Foundation, Concrete and Earthquake Engineering

Mechanism involved drifting of continents

Various stresses operating within the body of the earth result in regional crustal deformations or movements are known as diastrophic movements. This phenomenon is known as diastrophism.
Isostasy is used to explain various movements in the crust. The principle of isostasy is that the different masses of the earth’s crust are standing in equilibrium, tending to reach and maintain a hydrostatic balance. At certain depth these crustal blocks exert equal pressure
and stand equilibrium.

Holme’s Hypothesis of Convection Current:
Holme’s believed that neither tidal friction nor any force from outside can drift the continents and are related to mountain building. He explained the whole process with the help of convection current cycle which is operating within the earth.
This convection current theory assumes the formation of convection currents within the substratum which travel from the hotter parts near the core towards cooler parts under the crust.

Formation of Oceanic Deeps

When two ascending currents diverge under a continental block, they exert a tensional force. As the convective system gain strength the tensional force also increases, resulting in rupturing and splitting of the continental blocks leaving a gap in between. The broken fragments left in between sink into gap, reaching the ocean floor.

Drifting Continents

The currents flowing horizontally beneath the continental blocks drift them. The underlying basaltic layer which provides obstruction to the continental drift, move down into the substratum under the dragging effect of the currents directed downwards.

Mountain Building

When two currents coming from opposite sides meet and are directed downwards, there is deepening and narrowing effect. The continental mass, dragged downwards and compressed, gives rise to mountains and their and their roots. The underlying basaltic layer which also gets pushed downwards undergoes metamorphism, changing into a highly compressed type of rock called Eclogite.

Restoring the Basaltic Layer

The heavy matter (Eclogite) will descend further, merging with the substratum and further on heating, changing into magma which being a lighter material will again tend to move upwards. Most of this magma or basaltic material will rise up along with ascending currents, reaching the gaps left between the torn outstretched continents, restoring the basaltic layer, also forming the basaltic plateaus and causing volcanic activity. The basaltic layer in front of the continental mass when dragged down-wards, results in the formation of oceanic deeps.

Termination of system

The convective system as explained above, results in complete circulation of the basaltic matter beneath the crust. When the convection currents begin to wane out, the system loses its power. As a result the dragging and drifting action also diminishes and finally ceases. The lighter mountain roots which were being pulled downward continuously are left to rise up.

Holme’s Hypothesis and Isostasy

This convection current hypothesis provides appropriate explanation for the continental drift and the associated and related phenomenon of mountain building. Holme’s established this system depending on Isostasy which explains very successfully

Ø The circulation and recycling of basaltic materials

Ø Formation of ocean deep

Ø Presence of sima ( viscous substratum) sometimes with patches of sial at the ocean floors.

Ø Mountain building

Ø The lighter material of the mountains and their roots.

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