Earth System & Geomorphology

Earth's Interior & Plate Tectonics

Seismic discontinuities, isostasy and plate boundaries — the Part-A backbone that fixes the structure and dynamics of the planet.

PART 1

Topic Breakdown & Traps

The Engineering Principle

The Earth is layered — crust, mantle, outer (liquid) and inner (solid) core — boundaries first revealed by seismic-velocity jumps. The Mohorovičić discontinuity separates crust from mantle; the Gutenberg discontinuity marks the mantle–core boundary (where S-waves vanish, proving the outer core is liquid). Isostasy is the buoyant balance of crustal blocks floating on the denser mantle (Airy = variable-thickness roots; Pratt = variable density). Plate tectonics ties it together: lithospheric plates diverge at ridges (sea-floor spreading, recorded by symmetric magnetic stripes), converge at subduction zones, and slide past at transforms.

The Core Formula Matrix

Airy root:

r = \frac{ρ _{c}}{ρ _{m} - ρ _{c}} h

(root below a mountain of height

h

).

Lithostatic pressure:

P = ρ g z

.

Plate displacement:

d = v t

—

1 cm/yr \times 1 Myr = 10 km

.

Geothermal gradient (upper crust): ≈ 25–30 °C/km.

The ‘IIT Traps’

⚠
Moho vs Gutenberg. Moho = crust/mantle; Gutenberg = mantle/core. Do not confuse them.
⚠
S-wave shadow. The S-wave shadow zone proves the *outer* core is liquid — the inner core is solid.
⚠
Airy vs Pratt. Airy varies crustal *thickness*; Pratt varies crustal *density*.

📚 Standard references

Principles of Physical Geology — Arthur Holmes
Earth: An Introduction to Physical Geology — Tarbuck & Lutgens

PART 2

Progressive 3-Tier Question Suite

Q1BASIC1 Mark · MCQ

The seismic discontinuity separating the crust from the mantle is the

Q2MEDIUM2 Marks · NAT

Airy isostasy with

ρ_{c} = 2.67

and

ρ_{m} = 3.27

g/cm³. The crustal root beneath a

3

km-high mountain is _____ km.

Q3HARD2 Marks · NAT

A plate moves at

5

cm/yr. The displacement accumulated in

10

Myr is _____ km.