Distribution of oceans and continents Chapter 4 Continental

Distribution of oceans and continents Chapter 4

Continental drift theory • Alfred Wegener- A German Meteorologist came up with eth theory of continental drift. He was of the opinion that all the continents were joined together in the form of one super continent PANGAEA which was surrounded by the water on all sides he named that ocean PANTHALASSA. • He suggested that continents started to break and drift apart around 200 million years ago around carboniferous period. • He collected number of proofs to support his theory but the forces that he suggested for the movement of crust i. e. polar fleeing force and tidal force was inadequate for such a great movement. Therefore his theory was not accepted till the coming of plate tectonic.

Proofs in the support of continental drift as given by Alfred Wegener 1. The matching of continents

2. Rocks of same age across oceans

3. Tillite- Deposits made by glaciers

4. Placer deposits

5. Distribution of fossils

To Sum Up-

Forces suggested by Wegener for the drift • 1. Polar fleeing force- it relates to the rotation of the earth. Due to the rotation of the earth no landmass could remain stationary on poles for a long period of time and it would be pushed southwards. But scientist did not approve of the is force to be that strong. • 2. Tidal force – Tidal force is generated by the attraction of sun and moon and its effect could be seen on the ocean water, if this force has to work to move continents it has to be too strong that is not a possibility otherwise it would stop earths rotation as well. Forces suggested for the movement of continents were the weakest part of eth theory due to which it could not find place in the scientific world.

Post Drift Studies • 1. Convectional Current theory • Put forth by Arthur Holmes • He discussed the possibility of convectional currents as one of the source responsible for the movement of eth continents. • These currents are formed in mantle due to the heat coming from core and also due to heat generated by the disintegration of radioactive minerals.

2. Mapping of the oceans floor

Ocean floor configuration- It has three parts 1. Continental Margins 2. Abyssal Plains 3. Mid Oceanic ridge 1. Continental margins: The continental margin is the shallow water area found in proximity to continent. The continental margin consists of three different features: the continental rise, the continental slope, and the continental shelf. Continental margins constitute about 28% of the oceanic area.

2. Abyssal plains are the vast, flat, sediment-covered areas of the deep ocean floor. They are the flattest, most featureless areas on the Earth, and have a slope of less than one foot of elevation difference for each thousand feet of distance.

Mid Oceanic Ridge • A mid-ocean ridge or mid-oceanic ridge is an underwater mountain range, formed by plate tectonics. This uplifting of the ocean floor occurs when convection currents rise in the mantle beneath the oceanic crust and create magma where two tectonic plates meet at a divergent boundary.

Distribution of earthquakes and volcanoes • 1. Ring of fire- convergent plate margin around pacific accounts for more than 90% of active volcanoes and is called as ring of fire. It also experiences high seismic activity. • 2. Mid Continental belt- This extends from Himalayan region to Mediterranean region. This is also a convergent margin. • 3. Mid Atlantic ridge- This is a divergent plate boundary. New crust gets formed here and is prone to volcanic and seismic activity.

Sea floor spreading • This concept was propounded by Harry Hess. • It suggests that ocean floor is expanding due to divergence of plates. • Evidences: • Presence of volcanoes along mid oceanic ridges. • Same type of rocks in terms of composition, magnetic properties etc. on the both side of crest of mid oceanic ridge. • Ocean crust is younger than continental crust • Sediments on the ocean floor is thin. • Deep trenches and deep focus earthquake along mid oceanic ridge.

Theory of plate tectonics