rift valley is a lowland region that forms where Earth’s tectonic plates move apart, or rift. Rift valleys are found both on land and at the bottom of the ocean, where they are created by the process of seafloor spreading. Rift valleys differ from river valleys and glacial valleys in that they are created by tectonic activity and not the process of erosion.
 
Tectonic plates are huge, rocky slabs of Earth's lithosphere—its crust and upper mantle. Tectonic plates are constantly in motion—shifting against each other in fault zones, falling beneath one another in a process called subduction, crashing against one another at convergent plate boundaries, and tearing apart from each other at divergent plate boundaries.
 
Many rift valleys are part of “triple junctions,” a type of divergent boundary where three tectonic plates meet at about 120° angles. Two arms of the triple junction can split to form an entire ocean. The third, “failed rift” or aulacogen, may become a rift valley. The Atlantic Ocean, for instance, is a result of a triple junction that started in what is now the Gulf of Guinea on the west coast of Africa. Two arms of a triple junction on the supercontinent Pangaea  “opened” the ocean, while the aulacogen formed the rift valley known as the Benue Trough through what is now southern Nigeria.
 
Rift valleys can also form at transform faults, where tectonic plates are grinding past each other. The Salton Trough, which stretches through the states of California (United States) and Baja California (Mexico), is a rift valley created in part by the San Andreas Fault. The San Andreas is a transform fault that marks the roughly northward movement of the Pacific plate and the roughly southern movement of the North American plate.
 
Mid-Ocean Ridges
 
Many of Earth’s deepest rift valleys are found underwater, dividing long mountain ranges called mid-ocean ridges. As tectonic plates move away from one another at mid-ocean ridges, molten rock from the mantle may well up and harden as it contacts the frigid sea, forming new oceanic crust at the bottom of the rift valley. 

In the northern Mid-Atlantic Ridge, the North American plate and the Eurasian plate are splitting apart at a rate of about 2.5 centimeters (1 inch) per year. Over millions of years, the Mid-Atlantic Ridge has formed rift valleys as wide as 15 kilometers (9 miles).

In the Pacific Ocean, the East Pacific Rise has created rift valleys where the Pacific plate is separating from the North American plate, Cocos plate, Nazca plate, and Antarctic plate. Like many underwater rift valleys, the East Pacific Rise is dotted with hydrothermal vents. Geologic activity beneath the underwater rift valley creates these vents, which spew super-heated water and vent fluids into the ocean.

Continental Rift Valleys
 
Very few active rift valleys are found on continental lithosphere. The East African Rift, the Baikal Rift Valley, the West Antarctic Rift, and the Rio Grande Rift are Earth’s major active continental rift valleys. The East African Rift is part of the “Great Rift Valley” system discussed below.
 
The Baikal Rift Valley (sometimes called the Baikal Rift Zone) cuts through 2,000 kilometers (1,200 miles) of Siberia, in eastern Russia. The Baikal Rift Valley is formed by a divergent plate boundary, where the Amur plate is slowly tearing itself away from the Eurasian plate, and has been doing so for about 25 million years. The Amur plate is moving eastward at a rate of about 4 to 5 millimeters (.16 to .2 inch) a year.
 
The West Antarctic Rift is a series of smaller rifts that roughly separate the two regions of Earth’s southernmost continent, West Antarctica and East Antarctica. The West Antarctic Rift is one of the most difficult rift valleys to study, because it lies beneath the massive Antarctic Ice Sheet, which can be more than 2 kilometers (1.2 miles) thick.
 
The Rio Grande Rift is a series of rift valleys along faults in the Southwestern United States. The Rio Grande Rift separates the Colorado Plateau, which is generally moving in a clockwise direction, from the older part (craton) of the North American plate. The Rio Grande Rift stretches from central Colorado to the Mexican state of Chihuahua.
 
Great Rift Valley
 
The most well-known rift valley on Earth is probably the so-called "Great Rift Valley System" which stretches from the Middle East in the north to Mozambique in the south. The area is geologically active, and features volcanoes, hot springs, geysers, and frequent earthquakes.
 
Today, however, the Great Rift Valley exists as a cultural concept, not a scientific one. All of the rift valleys in the “system” are connected, but not part of a single unit.

