Earth Structures and the Earth's Interior

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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.  

Transcript of Earth Structures and the Earth's Interior

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Earth Structures and the Earth's Interior 

Introduction

In order to understand the structure we find near thesurface of the earth, we must understand the interior the earth and its gross structure. Much of the what weknow, or think we know, about the structure of the eais related to indirect observations we have made thathelp us make hypotheses about it. The structure of thearth appears to unique, at least within our solar system and may be related to the way we believe theearth formed.

 

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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.

 

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 The Gravity data is a little more straightforward. The surface rocks on the earth, fact the entire crust, appears to have adensity of about 2.7 gm/cc. The Earth'smass suggest an average density of abo7.0gm/cc. Thus the interior of the earth

must be denser.

The magnetic data is a little more complethan the gravity data. WE know that theearth has a dipole magnetic field with anorth and south magnetic pole. We alsoknow that throughout history the magnepoles have reversed.

 The reversal in magnetic fields is frozeninto the oceanic crust of the earth.

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 Structures on the Surface of the Earth

In order to understand the structures on the surface of the earth, it is important toexamine 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 volum

of a rock under load. Rocks behave either by brittle deformation and fracture, or bductile 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 tlink below.

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Deformation

Stress and Deformation

The way in which a rock deforms and a structure is created is a function of anumber of factors. These include the type of stress applied, the temperature andpressure 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 kilometeof depth. This trend is shown below. At the surface of the earth (lo P and T) rocksare brittle. The deeper you move into the earth (increasing P and T), the moreductile rocks become.

 Another important factor is the stress state, or the manner in which the load isapplied to the rock. There are three different types of stress states; Compressiontension, and shear. 

The final parameter affecting rock deformation the type of rock or earth material being deformeCertain rocks, like granite are much stronger th

others, like limestone.

To see how all of these parameters affect thedeformation of earth materials, click the linkbelow.

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Rock Deformation

Faults/Brittle Earth Structures

There are four types of faults, each corresponding to the stress state that producit. The types of faults are shown below. For the purposes of this discussion we w

treat strike slip and oblique slip faults as one.

 Reverse Faults

Reverse faults arecaused bycompresional stress

and are often found aconvergent plateboundaries. Theformation of a reversfault is shown in themovie link below.

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Normal Faults

Normal faults are caused by tension or extension and are often found at divergenboundaries. The formation of a normal fault is shown in the movie link below.

Normal Fault

 Strike-Slip Faults

Strike-slip faults are caused by shear stress and are often found at oblique-slip o

transform boundaries. The formation of a strike-slip fault is shown in the movie libelow.

Strike-slip Fault

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Folds/Ductile Earth StructuresWhen rocks areburied at depth,and are subject tohigh pressure antemperature, theytend to flow insteof fracture. Theresulting stucture

is usually a fold.The formation angeometry of a folare shown in thelink and figurebelow.

Fold

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To understand folding geometry we must also understand rock orientation. Wedefine the orientation of rock beds in terms of their strike and dip.

The strikis theorientatiof the linof 

intersection between thehorizontal plane and the rockbed. The dip is the angle that

the bed makes with horizontal,measured in the vertical plan

in the direction prependicular to the strike.

There are a variety of different styles of folding. Open folding is shown below in tgently buckled and bent bedding of the rock outcrops in the image.

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Folding may also be tight as is the case with the isoclinal folds shown in the imagbelow. In isoclinal folds the limbs and axial plane are parallel.

In overturned foldsthe axial plane andone limb dip in ondirection and theother limb is usuaclose to vertical, ashown in the imagbelow.

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In Recumbent folds the axial plane is horozontal, as shown in the image below.