Divergent Plate Boundaries
They slowly rifted apart and the Atlantic Ocean opened up in between them
As you have learned, the divergent plate boundary between them today is the mid-ocean ridge and it is covered by deep ocean wate
At the ocean ridge divergent plate boundaries, the crust is thinned and extended. How does this compare to the crust at the convergent plate boundaries?
At convergent boundaries, the crust is contracted and thickened.
What factor most determines the topography of the mid-ocean ridges at various places on Earth?
spreading rate
explosive volcanoes: Convergent Boundaries
Convergent zones are areas of explosive volcanoes because of the highly viscous silica-rich magma
quiet, non-explosive volcan oes: Divergent Boundaries
Divergent boundaries are characterized by non-explosive volcanic eruptions.
basaltic magma: Divergent Boundaries
Divergent boundaries are characterized by basaltic magmas.
shield volcanoes: Divergent Boundaries
Divergent boundaries have mostly shield volcanoes.
shallow-focus earthquakes only: Divergent Boundaries
Divergent boundaries have shallow focus earthquakes.
Seismicity at Divergent Plate Boundaries
Divergent boundary earthquakes are usually small in magnitude. (In contrast convergent boundary earthquakes, as you learned earlier, can be either deep or shallow-focus and can be high in magnitude.)
What event is characterized by the large-scal e downhill movement of loose rocks and boulders?
landslide
Which of the following is created at the mid-ocean ridges?
new oceanic crust
hydrothermal vent
opening in the crust that allows the upwelling of magma-heated water
The diagram below shows how hot molten magma lies beneath the ridge crest, while cooler material lies beneath areas outward from the crest. It also shows another feature—how sediments on the ocean floor are thin at the crest and grow thicker as you move outward.
pg 7
divergent plate boundary
place where two plates are moving away from each other
How do plates move at divergent plate boundaries?
rift away from each other
What is the visible breaking and displacement of the Earth's surf ace during an earthquake?
rupture
At mid-ocean ridges
seismicity is associated with two types of faults.
Where on Earth are the mid-ocean ridges the steepest?
slow spreading centers
What happens when loose, moist soil takes on fluid properties and becomes too weak to support buildings and bridges?
soil liquefaction
When Pangaea existed
the Americas were connected to Europe and Africa.
Because of the divergent plate boundary in the mid-Atlantic
the Atlantic Ocean opened up when Pangaea began breaking apart.
During the development of rift zones
the earth is stretched and thinned. Eventually, the crust thins so much that a small body of water develops, bordered by two distinct pieces of land. Rifting continues and the body of water between the two segments of land grows even bigger to form a juvenile ocean. Finally, after millions of years of rifting, the body of water has grown to a mature ocean with two separate continents on each side. The divergent plate boundary now between the Americas and Europe and Africa is a past continental rift zone. Current continental rift zones are future submarine divergent boundaries.
Near the rift valleys
there are numerous normal faults that cause earthquakes
Continental rift zones are bounded by normal faults with large vertical displacements;
they are regions where the crust has been arched upward and pulled apart.
since the mid-ocean ridges are places where fresh magma wells up from the mantle
they are also areas of high heat flow.
What is a giant wave of water triggered by an earthquake?
tsunami
basalt
type of rock most common at divergent plate boundaries
shield volcano
type of volcano common at divergent plate boundaries, characterized by a low broad shape and thin basaltic lava
seamount
underwater mountain range that does not break the water's surface; often extinct volcanoes
How is the sediment cover of the ocean floor characterized?
very thin, especially at the mid-ocean ridge crests
Because most divergent plate boundaries are at the mid-ocean ridges
volcanism associated with these boundaries is frequently under the sea
When that boundary first began
, however, it was covered by land, not ocean.
creation of ocean floor: Divergent Boundaries
Divergent boundaries are areas of ocean floor creation.
Seismicity at Divergent Plate Boundaries
Earthquakes at divergent plate boundaries are usually shallow-focus, with many taking place at a depth of 10 kilometers or less.
In this case, the heat causes the hot basaltic magma to move up
Moving away from the ridges, this heat flow diminishes rapidly. After nearly 300 kilometers, heat decreases to one-fifth its value at the ridge.
