Rift Valleys: Formation, Pictures, and Examples

What are rift valleys?

Rift valley is a long, narrow depression in the Earth's crust. They are formed by tectonic plates pulling away from each other, a process called rifting. The resulting valley is typically bordered by steep escarpments and can be the site of volcanic activity and earthquakes.

The Rift Valley is not a single valley, but rather a system of interconnected valleys, troughs, and lakes formed by the divergent movement of tectonic plates.

Where are rift valleys found?

Rift valleys are found primarily along tectonic plate boundaries, particularly divergent boundaries where plates are moving apart. These boundaries mark the zones where tectonic plates are actively pulling apart due to extensional tectonic forces.

How are rift valleys formed?

Rift valleys are formed by the separation of tectonic plates. As the plates pull apart, the Earth's crust stretches and thins, causing the land above to sink and form a long valley. Let's examine the sequential stages involved in rift valley formation.

1. Divergent Plate Boundaries and Extensional Tectonics

At divergent boundaries, tectonic plates slowly move away from each other. This movement creates extensional tectonics, a zone of stretching forces pulling the lithosphere, This stretching force thins and weakens the lithosphere, the rigid upper layer of the Earth composed of the crust and uppermost mantle.

2. Lithospheric Thinning and Normal Faulting

The extensional forces cause the lithosphere to thin and weaken. As it thins, the lithosphere experiences stress, eventually fracturing along normal faults. These are cracks in the Earth's crust where the rock on either side is displaced vertically. The central block of crust between the normal faults sinks downwards due to the extensional forces. This creates a depressed area called a graben, relative to the uplifted sides (footwalls) due to the extensional forces. This subsidence creates the initial depression of the rift valley.

3. Erosion and Sedimentation

Over geological timescales, wind, rain, and other erosional processes widen and deepen the rift valley. Additionally, sediments eroded from the uplifted flanks (shoulders) of the valley can accumulate on the valley floor.

4. Continued Divergence and Evolution

As the plates continue to diverge, the rift valley may widen and deepen. The graben floor may subside further, and volcanic activity might continue along the rift margins. In some instances, the rifting process may eventually lead to the formation of a new ocean basin if the divergence continues for millions of years (e.g., the Red Sea).

This rift valley in Iceland stretches along the Mid-Atlantic Ridge, where the North American and Eurasian tectonic plates are pulling apart from each other.

Types of Rift Valleys

Strictly speaking, there aren't different types of rift valleys based on their formation process. Since they all originate from the same phenomenon - divergent plate boundaries - they share a common geological mechanism. However, geologists sometimes categorize rift valleys based on their stage of development or their location:

Active Vs. Inactive Rift Valleys

Active Rift Valleys

Active rift valleys are those where the stretching and thinning of the crust is still happening. This is usually caused by the movement of tectonic plates at a divergent boundary. These areas are geologically young and are often characterized by:

Earthquakes
Volcanic activity
Steeper slopes and a rough valley floor

The East African Rift Valley: This is a massive rift system that stretches for thousands of kilometers from Ethiopia to Mozambique. It is home to a number of active volcanoes, including Mount Kenya and Mount Kilimanjaro.

Inactive Rift Valleys

Inactive rift valleys are those that are no longer undergoing extension and thinning. They may have been active millions of years ago, but they have since become stable. Inactive rift valleys are often filled with sediments, which can obscure their geological features.

Example of inactive rift valleys is The Rhine Rift Valley: This rift valley runs through western Europe, from the Netherlands to Switzerland. It was active millions of years ago, but it is now inactive.

Rift Valley, North of Masai Mara National Reserve, Kenya
Photo: Peter Prokosch

Continental Vs. Mid-Oceanic Rift Valleys

Continental Rift Valleys

Location: Occur within continents, where a single continental plate is being stretched and thinned.
Crust type: Thicker and lighter continental crust (mostly granite).
Volcanic activity is common, with eruptions consisting of basalt (lava rich in iron and magnesium).
As rifting progresses, the valley floor may drop down, forming large lakes (rift lakes). Examples include Lake Malawi and Lake Tanganyika in the East African Rift Valley.
Future: If rifting continues, the continental crust can become thin enough for molten rock from the mantle to erupt more frequently. This can eventually lead to the formation of a new ocean floor, separating the continents further. The Red Sea is an example of a seaway that began as a continental rift valley.

