Introduction (ആമുഖം)
Understanding the Earth's internal structure is a fundamental topic in Geography for all competitive exams, including those conducted by the Kerala PSC. We cannot directly observe the Earth's interior, so our knowledge is primarily based on indirect sources, with the most important being the study of Seismic Waves (P-waves and S-waves) from earthquakes. The speed and path of these waves change as they travel through different materials, revealing the layered structure of our planet.
1. The Layers of the Earth (ഭൂമിയുടെ പാളികൾ)
The Earth is broadly divided into three concentric layers based on its chemical composition:
- The Crust (ഭൂവൽക്കം)
- The Mantle (മാന്റിൽ)
- The Core (കാമ്പ്)
1.1 The Crust (ഭൂവൽക്കം)
This is the outermost, thinnest, and solid layer of the Earth. It constitutes less than 1% of the Earth's volume.
- Key Characteristics:
- It is brittle and can break, causing earthquakes.
- The thickness varies significantly.
- It is divided into two types:
- (a) Continental Crust (വൻകര ഭൂവൽക്കം):
- Thickness: Thicker, about 30-70 km.
- Composition: Lighter rock, primarily granitic. It is rich in Silica (Si) and Aluminium (Al).
- Mnemonic: SIAL (Silica + Aluminium).
- Density: Less dense (around 2.7 g/cm³).
- (b) Oceanic Crust (സമുദ്ര ഭൂവൽക്കം):
- Thickness: Thinner, about 5-10 km.
- Composition: Denser rock, primarily basaltic. It is rich in Silica (Si) and Magnesium (Mg).
- Mnemonic: SIMA (Silica + Magnesium).
- Density: Denser (around 3.0 g/cm³).
Because the Continental Crust (SIAL) is less dense, it "floats" higher on the mantle compared to the denser Oceanic Crust (SIMA).
1.2 The Mantle (മാന്റിൽ)
This layer lies just below the crust and extends up to a depth of 2,900 km. It is the thickest layer, making up about 84% of the Earth's volume.
- Key Characteristics:
- It is a semi-solid, rocky layer.
- It is divided into two parts:
- (a) Upper Mantle (ഉപരി മാന്റിൽ):
- This includes a very important zone called the Asthenosphere (അസ്തനോസ്ഫിയർ).
- Asthenosphere: A semi-molten, plastic-like (not fully liquid) layer on which the tectonic plates (Lithosphere) float. It is the main source of magma that comes to the surface during volcanic eruptions. The word 'Astheno' means weak.
- (b) Lower Mantle (അധോ മാന്റിൽ):
- Extends from the end of the Asthenosphere to the Core. It is more solid and denser due to immense pressure.
The combined solid part of the Crust and the uppermost rigid part of the Mantle is called the Lithosphere (ശിലാമണ്ഡലം). The Lithosphere is broken into tectonic plates.
1.3 The Core (കാമ്പ്)
This is the innermost layer of the Earth, with a radius of about 3,500 km. It is the densest layer.
- Key Characteristics:
- Composition: It is primarily made of heavy metals, mainly Nickel (Ni) and Iron (Fe).
- Mnemonic: NIFE (Nickel + Ferrum/Iron).
- It is divided into two parts:
- (a) Outer Core (പുറക്കാമ്പ്):
- State: Liquid. The temperature is so high that the metals are in a molten state.
- Importance: The movement of this liquid iron is responsible for creating the Earth's magnetic field (ഭൂമിയുടെ കാന്തികമണ്ഡലം).
- (b) Inner Core (അകക്കാമ്പ്):
- State: Solid. Despite being even hotter than the outer core, the immense pressure at the center of the Earth forces the material into a solid state.
- It is the densest part of the Earth.
2. Discontinuities (വിവിധ പാളികളെ വേർതിരിക്കുന്ന അതിരുകൾ)
Discontinuities are the boundaries between the Earth's layers, identified by abrupt changes in the speed of seismic waves.
- Conrad Discontinuity: Between Upper Crust (SIAL) and Lower Crust (SIMA).
- Mohorovičić Discontinuity (Moho): Separates the Crust and the Mantle. (Very Important)
- Repetti Discontinuity: Separates the Upper Mantle and the Lower Mantle.
- Gutenberg Discontinuity: Separates the Mantle and the Core. (Very Important)
- Lehmann Discontinuity: Separates the Outer Core (liquid) and the Inner Core (solid).
Summary of Important Points for Revision
- Crust: Thinnest layer. Divided into SIAL (Continental) and SIMA (Oceanic).
- Mantle: Thickest layer. Source of magma.
- Core: Densest layer. Made of NIFE (Nickel & Iron).
- Lithosphere: Crust + Upper rigid part of Mantle. These are the tectonic plates.