How are plate tectonics responsible for the formation of the oceans?
Plate tectonics drives the rifting of continents. At divergent plate boundaries and continental rift zones, a space opens up which ev entually fills with water and grows to form an ocean.
On the diagram above, why is the ocean floor at the ocean basin and ridge flask shown as older than the ocean floor at the ridge crest?
Remember that ridges are divergent plate boundaries and are spreading centers where plates are moving outward. The ocean floor is newer at the ridge because new material is constantly being generated there; then it gets slowly pushed outward by the diverging plates.
why the seafloor gets older as you move away from the ridge?
Remember that the seafloor is very young at the ridge because that is where new ocean crust is being created. Also, since the seafloor is spreading outward from the ridge, the older material gets pushed progressively farther and farther out.
How Old is the Atlantic Ocean?
So just how old is the Atlantic Ocean? Well you know that the distance of the Atlantic Ocean from the eastern coast of the U.S. to the western coast of Africa is 4550 kilometers. And you know that the average rate of spreading for the Mid - Atlantic ridge is 2.5 centimeters per year. Dividing the distance of 4550 kilometers by 2.5 centimeters per year sounds easy enough, but don't get caught working with kilometers and centimeters in the same problem.
Note that we do not really know the exact year that the Atlantic began forming
That's why a value that is approximately 180 million years ago is valid.
East African Rift Zone
The East African Rift is another great example of continental rifting, but it is further along than the Basin and Range rift zone of North America. The East African Rift extends from Ethiopia to Mozambique over a distance of nearly 3,000 kilometers. Like other rift zones, it is bounded by normal faults and has a thinned crust. The famous Mt. Kilimanjaro is a product of the rifting taking place in the rift zone as basaltic volcanoes are abundant here.
Which is an older continental rift zone—the Basin and Range or the Red Sea Rift? How do you know?
The Red Sea Rift zone is older because it already has a juvenile ocean forming between two land masses. The Basin and Range rift zone will event ually become like the Red Sea rift, with an ocean opening up and splitting North America into 2 separate land masses.
The Red Sea
The Red Sea separates Africa from Arabia and it is another example of a continental rift zone. In fact, the Red Sea is a juvenile ocean basin that is opening up as the two pieces of land are rifted apart. It is what the Basin and Range and East African Rift zones will eventually look like if rifting continues in those areas and the land splits apart to allow the formation of an ocean between them. Over time, the Red Sea will mature into a mature ocean.
Seismicity at Divergent Plate Boundaries
The divergent plate boundaries do not generally produce very strong or high magnitude earthquakes because they are areas of thin, weak lithosphere.
Features of the Mid-Ocean Ridges
There is very little fresh sediment on the ocean floor, especially near the ridges, because of the constant additions of basaltic lava.
Continental rifts have m any of the same characte ristics as those of the oceanic rifts.
They have uplifted, thinned crus t; normal faults; shallow earthquakes; basaltic volcanism; and they are under extension stress.
continental rift zone
a future divergent plate boundary; where a divergent boundary is forming within a continent
shallow and small
characterization of earthquakes at divergent plate boundaries
While convergent plate boundaries lead to the consumption of the lithosphere
divergent plate boundaries lead to the formation of new lithosphere.
three examples of continental rift zones. If rifting continues in each of the three areas you see,
eventua lly new continental margins will form.
Divergent plate boundaries are places where the crust is being
extended and thinned
This map is a good illustration of the fact
fact that large-magnitude earthquakes are rare at divergent plate boundaries.
What earthquake hazard is caused by damaged gas and electricity lines?
fire
pillow basalt
igneous formation common at the ocean floor; forms when magma erupts from ocean-floor fissures and solidifies
slow spreading centers
where the mid-ocean ridge is steepest
The mid-ocean ridges are the most pronounced topographic features on Earth
but they are completely hidden under water, so we do not often think of them when we think of Earth's tallest mountain ranges.
volcanoes at convergent plate boundaries are characterized
by silica-rich magma
low-silica, low viscosity
characterization of magma at divergent plate boundaries
Iceland, which is cut by an ocean ridge
has less than one per year
Notice that much of Northern Europe
has no large earthquakes at all
consumption of the ocean floor: Convergent Boundaries
Convergent boundaries are zones of ocean floor consumption.
