Global Geophysics and Geodesy-Assignment 1
Question 1-Global Earth’s Structure
The Earth is made up of two parts-the interior structure that lies beneath the crust, and
the outer surface that can be seen in the form of oceans and continents. The interior structure
of the Earth is responsible for many of its geophysical properties. Current knowledge about the
Earth’s interior structure has been determined by observing the travel time of seismic waves
when there is an earthquake. Figure 1 shows a schematic diagram of Earth’s interior structure.
Figure 1-Earth’s interior structure
As observable from Figure 1, the outermost ring is the Earth’s crust, also known as the
lithosphere. Directly below that is the upper mantle, followed by the lower mantle, then the
liquid outer core and then the solid inner core. The solid inner core is an iron-nickel alloy, and it
is responsible for the Earth’s magnetic field. Table 1 summarizes the properties of each of the
components in the Earth’s interior structure.
Table 1-Components of Earth’s Crust
Region Thickness(km) Radius Composition Phase Viscosity(rheology)
Inner Core 1200 1200 Iron-nickel Solid Viscous
Outer Core 2300 3500 Iron-nickel Liquid Fluid
Lower 2200 5700 Silicates Solid Viscous
Upper 700 6400 Silicates Solid Viscous
Mantle Scientists infer various physical parameters in the Earth’s interior, such as density and elasticity
constants through the use of a process called inversion. Inversion is carried out by measuring
the travel time of seismic waves that occur when there is an earthquake.
A plot of worldwide seismic epicentres has led to the theory of plate tectonics. The theory of
plate tectonics suggests that recently in the geologic past, present day continents were part of
a much larger supercontinent, known as Pangaea. The epicenter of an earthquake is directly
above the focus of an earthquake, where an earthquake originates (USGS, 2012). The epicenter
is the site where the liquid outer core absorbs secondary shear waves. Seismic epicenters are
located on Earth’s plates, on plate boundaries. According to the theory of plate tectonics, the
earth is divided into several plates that are in constant motion with one another and compose
the Earth’s crust. At these plate boundaries are seismic epicenters where earthquakes form,
volcanic activity takes place, and mountain building occurs.
The theory of plate tectonics suggests that all the plates in the Earth’s crust are connected t