Consider the following statements i. Tectonic plates include the crust and the uppermost mantle of the solid earth

Question

Consider the following statements:

i. Tectonic plates include the crust and the uppermost mantle of the solid earth

ii. Tectonic plates are rigid, solid layers about 100 km thick

iii. Tectonic plates include the crust and the entire mantle

iv. Tectonic plates are confined to continental land masses only

Which of the following statements is/are correct?

1. A. i, and ii (Correct answer)
2. B. ii only
3. C. iii and iv only
4. D. iv only

Correct Answer

Option A — i, and ii

Detailed Solution & Explanation

The correct answer is i, and ii.

Key Points

• Statement i is correct. A tectonic plate is a slab of the lithosphere, which comprises the crust together with the uppermost, rigid part of the mantle.
• Statement ii is correct. Plates are rigid and solid, with an average thickness of about 100 km. Oceanic lithosphere is thinner — around 5–100 km depending on age — while continental lithosphere reaches roughly 200 km beneath old shields.
• Statement iii is incorrect. Plates do not include the entire mantle. Below the lithosphere lies the asthenosphere, a partially molten, ductile layer over which the plates move; the mantle continues to a depth of about 2,900 km.
• Statement iv is incorrect. Plates are not confined to continents. Most of the Earth's surface is covered by oceanic plates, and the Pacific Plate is almost entirely oceanic. A single plate — the Indo-Australian, for instance — carries both continental and oceanic crust.

Additional Information

• There are seven major plates — Pacific, North American, South American, Eurasian, African, Indo-Australian and Antarctic — along with numerous minor plates such as the Nazca, Cocos, Philippine, Arabian, Caribbean, Juan de Fuca and Scotia plates.
• Three kinds of plate boundary:
  • Divergent (constructive) — plates move apart and new crust forms; the Mid-Atlantic Ridge and the East African Rift.
  • Convergent (destructive) — plates collide; subduction produces trenches, island arcs and volcanic mountains, while continent–continent collision builds fold mountains such as the Himalayas.
  • Transform (conservative) — plates slide past each other, as at the San Andreas Fault; crust is neither created nor destroyed, but earthquakes are frequent.
• The driving mechanisms are mantle convection, ridge push at spreading centres and slab pull from the dense sinking edge of a subducting plate — slab pull being generally regarded as the strongest.
• The intellectual lineage: Alfred Wegener's Continental Drift (1912), with Pangaea breaking into Laurasia and Gondwanaland; Harry Hess's Sea Floor Spreading (1960s), confirmed by the symmetrical palaeomagnetic stripes either side of ocean ridges; and finally Plate Tectonics, which unified both.