Features of the Mid-Ocean Ridges
. But if they were not covered with water, the mid-ocean ridges would be visible from the moon.
Features of the Mid-Ocean Ridges
. Photographs of the ocean floor show that ridges are covered with fresh lava flows and pillow basalt
How is the lithosphere directly under mid-ocean ridge crests characterized?
. hot and thin
Features of the Mid-Ocean Ridges
Along the ridges, the ocean crust is being extended and thinned. The ridges are characterized by numerous normal faults, as well as a number of transform faults that offset the ridge in several areas. Note in the drawing how the ridge is not linearly continuous, but broken and offset from time to time by the transform faults.
It took millions of years for it to open up completely and split the continents
At its earliest stage, the divergent plate boundary was what we call a continental rift zone.
Why do you think the ridge is less rugged where the spreading rate is faster?
At the ridges, the crust is being thinned and extended. The faster spreading creates more thinning and leveling of the topography.
shallow, intermediate, and deep-focus quakes: Convergent Boundaries
Convergent boundaries have earthquakes of all depths
high-silica magmaa: Convergent Boundaries
Convergent boundaries have high-silica magmas.
Seismicity at Divergent Plate Boundaries
Because the lithosphere is thin and weak in these areas, it breaks easily and does not support the build-up of pressure necessary to trigger massive earthquakes. Because of this, the deep mantle is not as fractured as the overlying crust.
The high heat flow and low gravity at the ridge crests lead scientists to believe that the lithosphere is very thin just below the ridges and that the hot asthenosphere rises nearly to the surface there.
By mapping variations in heat flow and gravity along the ocean ridges, we can identify specific areas where magma bodies are moving upward.
In what way is a continental rift zone a future divergent plate boundary?
Continental rift zones are where divergent plat e boundaries first begin to develop. They are where the crust is being thinned and extended. Over time, thinning and extension carry on to the point that the rift opens up, the land splits, and an ocean develops between two separate pieces of land. A divergent plate boundary is between t he two land masses that are moving in opposite directions.
stratovolcanoes: Convergent Boundaries
Convergent boundaries are characterized by steep stratovolcanoes.
(features of submarine volcanism.) Fissures and Pillow Basalt
Divergent plate boundaries under the ocean are marked by many fissures, or open areas where magma wells upward. The magma cools and solidifies to form the pillow basalt that is common on the ocean floor. These features are named pillow basalt because their size and appearance are similar to a pillow.
How do you think earthquakes at divergent plate boundaries compare to those at convergent plate boundaries in terms of their destructiveness?
Earthquakes at divergent plate boundaries are often of smaller magnitude even though they are shallow. Because of their smaller magnitude they are not nearly as destructive
Seismicity at Divergent Plate Boundaries
For divergent plate boundaries, seismicity is specifically concentrated along the ridge crests.
What do measurements of heat flow tell us about the crust at continental rift zones?
Heat flow measurements show t hat there is about 3 times more heat flow at continental rift zones than at other places—indicating that the cr ust is thin at continental rift zones.
( features of submarine volcanism.) Hydrothermal Vents
Hydrothermal vents are commonly found at submarine volcanoes. These are openings in the crust from which magma-heated water wells upward. Though they can also occur on the continents, they are most common at submarine volcanoes, where they can form billows of black smoke like the ones you see here. The waters that well up are rich in minerals, which means that undersea hydrothermal vents are rich in aquatic life. They are centers of entire ecosystems and support large communities of organisms.
Features of the Mid-Ocean Ridges
In contrast to mountains on Earth's continents, the mountains of the mid-ocean ridges are not made of folded and deformed sedimentary rock strata.
Features of the Mid-Ocean Ridges
Instead, they are made of un-deformed igneous basalt and metamorphosed basalt.
