Seafloor Spreading and Marine Magnetic Anomalies

Earth Science 

(1) Mountain Building

(a) Begin by briefly (2-4 sentences) summarizing the concept of isostasy and how it affects the elevation of the Earth’s surface and therefore mountains (and lowlands). Use a simple sketch to aid your explanation. 

(b) With the aid of a sketch, explain the processes by which (1) crustal shortening and thickening and (2) removal of the mantle lithosphere can lead to the uplift and the formation of mountainous topography. Include in your explanations the role of density, crustal/mantle roots, how the mantle depth and surface elevation change with each process, and the effect of crustal thickness changes, etc. 

(2) Seafloor spreading and marine magnetic anomalies

With the aid of text tied to simple sketches, explain how marine magnetic anomalies reveal and record the process of seafloor spreading. Your explanation should include brief discussions of magnetic (normal/reversed) polarity, magnetic-reversal chronology, and marine magnetic anomalies, and the process of seafloor spreading.

As in question (1), you should approach this as if you are teaching it to someone. Again, if someone were to read this would they understand the concept of magnetic polarity reversals and how these have been used to explain the concept of seafloor spreading?

1: Mountain Building 44

a:Isostasy: It is the phenomenon in which balance occurs between mantle and crust. It is also helpful in describing the mountain building process. Due to the low density of crustal plates floats over apparently high density mantle. The thickness, roots and elevation of crustal plates are described with the help of Isostasy. There are important role of density through which crustal rocks are uplift and deepens accordingly.
b:1 Crustal shortening and thickening occurs due to the movement of crustal plates over mantle, when two blocks of crustal blocks collides each other and forms thick crustal belt such as can be seen Himalyas. The collision of two blocks over mantle can be seen in below figure. As we can see in the below figure elevation of mountain increases due to upliftment of colliding crustal plates which makes suture zone i:e welding zone between two crustal blocks, it requires very huge amount of force for collision, and the process is called orogeny. Crustal shortening, the term is using because the crustal plates shortens due to the collision and thickening is used because collision zone shows the thickening of crustal plates the suture zone.
b:2 The topography and elevation of mountain is depend upon the intensity of force between colliding crustal blocks, density of blocks etc. The crustal roots are forms due to the inward deep collision among plates or blocks, this causes removal of mantle lithosphere. When two heavy crustal plates moves towards each other then the junction get welding of plates and due to almost same density of plates they do not over ride each other but they weld each other and due further movement of plates they moves upward and inward also. The inward movement of welded plates causes removal of some part of mantle lithosphere. And on opposite side the upward movement continue to takes place due to which elevation of mountain increases. In continuous process of collision the elevation of mountain and crustal roots increases. (See below figure).

2: Sea floor spreading and magnetic anomalies

1: Sea floor spreading is the movement of oceanic plates away from each other and lava comes out along mid oceanic ridges. The plate tectonic theory proved the movement of plates in which divergent plates are of one type.  This phenomenon proved with the help of magnetic anomaly. Magnetic anomaly is  the change in magnetic field from normal field of earth. Magnetic polarity of earth is kept changing in the past and would be in future also. It is the record of magnetic dipole which recorded by the rock during rock formation such as basalt. Normal polarity is used  when magnetic polarity of earth is same the geographic north and south of earth respectively and if not, then reverse polarity term is used. And magnetic reversal chronology is the method to date the age of layered rocks by isotopic dating technique. The dark and light color bands of magnetic stripes shows normal and reverse magnetic polarity. (See below figure)


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