Understanding Plate Boundaries: Convergent, Divergent, and Transform

The Earth's crust is broken into large pieces called tectonic plates. These plates are constantly moving, albeit very slowly. The areas where these plates meet are called plate boundaries. Different types of boundaries result in different geological phenomena like earthquakes, volcanoes, and mountain formation.

The question asks about the type of boundary where plates collide and one plate is pushed under the other. Let's examine the options provided:

Types of Plate Boundaries

• Divergent Boundaries: These occur when plates move apart from each other. New crust is created as magma rises from the mantle to fill the gap. The Mid-Atlantic Ridge is an example of a divergent boundary.
• Convergent Boundaries: These occur when plates move towards each other and collide. What happens at a convergent boundary depends on the type of crust involved (oceanic or continental). If oceanic crust meets continental crust, the denser oceanic plate is typically pushed underneath the continental plate. This process is called subduction. If two oceanic plates meet, one is usually subducted under the other. If two continental plates meet, neither is easily subducted, leading to mountain building (like the Himalayas). The process described in the question, where one plate is pushed under the other, is characteristic of subduction at a convergent boundary.
• Transform Boundaries: These occur when plates slide horizontally past each other. Crust is neither created nor destroyed at these boundaries. The San Andreas Fault in California is a well-known example of a transform boundary.
• Continental Boundaries: "Continental" refers to a type of crust, not a type of plate boundary itself. Boundaries occur where plates meet, regardless of whether they are made of continental or oceanic crust (or both).

Analyzing the Question and Options

The question specifically mentions plates colliding ("when plates collide") and one plate being pushed under the other ("one plate is pushed under the other"). This process of one plate descending beneath another is known as subduction. Subduction is a defining characteristic of certain types of convergent plate boundaries (specifically, oceanic-continental or oceanic-oceanic convergence).

Let's revisit the options in light of this understanding:

• Divergent boundaries involve plates moving apart, not colliding.
• Convergent boundaries involve plates colliding, and often, one plate subducts under the other.
• Transform boundaries involve plates sliding past each other, not colliding head-on or subducting.
• "Continental" is a type of crust, not a boundary name describing collision and subduction.

Based on the description of plates colliding and one being pushed under the other, the correct type of plate boundary is a convergent boundary.

Summary of Plate Boundary Characteristics

Therefore, the boundary type where plates collide and one is pushed under the other is a convergent boundary.

Revision Table: Key Plate Tectonics Concepts

Additional Information: Processes at Convergent Plate Boundaries

Convergent plate boundaries are zones of intense geological activity due to the collision of plates. There are three main types of convergent boundaries, depending on the type of crust involved:

• Oceanic-Continental Convergence: An oceanic plate converges with a continental plate. The denser oceanic plate subducts beneath the less dense continental plate. This often creates a volcanic mountain range on the continental crust (e.g., the Andes Mountains) and a deep oceanic trench offshore. Earthquakes are also common.
• Oceanic-Oceanic Convergence: Two oceanic plates converge. The older, colder, and therefore denser oceanic plate typically subducts beneath the younger one. This often results in the formation of a chain of volcanic islands (island arc) parallel to an oceanic trench (e.g., the Mariana Islands and Trench). Earthquakes occur frequently.
• Continental-Continental Convergence: Two continental plates converge. Because continental crust is relatively light and buoyant, neither plate easily subducts significantly into the mantle. Instead, the crust is crumpled and uplifted, forming large mountain ranges (e.g., the Himalayas, formed by the collision of the Indian and Eurasian plates). Earthquakes are common, but volcanic activity is usually minimal compared to the other types of convergent boundaries.

The question's description of "one plate is pushed under the other" most directly applies to the subduction process seen in oceanic-continental and oceanic-oceanic convergent boundaries, though continental collision is also a type of convergent boundary resulting from plates moving towards each other.