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

Basaltic lavas originate from mantle peridotite. Melting occurs due to decompression. Iceland's divergents have diverse lavas due to crust interactions.

Step-by-step solution

Identify the Source of Basaltic Lava

At divergent boundaries like the Mid-Atlantic Ridge, volcanic activity is primarily due to the upwelling of magma from the Earth's mantle. The source of basaltic lavas is the partial melting of the upper mantle, which predominantly consists of a type of rock known as peridotite.

Understand the Melting Process

The source rocks melt due to decompression melting. As tectonic plates move apart at divergent boundaries, the mantle beneath experiences a drop in pressure. This reduction in pressure lowers the melting point of the rocks, causing the mantle to partially melt and form magma that eventually erupts as basaltic lava.

Consider Other Types of Divergent Boundaries

Most divergent boundaries produce basaltic lava, but some may be associated with more varied volcanic compositions. For example, while not a divergent boundary in the traditional sense, intraplate volcanism like that of Iceland can exhibit more diverse lava compositions including rhyolitic or andesitic lavas due to crustal assimilation and fractional crystallization. However, most true oceanic divergent boundaries like the Mid-Atlantic Ridge produce primarily basalt.

Key concepts

Basaltic Lava

Basaltic lava is a type of magma that is rich in iron and magnesium as well as low in silica. This makes it exceedingly fluid, which means it can cover extensive areas with a thick layer of basalt when it cools and solidifies. Its propensity for fluidity primarily allows it to spread out into thin, expansive sheets while maintaining a high temperature.

At divergent boundaries, basaltic lava is most commonly found due to the underlying processes occurring beneath these zones. It is derived from magma that has its origins deep within the Earth's mantle, specifically from the partial melting of peridotite rocks. This process ensures that basaltic lava is consistently produced at locations such as the Mid-Atlantic Ridge, where tectonic plates are constantly moving apart.

Partial Melting

Partial melting is a crucial process in the formation of basaltic lava at divergent boundaries. The term 'partial melting' refers to a phenomenon where only a portion of solid rock melts to form magma. This occurs at varying temperatures because different minerals within the rock have different melting points.

In the case of the upper mantle rocks, which include peridotite, the minerals that melt first form basaltic magma. The molten portion rises because it is less dense than the remaining solids. Partial melting in the mantle is triggered at divergent boundaries where materials are less confined by pressure, creating conditions conducive for this kind of melting.

Decompression Melting

Decompression melting is a geological process that plays a central role in creating magma at divergent boundaries. It occurs as tectonic plates move apart, which decreases the pressure on the rocks in the upper mantle.

Normally, high pressure keeps the rocks in a solid state, even at high temperatures. However, as the pressure drops due to tectonic activity, the rocks' melting point decreases, allowing them to begin melting even without an increase in temperature.

This results in the formation of magma that can ascend through the crust and erupt as basaltic lava, making decompression melting a key mechanism in volcanic activity at these sites.

Upper Mantle

The upper mantle is a section of the Earth's interior that extends from just below the Earth's crust to a depth of about 410 kilometers. Below that lies the transition zone. This area is composed primarily of dense, solid rock but also contributes significantly to tectonic and volcanic activity due to its dynamic properties.

At divergent boundaries, convection currents within the upper mantle cause upwellings of partially melted rock to form magma. These currents result in the partial melting necessary for basaltic magma production.

• The upper mantle is mostly composed of peridotite, a rock that is rich in minerals such as olivine and pyroxenes.
• The heat and pressure conditions are such that they facilitate decompression melting behind the tectonic activity at divergent boundaries.

Oceanic Ridge

Oceanic ridges are underwater mountain ranges formed by tectonic plate movements at divergent boundaries. They are characterized by a high level of volcanic activity because of the constant creation of new oceanic crust as the plates pull apart.

One of the most renowned oceanic ridges is the Mid-Atlantic Ridge, which is central to the study of divergent boundaries. This ridge is an example of where basaltic lava is regularly produced, contributing to the constant renewal of the ocean floor.

• Oceanic ridges span thousands of kilometers, appearing mostly underwater with periodic appearances above sea level, such as Iceland.
• The unique geological activity at these ridges leads to frequent and informative studies in plate tectonics and volcanic processes.