (22.8%), Calcium (2.5%), Oxygen (44%)
Average Density: ~3.3 g/cm³
Outer Core:
Composition:
Nickel (6%), Iron (94%)
Density Range: 10 - 12 g/cm³
Inner Core:
Composition: Primarily Iron
Average Density: ~13 g/cm³
The Core
Description:
An iron-rich sphere with a radius of 3,471 km
Divided into:
Inner Core (Radius = 1,220 km): Solid sphere due to immense pressure.
Outer Core (2,255 km thick): Liquid layer.
Composition of Core:
Material: Iron-nickel alloy with minor components such as oxygen, silicon, and sulfur.
Density and Temperature:
Inner Core Density: 13 g/cm³
Outer Core Density: 10 - 12 g/cm³
Temperature of Inner Core: Approximately 6,000 °C (similar to the surface of the sun).
Core Rotation and Convection
Both inner core and outer core are spinning.
The heat from the inner core causes convection in the outer core, contributing to the generation of electric currents, which in turn create the Earth's magnetic field.
Evidence for Outer Core Convection
The presence of Earth's magnetic field indicates the movement of molten iron within the core, generating an electrical current.
The magnetic field acts as a magnetic dipole similar to a bar magnet.
Magnetic Pole Reversals
Definitions:
Secular Variation: Movement of magnetic poles geographically over time.
Spontaneous Reversals: Infrequent flips of the north and south magnetic poles.
Evidence for Pole Reversal:
Thermoremanent Magnetization: Rocks forming under specific conditions preserve their magnetic orientation, which aids in understanding historical magnetic conditions.
Paleomagnetic Time Scale: Recorded history of magnetic pole reversals.
The Mantle
Description:
Solid rock layer between the crust and the core, approximately 2,885 km thick (82% of Earth’s volume).
Composition: Predominantly ultramafic rock called peridotite.
Convection: Slow movement of mantle material; hot mantle rises while cold mantle sinks, leading to tectonic plate movements.
Subdivisions:
Upper Mantle: Cool and brittle (50 – 120 km thick).
Transitional Mantle: Ductile behavior extending up to 400 km.
Lower Mantle: Denser and hotter.
Behavior of the Mantle
The mantle can behave like a solid and a viscous fluid depending on the time scale.
Examples:
Glaciers: Flow slowly over time, but shatter when struck hard.
Silly Putty: Flows slowly but hardens or breaks with impact.
The Crust
Description: Thin, rocky outer layer with varying thickness.
Two Types of Crust:
Continental Crust: Average thickness of 35 – 40 km (up to 70+ km in mountain ranges), made up of a variety of rock types.
Oceanic Crust: Mainly igneous rock (basalt), with an average thickness of 7 – 10 km.
Density Comparison: Continental crust has a density of approximately 2.7 g/cm³ compared to oceanic crust's density of about 3 g/cm³.
Crust Characteristics
Buoyancy: The massive continental crust floats on the mantle, leading to its greater thickness compared to oceanic crust.
Regions of high elevation typically associated with thicker crust; oceanic crust is denser and thinner, thus sits lower than sea level.
Layers of Earth: Composition vs. Behavior
Compositional Layers: Core, mantle, and crust respectively characterized by their materials.
Behavioral Layers: Lithosphere (solid and brittle) vs. Asthenosphere (ductile and capable of flow) based on physical properties.
Types of Seismic Waves
Body Waves: Includes P-waves and S-waves.
P-waves (Primary waves):
Compressional waves, fastest type of seismic wave, travel through solid and liquid.
Wave motion in the direction of travel causing material contraction and extension; speeds up to ~13,000 mph.
S-waves (Secondary waves):
Shear waves that move perpendicular to the direction of travel, cannot traverse through liquids, causing material to shear.
Slower than P-waves, with speeds around 9,000 mph.
Importance of Seismic Waves
Seismic waves help determine the Earth’s interior structure through their velocities and paths:
Changes in wave velocity indicate different density materials.
The presence or absence of S-waves can confirm solid or liquid states in various layers.
Shadow Zones
Areas where seismic waves are not recorded due to the change in material state (e.g., liquid outer core).
P-waves experience shadow zones at angles between 103° and 150°; S-waves are blocked, indicating core properties.
Studying Earth’s Structure
Seismic velocities are essential in determining the composition of Earth's layers and their properties.
Estimate Earth’s Average Density: ~5.5 g/cm³, indicating denser materials in the inner Earth to qualify for this average when factoring surface rock densities (2.0 to 2.7 g/cm³).
Concluding Activity
Group discussion focusing on key concepts around Earth's layers, composition, seismic waves, and shadow zones.
Sketch and label a diagram representing the major layers of the Earth indicating P and S wave paths, illustrating their behavior in relation to each layer's properties.