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

Mantle material is the lava source, melting due to decompression; a continental rift like the East African Rift can feature rhyolitic lava.

Step-by-step solution

Understanding the Source of Lava

The lavas at divergent boundaries, like the Mid-Atlantic Ridge, primarily come from partial melting of the mantle. This occurs because divergent boundaries are locations where tectonic plates are moving apart, allowing material from the mantle to rise upward.

Identifying the Cause of Melting

The source rocks melt due to decompression. As the mantle material rises toward the surface at a divergent boundary, the pressure decreases, causing the solid rocks of the mantle to partially melt and form magma that can erupt as basaltic lava.

Exploring Alternative Divergent Boundaries

While basaltic lava is the norm at mid-ocean ridges, a divergent boundary in a region with a thicker continental crust may emit rhyolitic or andesitic lava. This alternative forms if continental rifting involves melting at various levels on the crust. For example, East African Rift is a continental divergent zone where rhyolitic lava can be found due to interaction with continental materials.

Key concepts

Mid-Atlantic Ridge

The Mid-Atlantic Ridge is a prominent example of a divergent boundary, where two tectonic plates are slowly moving apart. This boundary stretches across the floor of the Atlantic Ocean, acting as a marker of separation between the Eurasian Plate and the North American Plate to the north and the South American Plate and the African Plate to the south.
The continuous pulling apart of these plates allows mantle materials to rise closer to the Earth’s surface, leading to volcanic activity along the ridge.

• It is the longest mountain range in the world, though it is mostly underwater.
• This boundary is crucial for the creation of new oceanic crust as magma rises, cools, and solidifies into basaltic rock.

The Mid-Atlantic Ridge is thus a geologically active zone, continually reshaping the ocean floor.

basaltic lava

Basaltic lava is the primary type of lava found at divergent boundaries like the Mid-Atlantic Ridge. Characterized by its low viscosity, basaltic lava can flow over great distances before solidifying, making it responsible for forming extensive lava flows.
Its relatively low silicon dioxide content contributes to its fluid nature compared to other lava types.

• This kind of lava primarily consists of minerals such as olivine and pyroxene.
• It cools to form dense, dark-colored rocks called basalt.

Basaltic lava is an integral part of oceanic crust formation and underscores the differences in composition and behavior of lava that result from different tectonic settings.

decompression melting

Decompression melting is the process that causes mantle rocks to melt as they ascend toward the Earth's surface at divergent boundaries. This melting occurs because when mantle material rises, it experiences a drop in pressure without a corresponding loss of heat.
As pressure decreases, the rocks' melting points decrease, leading to partial melting.

• This phenomenon is critical for the production of magma at divergent boundaries.
• It does not require the addition of heat to cause melting, unlike other types of melting.

Decompression melting is significant in understanding how new oceanic crust forms, particularly along mid-ocean ridges where basaltic lava emerges.

mantle partial melting

Mantle partial melting refers to the incomplete melting of mantle rocks, which generates magma beneath the Earth's crust. At divergent boundaries, as mantle material is brought closer to the surface due to plate tectonics, some of it melts partially due to decompression.

• Only a fraction of the mantle rock melts, creating a magma with distinct geochemical characteristics.
• The molten rock that forms is less dense and rises to fill in the gaps created by diverging plates.

This process plays a pivotal role in the formation of the oceanic crust as the magma cools and solidifies, forming new basaltic rock along mid-ocean ridges.