- Asthenosphere: Semi-molten layer below the Lithosphere. Tectonic plates float on it.
- Outer Core: Liquid. Responsible for Earth's magnetic field.
- Inner Core: Solid due to extreme pressure.
- Mohorovičić (Moho): Boundary between Crust and Mantle.
- Gutenberg: Boundary between Mantle and Core.
Important Questions Asked by Kerala PSC
Here are some actual questions from previous Kerala PSC exams on this topic.
1. The boundary between Earth's crust and mantle is called _______?
(a) Conrad Discontinuity
(b) Mohorovicic Discontinuity
(c) Gutenberg Discontinuity
(d) Lehmann Discontinuity
Answer: (b) Mohorovicic Discontinuity
Exam: University Assistant - 2016 (Cat No: 471/2016) & Several LDC/VEO Exams.
2. SIAL, SIMA and NIFE are the layers of the _______?
(a) Atmosphere
(b) Hydrosphere
(c) Interior of the Earth
(d) Exterior of the Earth
Answer: (c) Interior of the Earth
Exam: LDC (Various Districts) - 2017 (e.g., Cat No: 414/2016)
3. The upper part of the mantle upon which the crust of the earth floats is _______?
(a) Barysphere
(b) Mesosphere
(c) Asthenosphere
(d) Gutenberg discontinuity
Answer: (c) Asthenosphere
Exam: Secretariat Assistant / Auditor - 2018 (Cat No: 001/2018)
4. The part of the earth which is rich in Nickel and Iron (Nife) is _______?
(a) Crust
(b) Core
(c) Mantle
(d) Sial
Answer: (b) Core
Exam: Fireman Trainee - 2015 (Cat No: 139/2015)
5. Which part of the Earth's interior is in liquid form?
(a) Mantle
(b) Inner Core
(c) Outer Core
(d) Crust
Answer: (c) Outer Core
Exam: Degree Level Common Preliminary Exam - Stage 1 (2021) (Cat No: 147/2021)
6. വൻകര ഭൂവൽക്കത്തിൽ പ്രധാനമായും അടങ്ങിയിരിക്കുന്ന ധാതുക്കൾ ഏതെല്ലാമാണ്? (Which are the major minerals found in the Continental Crust?)
(a) സിലിക്ക, അലൂമിനിയം (Silica, Aluminium)
(b) സിലിക്ക, മഗ്നീഷ്യം (Silica, Magnesium)
(c) നിക്കൽ, ഇരുമ്പ് (Nickel, Iron)
(d) അലൂമിനിയം, മഗ്നീഷ്യം (Aluminium, Magnesium)
Answer: (a) സിലിക്ക, അലൂമിനിയം (Silica, Aluminium)
Exam: This is a frequently asked question in 10th and +2 Level exams, for example in mock tests and previous papers for LGS/LDC Mains.
Practice Questions on Earth's Structure (ഭൂമിയുടെ ഘടന)
1. Which layer of the Earth constitutes about 84% of the planet's total volume?
(a) Crust (ഭൂവൽക്കം)
(b) Mantle (മാന്റിൽ)
(c) Outer Core (പുറക്കാമ്പ്)
(d) Inner Core (അകക്കാമ്പ്)
Answer: (b) Mantle (മാന്റിൽ)
Hint: The Mantle is the thickest layer.
2. The term 'SIAL' refers to the composition of which of the following?
(a) Oceanic Crust
(b) Continental Crust
(c) The entire Mantle
(d) The Core
Answer: (b) Continental Crust
Hint: SIAL stands for Silica and Aluminium, the main components of the lighter continental crust.
3. The Gutenberg Discontinuity separates which two layers of the Earth?
(a) Crust and Mantle
(b) Upper Mantle and Lower Mantle
(c) Mantle and Core
(d) Outer Core and Inner Core
Answer: (c) Mantle and Core
Hint: This is one of the most important discontinuities to remember.
4. The Earth's magnetic field is a result of the movement within which layer?
(a) The solid Inner Core
(b) The plastic Asthenosphere
(c) The rigid Lithosphere
(d) The liquid Outer Core
Answer: (d) The liquid Outer Core
Hint: The movement of molten iron generates the magnetic field.
5. The combination of the Earth's Crust and the solid upper part of the Mantle is called the ______.
(a) Asthenosphere
(b) Lithosphere
(c) Hydrosphere
(d) Barysphere
Answer: (b) Lithosphere (ശിലാമണ്ഡലം)
Hint: 'Litho' means stone or rock. The tectonic plates are pieces of the lithosphere.
6. Which of the following statements about the Earth's Inner Core is correct?
(a) It is in a liquid state.
(b) It is primarily made of Silica and Magnesium.
(c) It is solid due to immense pressure.
(d) It is the source of volcanic magma.