Mid-Oceanic Ridges

Location: Found in the middle of oceans, marking the divergent boundaries between oceanic plates.
Crust type: Underlain by thinner and denser oceanic crust (mostly composed of basalt).
Activity: Always active with ongoing seafloor spreading due to upwelling magma.
Examples: Mid-Atlantic Ridge, East Pacific Rise.
Future: If rifting continues, the continent can split, and a mid-oceanic ridge can form in the resulting ocean basin.

Aulacogens

Aulacogens are "failed" or inactive continental rift valleys that haven't progressed to the point of seafloor spreading. They can mark ancient divergent boundaries and are sometimes included in discussions of rift valleys. The key difference between aulacogens and active rift valleys lies in their tectonic activity. Rift valleys are zones of ongoing extension, while aulacogens are essentially inactive remnants.

Lake Baikal: This rift valley is famous for housing Lake Baikal

Features of Rift Valleys

Rift valleys boast several distinct features that arise from the unique way they form:

Shape and Size: Typically, rift valleys are elongated, narrow depressions stretching for hundreds of kilometers. Their floors are often relatively flat, a consequence of accumulated volcanic deposits and sediments. The sides, in contrast, tend to be steep and dramatic, sometimes exhibiting steps or terraces formed during the rifting process.

Volcanic Activity: The thinning crust associated with rift valleys can create pathways for molten rock (magma) to rise from the mantle. This often leads to significant volcanic activity along the rift, forming volcanoes and volcanic features like lava flows. The Great Rift Valley in Africa is a prime example of this.

Lakes: Due to the depression and potential for water blockage by volcanic activity, rift valleys can contain large and deep lakes. These lakes often have unique ecosystems due to their isolation and can be significant sources of freshwater. The Baikal Rift Valley in Siberia is home to Lake Baikal, the deepest freshwater lake on Earth.

Geothermal Activity: The thinning of the crust and presence of magma chambers can lead to elevated geothermal activity in rift valleys. This can manifest as hot springs, geysers, and fumaroles, creating a unique and sometimes harsh environment.

Biodiversity: Rift valleys can act as ecological islands, fostering the development of unique species due to their isolation and varied environments. The combination of freshwater lakes, volcanic landscapes, and diverse topography can create habitats for a wide range of plants and animals.

East African Rift valley

Examples of Rift Valleys

Some famous examples of rift valleys around the world:

East African Rift Valley: This is a major branch of the Great Rift Valley, running from Ethiopia in the north to Mozambique in the south. It is known for its stunning scenery, diverse wildlife, and active volcanoes. The East African Rift Valley is home to some of the deepest lakes in the world, including Lake Turkana and Lake Malawi. East African Rift Valley stretches thousands of kilometers through eastern Africa, encompassing countries like Ethiopia, Kenya, Tanzania, Rwanda, Burundi, Uganda, Eritrea, Djibouti, Somalia, Mozambique, Malawi, and Zambia.

Great Rift Valley: This massive rift valley system stretches for thousands of kilometers through eastern Africa, the Middle East, and southwest Asia. It's a series of connected rift valleys and is considered one of the largest geological features on Earth. Great Rift Valley include all those mentioned for the East African Rift Valley, along with Jordan, Israel, Lebanon, Syria, and Turkey.

The Baikal Rift Valley: Located in southern Siberia, this rift valley is the deepest continental rift valley on Earth, reaching depths of over 16,000 feet (5,000 meters). It's the result of a divergent plate boundary, where the Eurasian Plate and the Amur Plate are slowly pulling apart. Lake Baikal, the world's deepest freshwater lake by volume, fills much of the valley.

The Mid-Atlantic Ridge: This is an underwater rift valley that runs along the center of the Atlantic Ocean. It is a divergent plate boundary, where the North American and Eurasian plates are pulling away from each other. The Mid-Atlantic Ridge is not technically a continental rift valley because it is located on the ocean floor, but it is an important example of a rift valley formed by plate divergence.

Rift Valleys FAQ

What is the largest rift valley?

The East African Rift Valley stretches over 3,000 kilometers from Ethiopia to Mozambique.

What is the lowest point on Earth's surface located in a rift valley?

The Dead Sea, which lies in the Jordan Rift Valley, sits at over 400 meters below sea level.

Is Africa Splitting?

Africa is slowly splitting due to the East African Rift Valley, where tectonic plates are pulling apart at a rate of 7 millimeters per year. This won't cause a complete split for tens of millions of years, but the Red Sea and Gulf of Aden show where this process might lead in the distant future.