Earth Movements
Short Questions
1. What are the two main types of Earth movements?
Answer: Slow movements and sudden movements.
2. What is the primary cause of an earthquake?
Answer: Release of accumulated stress in the Earth’s crust.
3. What is the point on the surface directly above the seismic focus called?
Answer: Epicenter.
4. Which scale measures the intensity of an earthquake based on observed effects?
Answer: Mercalli scale.
5. What type of fold has limbs inclined at the same angle?
Answer: Symmetrical fold.
6. What landform is formed by the subsidence of the crust between two faults?
Answer: Rift valley.
7. Which type of fault involves horizontal movement along the fault plane?
Answer: Tear fault.
8. What is the name of the major volcanic belt around the Pacific Ocean?
Answer: Ring of Fire.
9. What type of lava has a high silica content and moves slowly?
Answer: Acidic lava.
10. Which volcanic landform is formed by alternating layers of lava and cinder?
Answer: Composite cone.
11. What is the shadow zone for S-waves?
Answer: The zone beyond 105° from the epicenter where S-waves are not recorded.
12. Which type of movement leads to the formation of continents?
Answer: Vertical (epeirogenic) movement.
13. What is the name of the volcanic material that solidifies in the air before falling?
Answer: Volcanic bombs.
14. Which mountain range is an example of a young fold mountain?
Answer: Himalayas.
15. What instrument records earthquake waves?
Answer: Seismograph.
Long Questions
1. What are the differences between slow and sudden Earth movements?
Answer: Slow Earth movements, such as vertical and horizontal tectonic movements, occur gradually over millions of years and are responsible for forming continents, plateaus, and mountains like the Himalayas. Sudden movements, like earthquakes and volcanic eruptions, happen rapidly within seconds or hours, causing immediate surface changes such as ground shaking or the formation of new landforms like calderas. The key difference lies in their speed, with slow movements being continuous and sudden movements being episodic.
2. How do compressional forces lead to the formation of fold mountains?
Answer: Compressional forces occur when tectonic plates move towards each other, exerting pressure on the Earth’s crust, which causes the rock layers to buckle and fold. These forces are most effective on ductile rocks deep within the crust, where high pressure allows the rocks to bend without breaking, forming structures like anticlines and synclines. Over millions of years, this folding process results in fold mountains, such as the Himalayas and the Alps, which are characterized by their layered, folded rock strata.
3. What are the characteristics of a normal fault, and where is it commonly found?
Answer: A normal fault occurs when tensional forces pull the Earth’s crust apart, causing one block of land to slide downward along the fault plane, with the exposed plane facing the sky. This type of fault is typically associated with the stretching and thinning of the crust, leading to the formation of rift valleys or grabens. Normal faults are commonly found in regions like the African Rift Valley, where tectonic plates are diverging.
4. How does the shadow zone help in understanding the Earth’s internal structure?
Answer: The shadow zone is a region where seismic waves from an earthquake are not detected, revealing the Earth’s internal composition, particularly the liquid outer core. P-waves are refracted and S-waves are blocked by the liquid core, creating a shadow zone between 105° and 140° from the epicenter for P-waves, and beyond 105° for S-waves. This pattern indicates that S-waves cannot travel through liquids, confirming the presence of a liquid outer core in the Earth.
5. What are the types of volcanic materials ejected during an eruption?
Answer: Volcanic eruptions release materials in three forms: liquid, solid, and gaseous. Liquid materials include magma (below the surface) and lava (on the surface), with acidic lava being thick and slow-moving, and basic lava being more fluid. Solid materials consist of volcanic dust, ash, breccias, and volcanic bombs, while gaseous materials form dark cauliflower-shaped clouds containing inflammable gases that produce flames.
6. Why are the Himalayas more vulnerable to earthquakes?
Answer: The Himalayas are located at the convergent boundary between the Indian Plate and the Eurasian Plate, where intense tectonic activity generates frequent earthquakes. The region experiences compressional forces and faulting due to the ongoing collision of these plates, making it seismically active. Additionally, being a young fold mountain range, the Himalayas are still undergoing tectonic uplift, increasing earthquake vulnerability for its inhabitants.
7. How does faulting lead to the formation of block mountains?
Answer: Faulting occurs when tensional forces pull the Earth’s crust apart, creating faults along which blocks of crust move relative to each other. If the block between two faults rises (forming a horst) or the surrounding land subsides, it creates a block mountain with steep faulted edges and a flat top. Examples include the Vosges in France and the Satpuras in Maharashtra, where such tectonic activity has shaped the landscape.
8. What are the different types of folds, and how do they differ?
Answer: Folds vary based on their structure: symmetrical folds have a vertical axial plane with limbs inclined at the same angle, while asymmetrical folds have an inclined axial plane with limbs at different angles. Overturned folds have one limb lying above the other, recumbent folds have a near-horizontal axial plane, and isoclinal folds have limbs sloping in the same direction at the same angle. These variations depend on the intensity of compressional forces and the nature of the rocks involved.
9. What are the causes of earthquakes as described in the chapter?
Answer: Earthquakes are primarily caused by the release of stress accumulated in the Earth’s crust, often due to tectonic movements along plate boundaries, such as in Indonesia or Assam, India. Volcanic activity can also trigger shallow earthquakes, as seen near Mount St. Helens in 1981, while anthropogenic causes like atomic explosions, mining, or large construction projects may induce localized earthquakes. These diverse causes highlight the dynamic nature of the Earth’s crust and human impact on seismic activity.
10. What are the different volcanic landforms, and how are they formed?
Answer: Volcanic landforms include lava domes, formed by solidified magma around the volcano’s mouth, and lava plateaus, created by extensive lava flows from fissure volcanoes, like the Deccan Trap in India. Calderas form as large depressions after massive eruptions, while crater lakes develop in extinct volcano craters filled with rainwater, such as Lonar Crater Lake. Cinder cones, like Mt. Nuovo in Italy, are built from ash and cinder, and composite cones, like Mt. St. Helens, consist of alternating lava and cinder layers, showcasing the diverse outcomes of volcanic activity.
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