SCIENCE

Definition: Major Plate

According to your lesson, a Major Plate is defined as one of the largest tectonic plates that make up most of Earth's surface, specifically having a surface area that exceeds 20 million square kilometers (20 million km2). 🌍✨

✅ The Answer

For the question "According to the classification system based on surface area, which of the following criteria defines a 'Major Plate'?", the correct choice is: An area exceeding 20 million square kilometers.


💡 Quick Study Tip:

Your notes also mention two other categories to keep in mind:

  • Minor Plates: Area between 1 and 20 million km2 (e.g., Nazca or Arabian plates).

  • Microplates: Area less than 1 million km2 and usually located within major plates (e.g., Juan de Fuca).


🌍 Lesson 1: Classification of Tectonic Plates

1. Major Plates

  • Definition: The largest tectonic plates that make up most of Earth's surface.

  • Criteria: Area exceeding 20 million square kilometers

  • Examples: Pacific, North American, Eurasian, African, Antarctic, Indo-Australian, and South American plates.


2. Minor Plates

  • Definition: Smaller than major plates but still large enough to affect regional geology.

  • Criteria: Area between 1 and 20 millionkm2.

  • Examples: Somali, Nazca, Philippine, Arabian, Indian, Caribbean, Cocos.

3. Microplates

  • Definition: Very small plates often located within major plates and affected by their movements.

  • Criteria: Area less than 1 million km2.

  • Examples: Panama, Juan de Fuca, Rivera, Mariana.

⚙ Driving Mechanism of Plate Motion

  • Definition: The primary force behind tectonic movement is Ancient Mantle Convection.

  • Mechanism: It operates like a conveyor belt, where melts arise from the asthenosphere to facilitate seafloor spreading.

🌊 Lesson 2: Plate Boundaries (Divergent & Convergent)

1. Divergent Plate Boundaries

  • Definition: Zones where plates move away from each other. Also known as spreading centers or constructive margins (because new ocean floor is generated).

  • Key Features:

    • Mid-ocean ridge: Elevated seafloor mountain systems.

    • Rift: Valleys linked in chains within these mountains.

    • Seafloor Spreading: The continuous process where both sides of the ridge move apart.causing the spreading.

2. Convergent Plate Boundaries

  • Definition: Areas where tectonic plates move towards each other.

  • Key Process: Subduction — where oceanic crust descends into the mantle and is destroyed along trenches.


Movements of the Convergent Boundaries

• Subduction oceanic

lithosphere is destroyed along trenches and is recycled back into the

asthenosphere.

• Arcs are long, sublinear chains of volcanoes following the orientation of ocean trenches.


Types of the Convergent Plate Boundaries

• In oceanic-continental convergent margins, the denser oceanic crust subducts under the lighter continental crust. This process forms continental volcanic arcs.


• In oceanic-oceanic convergent margins, the older and much denser plate subducts under the younger plate. This process forms volcanic

island arcs.


Oceanic-oceanic convergent forms

volcanic island arcs.


Convergent plate boundaries may also develop through the collision of two continental plates, by

continental-continental convergent margin. → forms mountain ranges with linear elevated regions.


↔ Transform Plate Boundaries

  • Along transform plate boundaries or transform faults, plates slide past one another in a horizontal motion parallel to the plate boundary

    separating the two plates.

• Transform boundaries are conservative margins. In these regions, no production or destruction of crust occurs.

  • Key Characteristics:

    • No Volcanism: There is no upwelling of magma, so you won't find active volcanoes here.

    • Earthquakes: These boundaries are generally defined by weak and shallow earthquakes.

  • Classic Example: The San Andreas Fault, which extends up to 1,300 km in length.