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 generate lavas with a large range of compositions.

Short answer

a. The mantle is the source of these lavas. b. Decompression melting causes the rocks to melt. c. Interaction with hotspots or varied crust, like in Iceland, generates diverse lavas.

Step-by-step solution

Identify the Source

Divergent boundaries, like the Mid-Atlantic Ridge, are formed where tectonic plates are moving apart. At these locations, the source of the basaltic lavas is the mantle, which is located beneath the Earth's crust. The mantle is primarily composed of ultramafic rocks which are rich in minerals like olivine and pyroxene.

Determine Causes of Melting

As tectonic plates separate at divergent boundaries, decompression melting occurs. This is because the pressure on the mantle decreases as it rises towards the Earth's surface due to the diverging plates. The reduced pressure lowers the melting point of mantle rocks, allowing partial melting to occur and form basaltic magma.

Describe Diverse Compositional Divergent Boundary

A divergent boundary that could produce lavas with a large range of compositions is one that interacts with different crustal material or has varying mantle source compositions. For example, a divergent boundary near a hotspot might introduce silica-rich magma from the continental crust or altered oceanic crust, leading to various magma compositions, such as those found in Iceland.

Key concepts

Basaltic Lava

Basaltic lava is a type of lava that is commonly associated with divergent boundaries. Typically, this lava is formed when the mantle partially melts as a result of tectonic activity. Basaltic lava is characterized by its low viscosity, which means it flows easily and can spread over large areas.
This is largely due to its composition, which is rich in iron and magnesium but low in silica. The ease with which basaltic lava flows impacts the landscape, creating features like shield volcanoes and vast plains.
Its dark color is indicative of the minerals it contains, such as olivine and pyroxene. These minerals further influence both the appearance and the behavior of basaltic eruptions. Understanding basaltic lava helps us comprehend the processes at divergent boundaries and the geological features they create.

Decompression Melting

Decompression melting is a crucial process that occurs at divergent boundaries. It happens when the pressure on the mantle decreases as it rises toward the Earth's surface.
As the tectonic plates separate, the mantle material rises into regions of lower pressure, which decreases its melting point. This phenomenon enables the mantle to begin melting without a significant increase in temperature.
The result is the formation of magma, particularly basaltic magma, which can then reach the surface. This process is vital for creating new oceanic crust at places like the Mid-Atlantic Ridge. Decompression melting underscores the dynamic nature of Earth's crust and mantle interactions.

Mid-Atlantic Ridge

The Mid-Atlantic Ridge is one of the most prominent examples of a divergent boundary on Earth. It stretches between the tectonic plates of the Atlantic Ocean and is several thousand kilometers long.
At this ridge, the African and South American plates, as well as the Eurasian and North American plates, are moving apart. This movement allows mantle material to rise and undergo decompression melting, resulting in the creation of new oceanic crust.
The basaltic lava emerging from this ridge constructs new seafloor as the plates continue to diverge. The activity at the Mid-Atlantic Ridge helps scientists understand plate tectonics and the constant reshaping of the Earth's surface. It's a key area for studying sea floor spreading and the geological processes associated with divergent boundaries.

Mantle Composition

The Earth's mantle is a significant source of basaltic lava, especially at divergent boundaries. Primarily, the mantle is composed of ultramafic rocks, which contain minerals such as olivine and pyroxene.
These minerals are rich in iron and magnesium, crucial for forming the low-viscosity basaltic lava. When conditions lead to decompression melting, it affects the mantle's complex mineral composition, resulting in the creation of magma.
The varying composition of the mantle can contribute to the range of lava types produced at different divergent boundaries. For instance, inclusions of other crustal components during melting may enhance the diversity of magma composition.
Understanding mantle composition is essential to studying the dynamics of Earth’s interior and its impact on surface geology.