Introduction to Earth Sciences I
1.4 Summary
We have learnt how several important macro-scale properties of the Earth were deduced. These provide a very basic description of the object we are dealing with and might represent some of the first entries made under "Earth" in the encyclopedia. In each case a set of observations were used in combination with a theory to infer the information we sought.
Information Available |
Theory |
Information Obtained |
Shadows (sun angle) in different locations |
Geometry |
Size of Earth |
Masses on balance beam |
Gravitation |
Mass of Earth, denser in middle |
Normal Mode frequencies |
Elasticity |
Strength of whole Earth |
Deformation caused by volcanic islands |
Elasticity of rigid bodies |
Strength of outer layers |
Rebound after de-glaciation |
Flow in viscous materials |
Viscosity of deep layers |
What we have deduced from these studies is that although the Earth is indeed very rigid in its outer layers, it is surprisingly soft in its inner layers. This inner softness is a critical feature of the Earth that governs many of its dynamic properties, and these are the subject of the next several topics.