What Is Not Likely To Happen At A Divergent Boundary

What's NOT Likely to Happen at a Divergent Boundary: A Deep Dive into Plate Tectonics

Divergent boundaries, where tectonic plates pull apart, are dynamic zones of Earth's crust responsible for creating new oceanic crust and driving significant geological processes. While characterized by volcanism, earthquakes, and rifting, certain geological events are highly improbable at these boundaries. This article delves into the fascinating world of divergent plate boundaries, explaining what typically occurs and, more importantly, what doesn't occur, providing a comprehensive understanding of these powerful geological features.

Understanding Divergent Boundaries: A Foundation

Before exploring the unlikelihoods, let's establish a firm understanding of what does commonly happen at divergent boundaries. These boundaries are primarily found along mid-ocean ridges, vast underwater mountain ranges where plates move apart, allowing magma from the Earth's mantle to rise and create new oceanic crust. The process involves several key elements:

1. Seafloor Spreading: The Birth of New Crust

As plates diverge, molten rock (magma) rises from the asthenosphere, the partially molten layer beneath the lithosphere. This magma cools and solidifies, forming new oceanic crust, a process known as seafloor spreading. This constant addition of new crust pushes the older crust outwards, away from the ridge.

2. Rifting and Formation of Rift Valleys: Land-Based Divergence

Divergent boundaries aren't limited to the ocean floor. On land, they manifest as rift valleys, vast elongated depressions formed by the stretching and thinning of the continental crust. The East African Rift Valley is a prime example of this process in action. Rift valleys are characterized by volcanic activity, faulting, and the gradual separation of continental plates.

3. Volcanic Activity: The Manifestation of Magma

The upwelling of magma at divergent boundaries is responsible for significant volcanic activity, both on the ocean floor and on land. This volcanism often leads to the formation of underwater volcanic mountains and, in some cases, volcanoes above sea level. The magma at divergent boundaries is typically basaltic, characterized by its low viscosity and relatively low silica content.

4. Shallow Earthquakes: Relatively Gentle Tremors

Earthquakes are common at divergent boundaries, but they are typically less powerful than those at convergent boundaries. These earthquakes are generally shallow, occurring in the upper crust as plates pull apart and rocks fracture.

What is Highly Unlikely to Happen at a Divergent Boundary?

Now, let's explore the geological events that are highly improbable at divergent boundaries. These events are often associated with other tectonic settings, highlighting the unique nature of divergent boundaries:

1. Subduction Zones: No Plates Plunging Underneath

Subduction, the process where one tectonic plate slides beneath another, is completely absent at divergent boundaries. Subduction zones are characteristic of convergent boundaries, where plates collide, and one is forced downwards into the mantle. At divergent boundaries, plates are moving away from each other, preventing any possibility of subduction.

2. Formation of High Mountain Ranges: No Collisional Uplift

The majestic Himalayas and Andes Mountains are testaments to the powerful forces of convergent plate boundaries. The collision of plates at these boundaries results in significant crustal shortening and uplift, creating towering mountain ranges. At divergent boundaries, plates are pulling apart, meaning there is no compressional force to create these impressive landforms. Instead, rifting leads to valleys and depressions.

3. Formation of Deep Ocean Trenches: No Subduction-Related Depressions

Deep ocean trenches are dramatic features found at convergent boundaries, where the subducting plate bends downwards, creating a deep depression in the ocean floor. These trenches are absent at divergent boundaries, as there is no subduction occurring.

4. High-Magnitude Earthquakes: Lower Seismic Potential

While earthquakes do occur at divergent boundaries, they are generally less powerful than those associated with convergent or transform boundaries. The lack of significant compressional stress means that the magnitude of earthquakes remains relatively low. The deeper, more powerful earthquakes are largely absent. The shallow depth of faulting also limits the energy release potential.

5. Formation of Accretionary Wedges: No Sediment Accumulation from Subduction

Accretionary wedges are masses of sediment scraped off the subducting plate at convergent boundaries. This accumulation of sediment is then piled up at the edge of the overriding plate, forming a significant landform. This process requires subduction, a process absent at divergent boundaries.

6. Andesitic or Rhyolitic Volcanism: Magma Composition Differences

The volcanic activity at divergent boundaries is overwhelmingly basaltic. Andesitic and rhyolitic volcanism, characterized by higher silica content and more viscous magma, are typically associated with convergent boundaries where subduction-related processes alter the magma composition. The relatively low silica content of the mantle magma rising at divergent boundaries precludes the formation of these more viscous, silica-rich magmas.

7. Metamorphism of High-Pressure, Low-Temperature Rocks: Different Pressure Regimes

Metamorphism, the transformation of rocks due to heat and pressure, occurs in different ways at various plate boundaries. At convergent boundaries, the high pressure associated with subduction leads to the formation of specific metamorphic rocks. Divergent boundaries, lacking the intense pressure of subduction, produce different types of metamorphic rocks, if any at all, characterized by lower pressure and possibly higher temperature transformations.

8. Formation of Ophiolites: Unique to Oceanic Crust Creation

Ophiolites, sections of oceanic crust and upper mantle that have been uplifted onto continental crust, are generally associated with the closure of ocean basins. While divergent boundaries create oceanic crust, the subsequent uplift and emplacement of ophiolites require more complex geological processes typically associated with convergent boundary activity.

9. Extensive Glaciation Directly Caused by Divergence: Climate Influences are Indirect

While the location of a divergent boundary can influence local climate (e.g., through the formation of rift valleys altering air circulation patterns), divergent plate boundaries themselves do not directly cause large-scale glaciation events. Glaciation is more closely tied to global climate change and other factors.

Conclusion: The Unique Character of Divergent Boundaries

Divergent boundaries are crucial for understanding plate tectonics, driving seafloor spreading, creating new crust, and shaping the Earth's surface. However, it's equally important to recognize the geological processes that are not likely to occur at these boundaries. Understanding these distinctions allows for a more complete picture of Earth's dynamic geological processes and the unique characteristics of each tectonic setting. The absence of subduction, high mountain ranges, deep trenches, and high-magnitude earthquakes, among other features, clearly distinguishes divergent boundaries from their convergent and transform counterparts. By focusing on these contrasts, we gain a much richer understanding of the powerful forces shaping our planet.