Question

Answer the following questions about divergent boundaries, such as the Mid- Atlantic Ridge, and their associated lavas: a. Divergent boundaries are characterized by outpourings of what type of lava: andesitic, basaltic lava, or rhyolitic? b. What is the source of these lavas? c. What causes the source rocks to melt?

Short answer

a) Basaltic lava. b) Upper mantle rocks. c) Decompression melting.

Step-by-step solution

Identifying Lava Type

Divergent boundaries are primarily characterized by outpourings of basaltic lava. Basaltic lava is more common at these locations due to the nature of the oceanic crust and mantle beneath them.

Understanding Lava Source

The source of basaltic lavas at divergent boundaries is the partial melting of the upper mantle material, specifically the peridotite mantle rocks. As these rocks melt, they form magma that rises to the surface and erupts as basaltic lava.

Explaining Source Rock Melting

The melting of source rocks at divergent boundaries is primarily caused by decompression melting. As tectonic plates pull apart, pressure is reduced on the underlying mantle material, allowing it to melt and form magma.

Key concepts

Basaltic Lava

Basaltic lava is a fascinating and crucial aspect of volcanic activity at divergent boundaries, such as the Mid-Atlantic Ridge. This type of lava is low in silica content, which makes it less viscous than other lava types like andesitic or rhyolitic lava. Because of this low viscosity, basaltic lava can flow over long distances before solidifying. This characteristic helps shape the oceanic floor, creating new crust at these divergent boundaries. The dark color of basaltic rock, once the lava cools, is due to its rich iron and magnesium content.

• Low silica content
• Flows easily and spreads widely
• Forms the oceanic crust

Partial Melting

Partial melting is a crucial process in the formation of magma in the Earth's mantle. This phenomenon occurs when only a portion of a solid is melted, which is often the case with mantle rocks like peridotite. During partial melting, minerals with lower melting points turn into liquid, while those with higher melting points remain solid. This selective melting is responsible for the creation of basaltic magma at divergent boundaries. As only part of a rock melts, the resultant magma has a different composition from the original rock. This is why the chemical makeup of basaltic lava can vary slightly from its source peridotite.

• Occurs when only some minerals melt
• Creates magma with a distinct composition

Decompression Melting

Decompression melting is a fascinating process triggered at divergent plate boundaries. As tectonic plates move apart, they cause a decrease in pressure on the mantle below. This drop in pressure occurs without a significant change in temperature, allowing the mantle materials to begin melting. In simpler terms, it's like taking the lid off a pressure cooker, where the pressure decreases, allowing the contents to boil more readily. This melting produces magma that rises to the surface.

• Caused by reduced pressure
• Key process at divergent boundaries
• Enables mantle materials to melt

Mid-Atlantic Ridge

The Mid-Atlantic Ridge is one of the most famous examples of a divergent boundary. It stretches over 10,000 miles and is essentially an underwater mountain range. This ridge marks the dividing line where the Eurasian and North American tectonic plates move apart. Basaltic lava flows are a common sight here, where new oceanic crust is continuously being created. This process contributes significantly to the phenomenon of seafloor spreading, which plays a key role in the theory of plate tectonics.

• Underwater mountain range
• Sign of seafloor spreading
• Continuous creation of oceanic crust

Upper Mantle

The upper mantle is a layer of the Earth that lies just beneath the crust and is primarily composed of solid rock, but it behaves like a viscous fluid over geological timescales. This layer is crucial because it is the source of basaltic magma found at divergent boundaries. The upper mantle's temperature and pressure conditions allow partial melting to occur, particularly of rocks such as peridotite, leading to the formation of basaltic lava. This layer is key to understanding the dynamic processes involved in plate tectonics and volcanic activity.

• Located below Earth’s crust
• Composed of solid but flowable rock
• Source of basaltic magma

Peridotite

Peridotite is a dense, coarse-grained rock largely made up of the minerals olivine and pyroxene, and it is the principal rock type found in the Earth’s upper mantle. Its composition is essential for the creation of basaltic magma through the process of partial melting. When peridotite melts under the right conditions, it forms basaltic magma, which can then rise to the surface at divergent boundaries. The high melting point of peridotite naturally ensures that only under specific circumstances, such as decreased pressure via decompression melting, will it begin to melt.

• Rich in olivine and pyroxene
• Main rock type of the upper mantle
• Formation source of basaltic magma