Week 9
Earth Structures
and the Earth's
Interior
Introduction
In order to understand the structure we find near the surface of the earth, we must understand the interior of the earth and its gross structure. Much of the what we know, or think we know, about the structure of the earth is related to indirect observations we have made that help us make hypotheses about it. The structure of the earth appears to unique, at least within our solar system and may be related to the way we believe the earth formed.
Evidence about the Interior Structure of the Earth
The evidence for the structure of the Earth's interior comes from three sources; gravity data, seismic data, and magnetic data.
The Gravity data is a little more straight forward. The surface rocks on the earth, in fact the entire crust, appears to have a density of about 2.7 gm/cc. The Earth's mass suggest an average density of about 7.0gm/cc. Thus the interior of the earth must be denser.
The magnetic data is a little more complex than the gravity data. WE know that the earth has a dipole magnetic field with a north and south magnetic pole. We also know that throughout history the magnetic poles have reversed.
The reversal in magnetic fields is frozen into the oceanic crust of the earth.
Structures on the Surface of the Earth
In order to understand the structures on the surface of the earth, it is important to examine the ways in hitch rocks react to the loads and stresses placed on them. Rocks respond to stress by deformation, which is the change in shape and volume of a rock under load. Rocks behave either by brittle deformation and fracture, or by ductile deformation and flow.
The structures we observe on the surface of the earth are a reflection of either brittle fracture of ductile flow. To examine rock deformation in more detail, click the link below.
Stress and Deformation
The way in which a rock deforms and a structure is created is a function of a number of factors. These include the type of stress applied, the temperature and pressure under which the rock is deformed, and the type of rock.
Pressure and temperature increase as you move down in the earth. The rate of increase is roughly 30 degrees centigrade per kilometer and 5000 psi per kilometer of depth. This trend is shown below. At the surface of the earth (lo P and T) rocks are brittle. The deeper you move into the earth (increasing P and T), the more ductile rocks become.
Another important factor is the stress state, or the manner in which the load is applied to the rock. There are three different types of stress states; Compression, tension, and shear.
The final parameter affecting rock deformation is the type of rock or earth material being deformed. Certain rocks, like granite are much stronger than others, like limestone.
To see how all of these parameters affect the deformation of earth materials, click the link below.
Faults/Brittle Earth Structures
There are four types of faults, each corresponding to the stress state that produces it. The types of faults are shown below. For the purposes of this discussion we will treat strike slip and oblique slip faults as one.
Reverse Faults
Reverse faults are caused by compresional stresses, and are often found at convergent plate boundaries. The formation of a reverse fault is shown in the movie link below.
Normal Faults
Normal faults are caused by tension or extension and are often found at divergent boundaries. The formation of a normal fault is shown in the movie link below.
Strike-Slip Faults
Strike-slip faults are caused by shear stress and are often found at oblique-slip or transform boundaries. The formation of a strike-slip fault is shown in the movie link below.
Folds/Ductile Earth Structures
When rocks are buried at depth, and are subject to high pressure and temperature, they tend to flow instead of fracture. The resulting stucture is usually a fold. The formation and geometry of a fold are shown in the link and figure below.
To understand folding geometry we must also understand rock orientation. We define the orientation of rock beds in terms of their strike and dip.
The strike is the orientation of the line of intersection between the horizontal plane and the rock bed. The dip is the angle that the bed makes with horizontal, measured in the vertical plane, in the direction prependicular to the strike.
There are a variety of different styles of folding. Open folding is shown below in the gently buckled and bent bedding of the rock outcrops in the image.
Folding may also be tight as is the case with the isoclinal folds shown in the image below. In isoclinal folds the limbs and axial plane are parallel.
In overturned folds the axial plane and one limb dip in one direction and the other limb is usually close to vertical, as shown in the image below.
In Recumbent folds the axial plane is horozontal, as shown in the image below.
Review Exercises
In the exercises below you will be asked to interpret the key factors involved in creating earth structures. You will be given a tec tonic setting (plate margin) with the location of the deformation marked with an "X". You will then be asked to determine the stress state, temperature and pressure, and resulting structure. Click the links below to access the problems.
All multimedia applications contained in this course are the property of Arjuna Multimedia , copyright 2005