The northern part of the system is the Jordan Rift Valley. The Jordan Rift Valley stretches from the Golan Heights, near Israel’s border with Syria and Lebanon, to the Dead Sea, to the Gulf of Aqaba—an inlet of the Red Sea that separates the Sinai Peninsula from the Arabian Peninsula.

Associated with the Jordan Rift Valley to the south is the Red Sea Rift. Millions of years ago, the Arabian Peninsula was connected to Africa. Seafloor spreading caused the Arabian and African plates to rift apart. The Indian Ocean flooded the rift valley between the continents, creating the Red Sea. Today, Africa and Asia are connected by the triangle of the Sinai Peninsula. Eventually, the Red Sea Rift will separate Africa and Asia entirely and connect the Mediterranean and Red Seas.

East African Rift
South of the Red Sea Rift lies the massive, complex East African Rift. Throughout the East African Rift, the continent of Africa is splitting in two. The African plate, sometimes called the Nubian plate, carries most of the continent, while the smaller Somali plate carries Horn of Africa.
 
The two major rift valley systems of the East African Rift are the Gregory Rift and the Western Rift. These rift valleys are dotted by volcanoes: Erta Ale, Ethiopia; Mount Kenya, Kenya (an extinct stratovolcano); Ol Doinyo Lengai, Tanzania; Mount Kilimanjaro, Tanzania (a dormant stratovolcano); and Mount Nyiragongo, Democratic Republic of Congo.
 
The Gregory Rift stretches from the Red Sea and the Arabian Sea to as far south as Mount Kilimanjaro. One of the most important features of the Gregory Rift is the Afar Triple Junction, found where the Horn of Africa straddles the Red Sea and the Gulf of Aden in the Arabian Sea. At the Afar Triple Junction, the Arabian plate, Nubian plate and Somali plate are all tearing away from each other.
 
Two arms of the Afar Triple Junction continue to widen in the process of seafloor spreading—the arm extending into the Red Sea and the arm extending into the Gulf of Aden. As these rifts continue, the narrow valley created by the Gregory Rift (the arm of the Afar Triple Junction located above sea level) may sink low enough that the Arabian Sea will flood it. Separated from Africa by this new strait, Horn of Africa (sitting on the Somali plate) would become a continental island, like Madagascar or New Zealand.
 
The Western Rift, also called the Albertine Rift, includes many of the African Great Lakes. The Western Rift is one of the most biodiverse regions in Africa, featuring a narrow corridor of highland forests, snow-capped mountains, savannas, and chains of lakes and wetlands.

Rift Lakes
 
Rift lakes, formed as freshwater floods rift valleys, often mark rift valley systems. More than a billion years ago, for instance, the North American plate began a rifting process. A triple junction formed in the middle of the young continent, and deep rift valley developed. Freshwater drained and collected in this rift valley, creating a lake. After millions of years, however, the rift failed. The continent remained intact and the rift’s arms failed to open up a new ocean. Today, the remains of that ancient rift lake, Lake Superior, rest atop one of the oldest and deepest rift valleys in the world.
 
Lake Baikal, the rift lake over the Baikal Rift Valley in Siberia, is the deepest and oldest freshwater lake in the world. The deepest parts of Lake Baikal are 1,642 meters (5,387 feet), and are getting deeper every year. In addition, over the past 25 million years, layers of soft sediment have accumulated on the lakebed. The actual floor of the rift valley is more than 5 kilometers (3 miles) deep. Lake Baikal also has the largest volume of liquid freshwater in the world—a staggering 23,615 cubic kilometers (5,700 cubic miles).
 
The Dead Sea is a rift lake in the Jordan Rift Valley. Although the Dead Sea is not the world's deepest lake, the deep Jordan Rift makes it the lowest land elevation on Earth. The surface of the Dead Sea is 429 meters (1,407 feet) below sea level, and the lake’s depth is another 304 meters (997 feet). Unlike Lake Baikal, however, the Dead Sea is not a true rift lake as it was not formed entirely by the rift beneath it. The so-called Dead Sea Transform is a geologically complex area, where tectonic plates interact in many ways.
 
