Table of Contents
Short Answer Type Question:
Q1. What were the forces suggested by Wegener for the movement of the continents?
Answer
Wegener suggested that the movement responsible for the drifting of the continents was caused by pole-fleeing force and tidal force. The polar-fleeing force relates to the rotation of the earth. The earth is not a perfect sphere; it has a bulge at the equator. This bulge is due to the rotation of the earth. The second force, the tidal force is due to the attraction of the moon and the sun that develops tides in oceanic waters. Wegener believed that these forces would become effective when applied over many million years.
Q2.How are the convectional currents in the mantle initiated and maintained?
Answer
The convectional currents in the mantle are generated due to radioactive elements causing
thermal differences in the mantle portion. The hot material rises up from greater depths and comparatively cold material goes down from above. The occurrence of this process repetitively give birth to convectional currents.
Q3. What is the major difference between the transform boundary and the convergent or divergent boundaries of plates?
Answer
| Transform Boundary | Convergent Boundary | Divergent Boundary |
| Where the crust is neither produced nor destroyed as the plates slide horizontally past each other. | Where the crust is destroyed as one plate dived under another. | Where new crust is generated as the plates pull away from each other. |
Q4.What was the location of the Indian landmass during the formation of the Deccan Traps?
Answer
The Deccan Traps were formed during the movement of the Indian plate towards the Asiatic plate. This started somewhere around 60 million years ago and continued for a long period of time. At that time, Indian landmass was located in the south of the equator.
Q5.What are the evidences in support of the continental drift theory?
Answer
The evidences in support of the continental drift theory are:
• The Matching of Continents (Jig-Saw-Fit): The shorelines of Africa and South America facing each other have a remarkable and unmistakable match.
• Rocks of Same Age Across the Oceans: The belt of ancient rocks of 2,000 million years from Brazil coast matches with those from western Africa. The earliest marine deposits along the coastline of South America and Africa are of the Jurassic age. This suggests that the ocean did not exist prior to that time.
• Tillite: It is the sedimentary rock formed out of deposits of glaciers. The Gondawana system
of sediments from India is known to have its counter parts in six different landmasses of the Southern Hemisphere. The glacial tillite provides unambiguous evidence of palaeoclimates and also of drifting of continents.
• Placer Deposits: The occurrence of rich placer deposits of gold in the Ghana coast and the absolute absence of source rock in the region is an amazing fact. The gold bearing veins are in Brazil and it is
obvious that the gold deposits of the Ghana are derived from the Brazil plateau when the two continents lay side by side.
• Distribution of Fossils: When identical species of plants and animals adapted to living on land or in fresh water are found on either side of the marine barriers, a problem arises regarding accounting for such distribution. The observations that Lemurs occur in India, Madagascar and Africa led some
to consider a contiguous landmass “Lemuria” linking these three landmasses.
Q6. Bring about the basic difference between the drift theory and Plate tectonics.
Answer
| Drift Theory | Plate tectonics |
| Drift theory suggested that in the past, there was a super continent called Pangaea. Over time, this super continent split apart to form the seven continents we have today. | Plate Tectonics is a theory of global tectonics which proposes that the earth’s lithosphere is divided into seven major and some minor plates. |
| It only covers the movement of continents. | It covers movements of both Continents and Oceans. |
| The evidences in support of drift theory are Jig-Saw-Fit, Placer Deposits, Distribution of Fossils and others. | It is based on scientific analysis of the processes inside the earth’s surface. |
| It does not give any future prediction. | This theory says that all plates shall continue to move in the future period as well. |
Q7. What were the major post-drift discoveries that rejuvenated the interest of scientists in the study of distribution of oceans and continents?
Answer
A number of post-drift discoveries during the post-war period added new information to geological literature. Particularly, the information collected from the ocean floor mapping provided new dimensions for the study of distribution of oceans and continents.
• All along the midoceanic ridges, volcanic eruptions are common and they bring huge amounts of
lava to the surface in this area.
• The rocks equidistant on either sides of the crest of mid-oceanic ridges show remarkable similarities in terms of period of formation, chemical compositions and magnetic properties. Rocks closer to the
mid-oceanic ridges are normal polarity and are the youngest. The age of the rocks increases as one moves away from the crest.
• The ocean crust rocks are much younger than the continental rocks. The age of rocks in the oceanic crust is nowhere more than 200 million years old.
• The sediments on the ocean floor are unexpectedly very thin.
• The deep trenches have deep-seated earthquake occurrences while in the midoceanic ridge areas, the quake foci have shallow depths.
Long Answer Type Questions:
Q1.According to tectonic plates theory in how many plates has the earth been divided? Explain.
