CUET UG Geography: Physical Geography - Geomorphology, Interior of Earth, Rocks and Minerals, Earthquakes

Must-Know (15–20 detailed bullets)

• Earth’s radius is approximately 6,371 km; verified from seismic wave studies and geodetic measurements (NCERT Class 11 Fundamentals of Physical Geography).
• The crust-mantle boundary is called the Mohorovi?i? discontinuity (Moho), identified by a sudden increase in seismic wave velocity at an average depth of 30–40 km under continents and 5–10 km under oceans.
• The mantle extends from the Moho to a depth of about 2,900 km and is composed mainly of silicate minerals rich in iron and magnesium, such as olivine and pyroxene.
• The outer core is liquid and extends from 2,900 km to 5,150 km depth; its fluid motion generates Earth’s magnetic field via the dynamo effect.
• The inner core is solid due to immense pressure despite temperatures exceeding 5,000°C; it is composed primarily of iron and nickel.
• P-waves (primary waves) are compressional waves that travel through solids, liquids, and gases; they are the fastest seismic waves (~8 km/s in crust).
• S-waves (secondary waves) are shear waves that travel only through solids; they cannot pass through the outer core, creating an S-wave shadow zone between 105° and 140° from the epicenter.
• The Gutenberg discontinuity lies at 2,900 km depth and marks the boundary between the mantle and the core.
• The Lehmann discontinuity is found at about 5,150 km depth and separates the liquid outer core from the solid inner core.
• Igneous rocks form from the solidification of magma or lava; granite (intrusive) and basalt (extrusive) are common examples.
• Sedimentary rocks are formed by compaction and cementation of sediments; sandstone, shale, and limestone are key types.
• Metamorphic rocks result from the transformation of existing rocks under high temperature and pressure; slate (from shale) and marble (from limestone) are examples.
• The rock cycle illustrates the interconversion of igneous, sedimentary, and metamorphic rocks through geological processes over time.
• Quartz (SiO?) is a rock-forming mineral with a hardness of 7 on Mohs scale and is resistant to weathering.
• Feldspar is the most abundant mineral group in Earth’s crust, making up about 60% of it; it weathers to form clay minerals.
• Earthquakes originate at a point called the focus (hypocenter), while the point directly above on the surface is the epicenter.
• The Richter scale measures the magnitude of an earthquake logarithmically; each whole number increase represents a tenfold increase in amplitude of seismic waves.
• The Mercalli Intensity Scale measures earthquake effects on humans, structures, and Earth's surface using Roman numerals I to XII.
• The Pacific Ring of Fire is the most seismically and volcanically active zone, accounting for about 80% of Earth’s earthquakes.
• Body waves (P and S) travel through Earth’s interior, while surface waves (Love and Rayleigh) travel along the crust and cause the most damage during earthquakes.

Difficulty Level

Intermediate — Requires understanding of layered Earth structure, rock classification, and seismic wave behavior, with some numerical values and technical terms frequently tested.

Common CUET Traps (3 bullets)

• Trap: Students assume S-waves can travel through liquids because they hear “seismic waves move through Earth.”
  Avoid: Remember S-waves only travel through solids; their absence in the outer core proves it is liquid.

• Trap: Confusing the focus (hypocenter) with the epicenter.
  Avoid: Focus is the underground origin point; epicenter is the surface point directly above it.

• Trap: Believing that metamorphic rocks form only from sedimentary rocks.
  Avoid: Metamorphism affects all rock types—igneous, sedimentary, and even existing metamorphic rocks can be transformed.

Practice MCQs (5 questions)

1. Question: Which discontinuity marks the boundary between Earth’s mantle and core?
   A. Mohorovi?i?
   B. Gutenberg
   C. Lehmann
   D. Conrad
   Answer: B
   Explanation: The Gutenberg discontinuity lies at 2,900 km depth, separating the mantle from the core.
   Why others fail: Mohorovi?i? separates crust and mantle, a common confusion.

2. Question: Which type of seismic wave cannot pass through the outer core?
   A. P-waves
   B. Surface waves
   C. S-waves
   D. Body waves
   Answer: C
   Explanation: S-waves do not travel through liquids, and the outer core is liquid.
   Why others fail: P-waves do pass through the outer core, so students may misremember wave behavior.

3. Question: Which rock is formed by the metamorphism of limestone?
   A. Slate
   B. Quartzite
   C. Marble
   D. Schist
   Answer: C
   Explanation: Limestone recrystallizes into marble under heat and pressure.
   Why others fail: Quartzite comes from sandstone, a frequent mix-up.

4. Question: What is the approximate depth of the inner core boundary (Lehmann discontinuity)?
   A. 2,900 km
   B. 5,150 km
   C. 6,371 km
   D. 4,000 km
   Answer: B
   Explanation: The Lehmann discontinuity at ~5,150 km separates the outer core from the solid inner core.
   Why others fail: 2,900 km is the Gutenberg discontinuity, often confused with inner core boundary.

5. Question: Which of the following minerals is most resistant to chemical weathering?
   A. Calcite
   B. Feldspar
   C. Quartz
   D. Mica
   Answer: C
   Explanation: Quartz (SiO?) is highly stable and resists chemical breakdown better than other common minerals.
   Why others fail: Feldspar is abundant but weathers easily to clay, a key detail often overlooked.

Last?Minute Revision (15–20 one?liners)

• Moho discontinuity: separates crust and mantle; depth ~30–40 km (continent), ~5–10 km (ocean).
• Gutenberg discontinuity at 2,900 km: mantle-core boundary.
• S-waves do not travel through outer core-proves it is liquid.
• P-waves travel fastest and through all media; S-waves only through solids.
• Shadow zone: 105°–140° from epicenter; no S-waves, partial P-waves.
• Earth’s inner core is solid due to high pressure, not low temperature.
• Core is made of Fe-Ni (nife = nickel + iron).
• Crust is silica + alumina-“sial”; mantle is silica + magnesium-“sima”.
• Granite: intrusive igneous rock; forms below surface.
• Basalt: extrusive igneous rock; forms lava flows on surface.
• Sedimentary rocks: layered, often contain fossils (e.g., limestone).
• Metamorphic rocks: formed under heat and pressure; e.g., slate from shale.
• Quartz hardness = 7 on Mohs scale; scratches glass.
• Feldspar is most abundant mineral in Earth’s crust (~60%).
• Focus = origin point of earthquake; epicenter = point on surface above it.
• Richter scale: logarithmic; magnitude 6 is 10× stronger than magnitude 5 in wave amplitude.
• Mercalli scale: measures intensity from I (not felt) to XII (total destruction).
• Pacific Ring of Fire: most earthquakes and volcanoes occur here.
• Rock cycle: shows transformation between igneous, sedimentary, metamorphic rocks.
• Mnemonic: “I Saw My Dog Run”-Igneous, Sedimentary, Metamorphic, Dynamic (rock cycle order).