Because of plate tectonics
Pangaea broke apart and the continents began their slow migration to their present locations. As they did, the Atlantic Ocean opened up in between North America and the west coasts of Europe and Africa. Down the middle of the growing Atlantic Ocean is the culprit for all of this—the Mid-Atlantic Ridge, a divergent plate boundary, where two plates are rifting and moving away from each other. In contrast to the compression forces at convergent plate boundaries, divergent plate boundaries are places of extension stress, where the crust is being extended, thinned, and rifted.
Seismicity at Divergent Plate Boundaries
Plotting earthquakes on a map shows a band of shallow-focus earthquakes that coincide almost entirely with divergent plate boundaries.
Measurements of gravity across the ocean ridges show low values at the crest and higher values as you move outward from the crest.
This gravity anomaly indicates that materials below the ridge are less dense than materials in the adjacent ocean floor.
In contrast to stratovolcanoes (also called composite volcanoes) that occur at convergent plate boundaries, shield volcanoes that occur at divergent plate boundaries do not have explosive eruptions.
This is because they have thinner and more fluid magmas with lower silica contents.
( features of submarine volcanism.) Seamounts
Though they may form in a number of environments, seamounts are another feature of many divergent plate boundaries. Seamounts are mountain ranges under the sea that do not break the water's surface and are thus not islands. They are usually extinct volcanoes or volcanoes that no longer erupt.
continental rift zones
Up to this point, you have learned mostly about divergent plate boundaries at the mid-ocean ridges.
Shield volcanoes
have a relatively broad, flat profile and are named for their resemblance to a warrior's shield
those at divergent boundaries
have basaltic-rich magma
Forces That Shape Earth: Plate Movement
When you think about a mid-ocean ridge, do you imagine a giant underwater mountain range that encircles Earth? You aren't entirely incorrect. A mid-ocean ridge is frequently characterized by large mountains where frequent lava flows occur. These mountains are formed almost entirely of igneous basalt and metamorphosed basalt. This image demonstrates the immense size of these ridges. They are among the most distinctive features on our planet; we just cannot see them because they are covered with water
Features of the Mid-Ocean Ridges
Where the ridges are spreading out slowly, they are steep, more rugged and mountainous. The ridge in the North Atlantic, for example, is spreading less than 5 cm per year. It is one of the steepest and most prominent parts of the mid-ocean ridge. Where spreading is faster, such as in the east Pacific where plates are spreading at about 9 cm per year, the ridge is smoother and not as rugged.
Forces That Shape Earth: Plate Movement
While it may be easy to imagine a mid-ocean ridge as one continuous crack in the earth, this is not actually the case. Mid-ocean ridges are frequently offset by normal and transform faults. Study the image to better understand how a mid-ocean ridge functions.
Seismicity at Divergent Plate Boundaries
You already know that earthquakes are concentrated along tectonic plate boundaries
Forces That Shape Earth: Plate Movement
You have already learned that the plates that make up Earth's crust are in motion. This motion results in a constant recycling of Earth's crust. Crust is destroyed in some places while new crust forms in others. A mid-ocean ridge is a place where two plates are separating or moving in opposite directions. As the plates move away from each other, magma pushes to the surface, cools and forms new crust. Because these ridges are located under the ocean, this new crust is actually new ocean floor
The shallow-sloping volcanoes of divergent plate boundaries
are called shield volcanoes - They form from low-viscosity, highly fluid lavas that can travel great distances.
While steep stratovolcanoes
are common at convergent boundaries
that the mid-ocean ridges
are often offset by transform faults—these too cause earthquakes at divergent plate boundaries.
the volcanoes associated with divergent boundaries
are shallow-sloping.
Convergent plate boundaries
are sometimes called destructive boundaries because they consume crustal material at subduction zones.
Which rock is most common along the mid-ocean ridges?
basalt
By measuring heat flow from the ocean floor
we see that heat flow is highest at the ridge crests, implying that a large heat source lies beneath the ridge axis
This creation of new crustal material takes place at mid-ocean ridges,
where the oceanic crust is rifted open and magma wells up to fill the opening. The magma then hardens to form the igneous rocks that make up the oceanic crust.
. Divergent plate boundaries can also de velop on the continents
where they are called continental rift zones.