Question

Divergent boundaries, such as the Mid-Atlantic Ridge, are characterized by outpourings of basaltic lava. Answer the following questions about divergent boundaries and their associated lavas: a. What is the source of these lavas? b. What causes the source rocks to melt? c. Describe a divergent boundary that would be associated with lava other than basalt. Why did you choose it, and what type of lava would you expect to erupt there?

Short answer

a) Source is upper mantle; b) Melting caused by decompression; c) East African Rift may have rhyolitic lava.

Step-by-step solution

Understanding the Source of Basaltic Lavas

The source of basaltic lavas at divergent boundaries, like the Mid-Atlantic Ridge, is the partial melting of the upper mantle. This region of the mantle is composed mainly of peridotite, which consists mostly of olivine and pyroxene.

Mechanism of Melting

The melting of the source rocks at divergent boundaries is primarily due to decompression melting. As tectonic plates diverge, the mantle material rises to fill the gap. The pressure decreases as the material rises, leading to melting despite near-consistent high temperatures, allowing basaltic magma to form.

Exploring Non-Basaltic Divergent Boundaries

A divergent boundary associated with lavas other than basalt could be one experiencing significant interaction with continental crust, such as the East African Rift. In this scenario, rhyolitic lava may erupt due to the assimilation of silica-rich continental material, increasing silica content in the resulting magma.

Key concepts

Mid-Atlantic Ridge

The Mid-Atlantic Ridge is one of the most well-known examples of divergent plate boundaries on Earth. Located in the Atlantic Ocean, it serves as a massive underwater mountain range that zigzags between the continents. At this boundary, tectonic plates are moving apart, allowing magma to rise from the mantle and solidify into new oceanic crust. The constant movement and creation of crust lead to the ridge's formation, stretching over thousands of kilometers. As the plates diverge, basaltic lava pours out smoothly and cools to form the rugged terrain seen at this and other mid-ocean ridges. This geological feature is crucial in understanding the process of sea-floor spreading, which plays a vital role in plate tectonics. Key Points:

• Mid-Atlantic Ridge is a classic example of a divergent boundary.
• Magma rises between diverging plates, forming new oceanic crust.
• Sea-floor spreading is a continuous process at this ridge.

Basaltic Lava

Basaltic lava is a type of lava that prominently occurs at mid-ocean ridges like the Mid-Atlantic Ridge. It originates from the partial melting of the upper mantle, specifically a rock called peridotite. The lava is low in silica and is rich in iron and magnesium, making it less viscous compared to its more silica-rich counterparts like rhyolitic and andesitic lavas. This low viscosity allows basaltic lava to flow over long distances, forming expansive lava fields. Basaltic Lava Characteristics:

• Low silica content results in high fluidity.
• Originates from partial melting of the upper mantle.
• Forms new oceanic crust as it solidifies upon reaching the surface.

Decompression Melting

Decompression melting is a critical geological process at divergent boundaries. It occurs when solid mantle material rises within the Earth as tectonic plates move apart at places like the Mid-Atlantic Ridge. As the mantle material ascends, the pressure on it decreases. Despite remaining at much the same high temperature, this reduction in pressure allows the mantle to melt, forming magma. This magma then contributes to the creation of new geological features like mid-ocean ridges when it reaches the surface and solidifies. Key Aspects of Decompression Melting:

• Reduced pressure causes rocks to melt, not increased temperature.
• Responsible for the generation of basaltic magma at divergent boundaries.
• Facilitates the production of new oceanic crust as magma solidifies.

Upper Mantle

The upper mantle is a layer of the Earth beneath the crust that plays a crucial role in plate tectonics and magmatism, particularly at divergent boundaries. Composed mostly of peridotite, which includes minerals like olivine and pyroxene, it is a source of basaltic magma. This layer experiences specific conditions where decompression melting can occur, providing the material that forms new ocean crust. Features of the Upper Mantle:

• Located just beneath the Earth's crust.
• Mainly consists of peridotite, a dense, magmatic rock.
• Partial melting of this layer leads to basaltic lava formation.

East African Rift

The East African Rift is a unique example of a divergent boundary that occurs on a continent, unlike the oceanic Mid-Atlantic Ridge. It stretches over 6,000 kilometers in a complex series of rift valleys. Here, the African continent is slowly splitting apart as tectonic forces pull it in different directions. This rift is associated with more varied volcanic activity compared to oceanic spreading centers. While basaltic lava can be present, the interaction of the rising magma with the continental crust may lead to the formation of rhyolitic lava. This is due to the incorporation of silica-rich materials from the continent, altering the composition of the magma. Characteristics of the East African Rift:

• Occurs on land, across the African continent.
• Involves complex rift systems and diverse volcanism.
• Possible formation of rhyolitic lava due to continental material.