The most famous rift lakes in the world may be the series of narrow, deep rift valleys in the East African Rift known simply as the Rift Valley lakes. The Rift Valley lakes, stretching from Ethiopia to Malawi, are sites of amazing biodiversity. They include freshwater lakes, similar to Lake Baikal, as well as saltwater “soda lakes” similar to the Dead Sea.
 
Lake Tanganyika, whose long shores are shared by Burundi, Democratic Republic of Congo, Tanzania, and Zambia, is the largest of the Rift Valley lakes. Lake Tanganyika is the world’s second-deepest and second-biggest  (by volume of freshwater) lake in the world. Only Lake Baikal is deeper and holds more water. Like many freshwater Rift Valley lakes, Lake Tanganyika is home to hundreds of endemic species of cichlid fish.
 
Lake Natron, Tanzania, is one of the shallow, alkali-rich soda lakes of the East African Rift. Its dazzling red color is not a product of the region’s rocky geology, but the pink salt-loving bacteria that live in the briny water.
rift valley
Rift valleys are typically deep and narrow.

Grabens
Rift valleys are sometimes called grabens, which means “ditch” in German. While there is no official distinction between a graben and a rift valley, a graben usually describes a small rift valley.

Lakes in the Rift
Not all lakes located around the East African Rift are rift lakes. Lake Victoria, the largest freshwater lake in Africa, is not a rift lake, for instance. Lake Victoria’s basin formed as mountains uplifted around it. It did not sink as a result of the nearby East African Rift.

Finds in the Rift
Many, many important paleoanthropological discoveries have been made in the East African Rift, nicknamed  the “cradle of humanity.” “Lucy,” for instance, is a 3.2 million-year-old hominin skeleton that was discovered in Ethiopia, while “Turkana Boy” is a 1.5-million-year old hominin skeleton unearthed in Kenya. Scientists think that the tectonic activity that created the East African Rift also contributed to creating an environment that was ideal to the proliferation of life. The continual erosion of the cliffs of East African Rift also contribute to the dozens of discoveries.

Valles Marineris
The largest and deepest rift valley yet discovered is not on Earth—it’s on Mars. Valles Marineris was formed millions of years ago, when the rocky Martian lithosphere was still rifting and shifting. Valles Marineris reaches depths of up to 7 kilometers (4 miles) and spans about 4,000 kilometers (2,500 miles) long. That’s about 20% of the diameter of Mars itself!

Let Her Rift
With so much volcanic and tectonic activity going on there, the East African Rift Valley is a potent power source. The United Nations Environment Program is developing a geothermal energy program that would tap into this potential. The program would convert the heat created by the rift valley’s underground activity into electricity through a series of steam wells. One of the wells in Kenya produces enough power for 5,700 homes! If successful, this program would provide a sustainable energy source for millions of people, many of whom do not have access to electricity today.

accumulate
Verb

to gather or collect.

African Great Lakes
Noun

system of lakes in and around the Great Rift Valley: Lake Albert, Lake Edward, Lake Kivu, Lake Malawi, Lake Tanganyika, Lake Turkana, and Lake Victoria. Also called the Rift Valley Lakes and the East African Lakes.

alkali
Noun

chemical compound often found as mineral salt in soils and bodies of water.

ancient
Adjective

very old.

Antarctic Ice Sheet
Noun

thick glacier covering most of Antarctica.

aulacogen
Noun
failed or stunted arm of a triple junction in a plate tectonics rift system.
Plural Noun

(singular: bacterium) single-celled organisms found in every ecosystem on Earth.

Noun

all the different kinds of living organisms within a given area.

Noun

natural or artificial line separating two pieces of land.

briny
Adjective

salty.

cichlid
Noun

spiny-finned freshwater fish.

complex
Adjective

complicated.

concept
Noun

idea.

Noun

one of the seven main land masses on Earth.

continental island
Noun

land once connected to a continent but broken off by shifting tectonic plates.

convergent plate boundary
Noun

area where two or more tectonic plates bump into each other. Also called a collision zone.

corridor
Noun

hallway, or connecting passage of land.

craton
Noun

old, stable part of continental crust, made up of shields and platforms.

