notes of class 8th science chapter 12

 Here’s a summary of Class 8 Science Chapter 12: "Some Natural Phenomena". This chapter typically covers various natural phenomena related to Earth and the atmosphere, including earthquakes, volcanoes, and lightning.

Chapter 12: Some Natural Phenomena

1. Earthquakes

• Definition: An earthquake is the shaking of the Earth's surface caused by the sudden release of energy from the Earth's interior.
• Causes:
  • Movement of tectonic plates.
  • Volcanic activity.
  • Human activities like mining or reservoir-induced seismicity.
• Seismic Waves:
  • Primary Waves (P-waves): Fastest, travel through solids and liquids.
  • Secondary Waves (S-waves): Slower, travel only through solids.
• Measuring Earthquakes:
  • Seismometer/Seismograph: Instrument used to detect and record the seismic waves.
  • Richter Scale: Measures the magnitude of the earthquake.
  • Mercalli Scale: Measures the intensity based on observed effects.

2. Volcanoes

• Definition: A volcano is an opening in the Earth's crust where molten rock, ash, and gases from the Earth's interior escape to the surface.
• Types of Volcanoes:
  • Shield Volcanoes: Broad, gently sloping sides (e.g., Mauna Loa in Hawaii).
  • Stratovolcanoes (Composite Volcanoes): Steep, conical shape (e.g., Mount St. Helens).
  • Cinder Cones: Small, steep-sided volcanoes (e.g., Paricutin in Mexico).
• Volcanic Eruptions:
  • Lava Flow: Molten rock that flows from the volcano.
  • Ash Cloud: Fine particles ejected during an eruption.
  • Volcanic Gases: Include water vapor, carbon dioxide, sulfur dioxide.
• Effects of Volcanic Eruptions:
  • Can create new landforms.
  • Can lead to loss of life and property.
  • Affect climate by injecting ash and gases into the atmosphere.

3. Lightning and Thunder

• Lightning:
  • Definition: A sudden electrostatic discharge that occurs during a thunderstorm.
  • Formation:
    • Results from the buildup of electrical charges in the clouds.
    • When the charge difference becomes too great, it discharges as lightning.
  • Types of Lightning:
    • Intra-cloud Lightning: Between different parts of the same cloud.
    • Cloud-to-ground Lightning: From the cloud to the ground.
    • Cloud-to-cloud Lightning: Between different clouds.
• Thunder:
  • Definition: The sound produced by the rapid expansion and contraction of air heated by lightning.
  • Speed: Sound travels slower than light, so thunder is heard after lightning is seen.

4. Tornadoes

• Definition: A tornado is a rapidly rotating column of air extending from a thunderstorm to the ground.
• Formation:
  • Occurs in severe thunderstorms with strong wind shear.
  • A rotating updraft called a mesocyclone can develop into a tornado.
• Characteristics:
  • Can cause significant damage with high winds.
  • Often appear as a visible funnel cloud.

5. Important Safety Measures

• During an Earthquake:
  • Drop, Cover, and Hold On.
  • Stay indoors or in a safe place away from glass and heavy objects.
• During a Volcanic Eruption:
  • Follow evacuation orders.
  • Protect yourself from ashfall by covering your mouth and nose.
• During a Lightning Storm:
  • Avoid open fields and tall objects.
  • Stay indoors and avoid using electrical appliances.
• During a Tornado:
  • Seek shelter in a basement or a small, windowless interior room.
  • Avoid taking shelter under overpasses or bridges.

This summary should cover the key points of the chapter on natural phenomena. For detailed explanations and diagrams, refer to your textbook and class notes.

Detailed Overview of Key Topics

1. Earthquakes

• Tectonic Plates:

  • The Earth's outer shell (the lithosphere) is divided into several large and small pieces called tectonic plates.
  • These plates are constantly moving due to the flow of molten rock in the mantle (convection currents).
  • Most earthquakes occur at the boundaries of these plates.

• Faults:

  • A fault is a fracture in the Earth's crust where blocks of rock have moved relative to each other.
  • The point where the earthquake originates is called the focus or hypocenter.
  • The point on the Earth's surface directly above the focus is called the epicenter.

• Effects of Earthquakes:

  • Ground shaking.
  • Surface rupture.
  • Tsunamis (if the earthquake occurs under the sea).
  • Landslides and avalanches.

• Safety Tips:

  • If indoors, stay inside and take cover under a sturdy piece of furniture.
  • If outdoors, move to an open area away from buildings, trees, and utility wires.
  • If driving, pull over safely and stay in the vehicle until shaking stops.

2. Volcanoes

• Volcanic Material:

  • Lava: Molten rock that reaches the Earth's surface.
  • Tephra: Ejected material such as ash, cinders, and volcanic rocks.
  • Volcanic Gases: Gases such as water vapor, carbon dioxide, sulfur dioxide, and hydrogen sulfide.

• Volcano Formation:

  • Hot Spots: Areas where magma from the mantle rises through the crust, forming volcanoes (e.g., the Hawaiian Islands).
  • Convergent Boundaries: Where tectonic plates collide, one plate is forced under another, leading to volcanic activity (e.g., the Pacific Ring of Fire).
  • Divergent Boundaries: Where tectonic plates move apart, magma rises to create new crust (e.g., the Mid-Atlantic Ridge).

• Volcanic Hazards:

  • Lava Flows: Can destroy everything in their path but usually move slowly enough for people to evacuate.
  • Ashfall: Can cover large areas, leading to respiratory problems and disrupting air travel.
  • Pyroclastic Flows: Fast-moving currents of hot gas and volcanic matter that can be extremely destructive.

• Volcanic Eruption Indicators:

  • Increased seismic activity.
  • Gas emissions.
  • Ground deformation (swelling or tilting).

3. Lightning and Thunder

• Electricity in the Atmosphere:

  • Lightning is a result of electrical charges building up in thunderstorm clouds.
  • The top of the cloud typically becomes positively charged, while the bottom becomes negatively charged.
  • Lightning can also occur between clouds or within the same cloud.

• Lightning Safety:

  • Avoid using electrical appliances and stay away from water during a thunderstorm.
  • If you are caught outside, avoid open fields and tall trees.

• Thunder:

  • The sound of thunder travels at approximately 1 km every 3 seconds.
  • By counting the time between seeing lightning and hearing thunder, you can estimate the distance of the lightning strike.

4. Tornadoes

• Tornado Formation:

  • Tornadoes form from severe thunderstorms when there is a strong wind shear (difference in wind speed and direction) and a rotating updraft.
  • The rotation extends downward from the storm cloud, forming a visible funnel.

• Tornado Classification:

  • Enhanced Fujita Scale (EF Scale): Measures tornado intensity based on damage. Categories range from EF0 (weak) to EF5 (violent).

• Tornado Safety Tips:

  • Move to a small, windowless interior room or basement.
  • Avoid seeking shelter under overpasses or bridges as they can become wind tunnels.

5. General Safety Measures for Natural Phenomena

• Preparation:
  • Earthquake: Secure heavy furniture, create an emergency kit, and have a family emergency plan.
  • Volcano: Know evacuation routes, have masks ready to protect against ash, and stay informed about volcanic activity.
  • Lightning: Avoid open areas and tall objects, and stay indoors during thunderstorms.
  • Tornado: Have a tornado plan, including a safe place to go and a way to receive weather alerts.

Conclusion

Understanding these natural phenomena helps us better prepare for and respond to their impacts. Each phenomenon has specific characteristics and requires different safety measures to mitigate their effects. Always stay informed and follow safety guidelines to protect yourself and your community.