Alabama Landscapes

Geology, Geologic Processes and Geologic History

Table of Contents

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Internal Geologic Processes

Internal processes get their energy from the Earth's internal heat. They are:

Magmatism: generation, rise and solidification of molten rock (magma) formed by partial melting of the Earth’s interior.  Igneous rocks form by magmatism.

Metamorphism: change of pre-existing rocks by heat, pressure and fluids.  Metamorphic rocks are the product of this process

Deformation: change in size, shape and location of rocks and regolith. 

Brittle rocks are those that rupture. The results of rupture are:

(a) Joints form when rocks rupture with little movement of the resulting blocks (Figure IP1)

Figure IP1. Two sets of joints: one vertical and the other dipping (sloping) to the left. (Image © Mike Neilson)

 

 

 

 

 

 

(b) A fault is a fracture (called the fault plane) with movement of the resultant rock blocks parallel to the fault  plane. There are two common types of faults in Alabama:

      Normal fault: the hanging wall block (one with the acute angle to the fault plane) appears to move down the fault plane (Figure IP4).

      Figure IP4. Cross section of a normal fault. The hanging wall block is to the right of the fault plane. The yellow arrow shows the sense of movement of the hanging wall block. Red: rock stratum cut by fault

       

       

       

       

      Figure IP3. Normal fault cutting coal seams.  The fault plane is almost vertical and the hanging wall block (left) appears to have moved down.  (Image © Mike Neilson).

       

       

      Thrust fault: the hanging wall block (one with the acute angle to the fault plane) appears to move up the fault plane, which has a low angle (Figure IP5).

      . Figure IP5. Cross section of a thrust fault. The hanging wall block is to the right of the fault plane. The yellow arrow shows the sense of movement of the hanging wall block. Red: rock stratum cut by fault

Ductile rocks are those that bend rather than rupture.

      A fold is a bend in a rock layer. The two most common folds are (Figure IP6):

      Figure IP6.  Common folds in Alabama. An anticline is an uparch and a syncline is a downarch

       

       

      Figure IP7.Small syncline.  In this case the axial plane is vertical and the axis horizontal. (Image © Mike Neilson)

       

       

       

       

       

      The sides (limbs) of a fold commonly dip (tilt) in opposite directions, but in highly ductile rocks may be parallel, forming an isoclinal fold

      The axis is the (imaginary) line of maximum curvature along any bed in a fold. It may be horizontal or inclined at an angle to the earth’s surface (plunging). The axial plane is the plane containing all axes of a fold and may be vertical, inclined or horizontal (Figure IP8).

 

 

 

 

Figure IP8.  Axis and axial plane of a fold

 

 

 

 

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