Noun

rocky outermost layer of Earth or other planet.

divergent boundary
Noun

area where two or more tectonic plates are moving away from each other. Also called an extensional boundary.

dormant
Adjective

state of minimal growth or activity.

earthquake
Noun

the sudden shaking of Earth's crust caused by the release of energy along fault lines or from volcanic activity.

Noun

height above or below sea level.

endemic
Adjective

native to a specific geographic space.

Noun

act in which earth is worn away, often by water, wind, or ice.

extinct
Adjective

no longer existing.

fault
Noun

a crack in the Earth's crust where there has been movement.

forest
Noun

ecosystem filled with trees and underbrush.

freshwater
Noun

water that is not salty.

frigid
Adjective

very cold.

geologic
Adjective

having to do with the physical formations of the Earth.

Noun

natural hot spring that sometimes erupts with water or steam.

glacial valley
Noun

depression in the earth created by a moving glacier.

graben
Noun

small rift valley.

Great Rift Valley system
Noun

series of faults and other sites of tectonic activity stretching from southwestern Asia to the Horn of Africa.

Horn of Africa
Noun

large peninsula in northeast Africa, including the countries of Somalia, Djibouti, Eritrea, and Ethiopia. Also called the Somali Peninsula.

hot spring
Noun

small flow of water flowing naturally from an underground water source heated by hot or molten rock.

hydrothermal
Adjective

related to hot water, especially water heated by the Earth's internal temperature.

inlet
Noun

small indentation in a shoreline.

Noun

body of water surrounded by land.

lakebed
Noun

bottom of a lake.

Noun

outer, solid portion of the Earth. Also called the geosphere.

Noun

middle layer of the Earth, made of mostly solid rock.

massive
Adjective

very large or heavy.

Middle East
Noun

region of southwest Asia and northeast Africa.

mid-ocean ridge
Noun

underwater mountain range.

molten
Adjective

solid material turned to liquid by heat.

oceanic crust
Noun

thin layer of the Earth that sits beneath ocean basins.

Pangaea
Noun

supercontinent of all the Earth's landmass that existed about 250 million years ago.

Noun

piece of land jutting into a body of water.

Noun

large region that is higher than the surrounding area and relatively flat.

Noun

movement and interaction of the Earth's plates.

rift
Noun

break in the Earth's crust created by it spreading or splitting apart.

rift lake
Noun

lake formed as a result of plate tectonics separating the Earth's crust and water collecting in the depression.

Noun

depression in the ground caused by the Earth's crust spreading apart.

river valley
Noun

depression in the earth caused by a river eroding the surrounding soil.

savanna
Noun

type of tropical grassland with scattered trees.

Noun

rift in underwater mountain range where new oceanic crust is formed.

Noun

base level for measuring elevations. Sea level is determined by measurements taken over a 19-year cycle.

Noun

solid material transported and deposited by water, ice, and wind.

shore
Noun

coast.

Siberia
Noun

region of land stretching across Russia from the Ural Mountains to the Pacific Ocean.

soda lake
Noun

lake with a high concentration of carbonate salts. Also called an alkaline lake.

staggering
Adjective

overwhelming or shocking.

Noun

narrow passage of water that connects two larger bodies of water.

stratovolcano
Noun

steep volcano made of hardened lava, rock, and ash. Also known as a composite volcano.

subduction
Noun

process of one tectonic plate melting, sliding, or falling beneath another.

supercontinent
Noun

ancient, giant landmass that split apart to form all the continents we know today.

tectonic activity
Noun

movement of tectonic plates resulting in geologic activity such as volcanic eruptions and earthquakes.

tectonic plate
Noun

massive slab of solid rock made up of Earth's lithosphere (crust and upper mantle). Also called lithospheric plate.

transform fault
Noun

boundary between two tectonic plates, where the plates are moving horizontally or vertically in opposite directions, not against or away from each other. Also called a conservative plate boundary.

triple junction
Noun

region where the boundaries of three tectonic plates meet and interact.

valley
Noun

depression in the Earth between hills.

vent fluid
Noun

chemicals ejected by hydrothermal vents.

Noun

an opening in the Earth's crust, through which lava, ash, and gases erupt, and also the cone built by eruptions.

volume
Noun

space an object occupies.

well up
Verb

to swell or build up.

Noun

area of land covered by shallow water or saturated by water.