Answer: (c) It is solid due to immense pressure.
Hint: Despite the heat, the pressure at the center is so high that it forces the atoms into a solid state.
7. ഭൂമിയുടെ കേന്ദ്രഭാഗം 'NIFE' എന്ന് അറിയപ്പെടാൻ കാരണം എന്ത്? (Why is the core of the Earth known as 'NIFE'?)
(a) അവിടെ മർദ്ദം കൂടുതലായതുകൊണ്ട് (Because the pressure is high there)
(b) അവിടെ താപം കൂടുതലായതുകൊണ്ട് (Because the temperature is high there)
(c) അത് നിക്കലും ഇരുമ്പും കൊണ്ട് നിർമ്മിതമായതുകൊണ്ട് (Because it is made of Nickel and Iron)
(d) അത് സിലിക്കയും അലുമിനിയവും കൊണ്ട് നിർമ്മിതമായതുകൊണ്ട് (Because it is made of Silica and Aluminium)
Answer: (c) അത് നിക്കലും ഇരുമ്പും കൊണ്ട് നിർമ്മിതമായതുകൊണ്ട് (Because it is made of Nickel and Iron)
Hint: NIFE is the mnemonic for Nickel (Ni) and Ferrum (Fe - Iron).
8. The thinnest layer of the Earth is the ______.
(a) Crust
(b) Mantle
(c) Core
(d) Asthenosphere
Answer: (a) Crust
Hint: It is the outermost and thinnest shell.
9. The weak, semi-molten layer in the upper mantle which is the primary source of magma is known as:
(a) Lithosphere
(b) Moho Discontinuity
(c) Asthenosphere
(d) Outer Core
Answer: (c) Asthenosphere (അസ്തനോസ്ഫിയർ)
Hint: 'Astheno' means weak. The rigid plates float on this weak layer.
10. The boundary separating the Crust from the Mantle is the:
(a) Lehmann Discontinuity
(b) Gutenberg Discontinuity
(c) Conrad Discontinuity
(d) Mohorovičić Discontinuity
Answer: (d) Mohorovičić Discontinuity
Hint: Often shortened to 'Moho'. This is a very frequently asked question.
11. Which layer is denser?
(a) Continental Crust (SIAL)
(b) Oceanic Crust (SIMA)
(c) Both have the same density
(d) Density cannot be determined
Answer: (b) Oceanic Crust (SIMA)
Hint: The denser oceanic crust subducts (sinks) under the lighter continental crust at collision zones.
12. The Lehmann Discontinuity marks the boundary between:
(a) Upper and Lower Crust
(b) Mantle and Core
(c) Outer Core and Inner Core
(d) Upper and Lower Mantle
Answer: (c) Outer Core and Inner Core
Hint: This separates the liquid outer part of the core from the solid inner part.
13. Our most reliable information about the Earth's interior comes from:
(a) Deep-sea drilling projects
(b) Analysis of volcanic materials
(c) The study of seismic waves (earthquake waves)
(d) Analysis of meteorites
Answer: (c) The study of seismic waves (earthquake waves)
Hint: P-waves and S-waves travel differently through different materials, revealing the layered structure.
14. Identify the correctly matched pair:
(a) SIAL - Oceanic Crust
(b) NIFE - Mantle
(c) SIMA - Core
(d) Asthenosphere - Upper Mantle
Answer: (d) Asthenosphere - Upper Mantle
Hint: The Asthenosphere is a key part of the Upper Mantle. The other options are mismatched.
15. ശിലാമണ്ഡലം (Lithosphere) എന്നറിയപ്പെടുന്ന ഭാഗത്ത് താഴെ പറയുന്നവയിൽ ഏതൊക്കെ ഉൾപ്പെടുന്നു? (Which of the following are included in the part known as the Lithosphere?)
(a) പുറക്കാമ്പും അകക്കാമ്പും (Outer Core and Inner Core)
(b) മാന്റിലിന്റെ മുകൾഭാഗവും പുറക്കാമ്പും (Upper part of Mantle and Outer Core)
(c) ഭൂവൽക്കവും മാന്റിലിന്റെ മുകൾഭാഗവും (Crust and Upper part of Mantle)
(d) ഭൂവൽക്കം മാത്രം (Only the Crust)
Answer: (c) ഭൂവൽക്കവും മാന്റിലിന്റെ മുകൾഭാഗവും (Crust and Upper part of Mantle)
Hint: The lithosphere is the rigid, solid outer shell of the Earth, not just the crust.
16. The average thickness of the Continental Crust is approximately:
(a) 5-10 km
(b) 30-70 km
(c) 100-200 km
(d) 2900 km
Answer: (b) 30-70 km
Hint: Continental crust is significantly thicker than the oceanic crust (5-10 km).
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