Answer:
The theory of plate tectonics proposes that the earth’s lithosphere is divided into seven major and some minor plates. The major plates are as follows:
- Antarctica and the surrounding oceanic plate
- North American plate
- South American plate
- Pacific plate
- India-Australia-New Zealand plate
- Africa with the eastern Atlantic floor plate
- Eurasia and the adjacent oceanic plate. Some important minor plates are:
- Cocos plate: It is between Central America and Pacific plate
- Nazca plate: It is between South America and Pacific plate
- Arabian plate: It includes mostly the Saudi Arabian landmass
- Philippine plate: It is between the Asiatic and Pacific plate
- Caroline plate: It is between the Philippine and Indian plate (North of New Guinea)
- Fuji plate: It includes North-east of Australia.
- Pacific plate is largely an oceanic plate whereas the Eurasian plate may be called a continental plate. Plates are not static. Plates may converge or diverge. Plates may break as well.
Q2.Explain tectonic plate theory and its j working.
Answer:
These plates have been constantly moving over the globe throughout the history of the earth.
- The theory of plate tectonics was introduced by Mckenzie, Parker and , Morgan in 1967.
- A tectonic plate is also called as lithosphere plate.
- It is a massive irregularly shaped slab of solid rock.
- Consists of oceanic and continental sphere.
- Plates move horizontally over the asthenosphere.
- Average thickness is 100 km of oceanic part and 200 km of continental part.
- It may be oceanic or continental.
- Pacific plate is largest oceanic plate whereas Eurasian plate is the largest continental plate.
- These plates are moving constantly throughout geological time not the continent, believed by Wegener.
It creates three types of boundaries.
- Divergent boundaries
- New crust is generated
- Plates move away from each other
- These are called spreading sites
- Ex. Mid atlantic ridge
- Convergent boundaries |
- Crust is destroyed
- Sinking of plate is called “subduction zone”. There are three ways in which subduction occurs (i) between an oceanic and continental plates; (ii) between two oceanic plates; and (iii) between two continental plates.
- Transform boundaries: Where the crust is neither produced nor destroyed as the plates slide horizontally past each other.
Q3.Explain important theories associated with the movement of continents.
Answer:
Continental drift: Abraham Ortelius a Dutch map maker in 1596 first proposed the possibility of joining the continents such as America with Europe and Africa. Antonio Pellegrini drew a map showing the three continents together. Alfred Wegener, a German meteorologist put forth the continental drift theory. According to him, all continents formed a single continental mass called Pangaea.
All oceans formed a single universal ocean called Panthalassa around 200 million years ago. The Pangaea began to split into two large continental masses called Laurasia and Gondwanaland. By further splitting Laurasia formed northern continents and Gondwanaland formed southern continents.
Sea Floor Spreading: The deep trenches have deep-seated earthquake occurrences while in the mid- oceanic ridge areas, the quake foci have shallow depths. These facts and a detailed analysis of magnetic properties of the rocks on either sides of the mid-oceanic ridge led Hess in 1961 to propose his hypothesis. It was called the “sea floor spreading”. Hess argued that constant eruptions at the crest of oceanic ridges cause the rupture of the oceanic crust forces and the new lava wedges into it, pushing the oceanic crust on either side. The ocean floor, thus spreads. Two facts made Hess think about the consumption of the oceanic crust.
- The younger age of the oceanic crust.
- The spreading of one ocean does not cause the shrinking of the other.
He further maintained that the ocean floor that gets pushed due to volcanic eruptions at the crest, sinks down at the oceanic trenches and gets consumed. Plate Tectonics: It was in 1967, McKenzie and Parker and also Morgan, independently collected the available ideas and came out with another concept termed Plate Tectonics. The theory of plate tectonics proposes that the earth’s lithosphere is divided into seven major and some minor plates. These plates have been constantly moving over the globe throughout the history of the earth. It is not the continent that moves as believed by Wegener. Continents are part of a plate and what moves is the plate. All the plates, without exception, have moved in the geological past, and shall continue to move in the future as well.
Q4.The ocean floor may be segmented into how many divisions based on the depth as well as the forms of relief?
Answer:
The ocean floor may be segmented into three major divisions based on the depth as well as the forms of relief. These divisions are:
(i) Continental margins,
(ii) Abyssal plains and
(iii) Mid-oceanic ridges.
- Continental margins
- Form transitional zone between continental shore and deep sea basins
- They include continental slope , shelf, continental rise and deep oceanic trenches
- Abyssal plains
- Extensive plains
- Found between continental margin and mid oceanic ridge
- Continental sediments get deposited
- Mid-oceanic ridges
- Distribution of volcanoes and earthquakes
- All volcanoes and earthquakes are parallel to the coast
- This line also co-incides with mid- Atlantic ridge and Alpine Himalayan system
- Around the Pacific Ocean it is called ring of fire mid oceanic ridges.
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