Characteristics and Origins of the Solar System

Lecture 6, September 11, 2000

The Earth as a Planet

Purpose:

• To study the Earth within the context of the other solar system objects.
• To learn things about the solar system which are unavailable from other astronomical objects (so far).

Material from next few lectures from Chapter 7 of textbook, take it out and sing along!

First: “Just the facts, Maam” This information is from Table 7.1 of book.

Characteristic	Value
Distance from Sun	1.00 astronomical unit
Radius	6378 kilometers
Mass	5.98 ´ 10^24 kilograms
Density	5.52 grams/cc

Structure of the Earth

A few kilometers down and we have almost no direct measurements. Almost like astronomy in general. Best technique is seismology, ringing of Earth to Earthquakes.

>>>>> Demonstration with eigenmodes of tuning fork.

Basic structure as shown in Figure 7.2 of book. Structure of Earth divided into three parts

Terra in partes tres divisa est.

• Crust, the outer part we live on, very thin
• Mantle, large layer of molten rock
• Core, or inner metallic part of Earth

The crust is the part that most of geology deals with. However, it is a thin skin on the remainer of the planet.

Properties of the Earth’s Crust

• Extremely thin, and different in continents and oceans.
• Thickness 6 kilometers under oceans and 20 to 70 kilometers under continents.
• Rocks consist of basalts on ocean beds, granites in continents

Basalts and Granites are igneous rocks, associated with volcanic lava, hot flows.

Chemical compositions of these rocks is primarily silicon, oxygen, iron, aluminum, and magnesium. Different blends make different minerals.

>>>> “chocolate sampler” with different minerals.

An intriguing first result of this is that the chemical composition of the Earth differs so markedly from the Sun and the other stars. In those objects, the main elements are Hydrogen and Helium, followed at an abundance of about one atom in a thousand of oxygen, carbon, and nitrogen.

Types of Rocks

• Igneous (basalts and granites)
• Sedimentary
• Metamorphic
• Primitive (to an astronomer the most interesting. No examples on Earth. We will have a “laying on of the hands” of some samples from outer space.)

Characteristics of the Crust

Major fact is that it is fractured into tectonic plates, and that these are moving with respect to each other, in a process called continental drift. Book emphasizes importance of plate tectonics to field of geology, sic p151

“It (plate tectonics) is a concept as basic to geology as evolution by natural selection is to biology, or gravity is to understanding the orbits of the planets”

Personal aside: plate tectonics is actually in better shape experimentally than natural selection. It has been directly verified by measurements showing continental drift. In biology, we know evolution occurs, but natural selection is more of a suggestion as to why it occurs.

Look at Figure 7.6 to see where the plates are.

>>>>>> Map with tectonic plates.

Types of Plate Motions

Remember, in an astronomy class we are talking about all of this to grasp it as a potentially general, widespread planetary process!

· Rift

· Subduction

· Mountain building and plate collision

· Faulting and plate shear

The plate motions are driven by convection of material in the mantle.