Transform Plate Boundaries

Characteristics of Transform Plate Boundaries

The plates are under neither compression nor extension stress, but are under shear stress - the type of stress produced when friction builds up between two plates grinding horizontally past each other.

Seismicity and Volcanism at Transform Plate Boundaries

Volcanic eruptions at transform boundaries are non-explosive.

Seismicity and Volcanism at Transform Plate Boundaries

(A pull-apart basin is a basin that forms when two sliding plates literally pull apart a slab crust between them.)

Earthquakes at transform boundaries have the following characteristics:

- They can take place at either oceanic or continental transform boundaries. - They are shallow at oceanic transform faults (less than 10 km deep) and small in magnitude. - They are deeper (up to 20 km deep) and larger in magnitude along continental transforms. - They are rarely related to volcanism, as are earthquakes at divergent boundaries. - They are related to strike-slip motions perpendicular to the ridge crest at oceanic transforms.

In this image you see a piece of fault breccia

. Notice the fragments of other rocks in it. These came from rocks sheared off from the sliding plates.

Ridge-Ridge Transform Boundaries

A ridge-ridge transform plate boundary is simply a transform fault separating two segments of an oceanic ridge. This is by far the most abundant type of transform plate boundary. In fact, nearly all transform faults and transform plate boundaries found on the ocean floor are associated with the oceanic ridges in this way. Relative vertical motion along the transform fault takes place between the two segments of the ridge. This image above shows how movement occurs at a ridge-ridge transform plate boundary. Notice that the main boundary is a divergent plate boundary, where Plate A and Plate B are spreading apart. The transform boundary, shown from point x to point y, connects the ridge zones and creates a localized spot where plate segments slide past each other horizontally.

The San Andreas Fault, which runs down the length of California

As the two plates grind and slip past each other, friction builds up—sometimes to the point that rock bodies rupture and trigger massive earthquakes, which is why those who live near the San Andreas Fault always know that the next big one could be at any time.

How do you characterize plate movement at transform plate boundaries?

At transform plate boundaries, plates slide past each other.

Seismicity and Volcanism at Transform Plate Boundaries

Earthquakes are common at transform plate boundaries.

Characteristics of Transform Plate Boundaries

In contrast to convergent and divergent boundaries, transform plate boundaries neither create nor consume lithospheric material.

Ridge-Trench Transform Boundaries

In this diagram, notice how the transform fault marks a boundary where the two plates are moving sideways against each other. This is a ridge-trench transform plate boundary. It marks the transition between a diverging ridge zone and a converging trench zone. This type of transform plate boundary is rare, but they form an important boundary between spreading and converging plates.

The San Andreas Fault, which runs down the length of California

It is also a transform plate boundary, where horizontal movement between plates takes place.

How do earthquakes at oceanic transforms compare to earthquakes at the mid-ocean ridge crests?

Remember that earthquakes at the ridge crests are relatively rare and that they take place along normal faults. Earthquakes at oceanic transforms are more common and take place at transform faults, which are a special type of strike-slip fault. At normal faults, movement is vertical; at strike-slip faults, movement is horizontal.

Trench-Trench Transform Boundaries

The final type of transform plate boundary is the trench-trench transform plate boundary. These connect two trenches, or areas of convergence. They are also rare but result in some of the most famous transform faults on Earth. Notice that they are places where the direction of subduction changes along a convergent plate boundary. At the bottom of the diagram, the small black triangles show you that the plate on the left is being subducted under the other. At the top of the diagram, you see that the plate on the right is now being subducted.

Seismicity and Volcanism at Transform Plate Boundaries

The magma that fills in the pull-apart basins comes from partial melting of the mantle that occurs when it rises to fill the pull-apart basins

The widespread occurrence of earthquakes along oceanic transform faults is due to the cold and brittle crust along the transforms.

They break by brittle fracture and release the energy of an earthquake as they do.

Seismicity and Volcanism at Transform Plate Boundaries

They have localized pull-apart basins where small volumes of basaltic magma can erupt to form lava flows and cinder cones.

Volcanoes rarely develop at transform plate boundaries, as they do at convergent and divergent plate boundaries.

This is because transform boundaries do not allow for the upwelling or new creation of magma that other boundary types do.

Characteristics of Transform Plate Boundaries 1

Transform plate boundaries are not as long and extensive as divergent and convergent plate boundaries usually are. In fact, transform plate boundaries are really nothing more than faults. Specifically, they are transform faults, a special kind of strike slip fault, which is characterized by vertical, not horizontal, displacement of the crus t. All of the green areas on this map are transform faults, also called transform plate boundaries.

What is the relationship between transfor m plate boundaries and the mid-ocean ridges?

Transform plate boundaries—which are really nothing more than localized transform faults—are often found offsetting and intersecting the ocean ridges.

Divergent plate boundaries are often called constructive boundaries and convergent plate boundaries are often called destructive boundaries. Transform plate boundaries are called conservative boundaries. Can you think why transform plate boundaries are called conservative ?

While convergent and divergent plate boundaries result respectively in the consumption and creation of oceanic crust, transform plate boundaries do neither. At transform plate boundaries, the crust is conserved.

Characteristics of Transform Plate Boundaries

You have already learned that transform plate boundaries are places where two plates are sliding past each other.

Metamorphic rocks at transform plate boundaries

also develop by the horizontal shearing motion of the two plates sliding against each other

In fault breccias

angular pieces of rock that have broken off of larger rock bodies become cemented together by the flow of groundwater that seeps in at the fault.

Transform faults on the continents are not nearly as common

as those on the ocean floor because most transform faults develop at the ocean ridges.

Leaky transform faults erupt what type of magma?

basaltic

But some transform boundaries are called "leaky"

because they are associated with basaltic volcanism.

Which terms best describes tr ansform plate boundaries in terms of what happens to the crust?

conservative

As you learned in the Divergent Plate Boundaries section,

earthquakes associated with the mid-ocean ridges often occur along transform faults that run perpendicular to them

Friction at transform plate boundaries also creates

high heat and ductile deformation of rock bodies. This leads to mineral recrystallization of basaltic lava flows.

The San Andreas Fault, which runs down the length of California

is one of the largest strike-slip faults known

One common rock at transform boundaries on the continents

is the sedimentary rock fault breccia

In oceans, the influx of seawater into hot crust exposed by faulting can also create

metamorphic rocks by causing contact metamorphism and recrsytallization of the pre-existing rocks.

Volcanoes at transform faults are

non-explosive

Where are most transform faults found?

oceanic ridges

Click each image below to learn more about some prominent continental transform faults.

pg 8

Earthquakes at oceanic transforms are

shallow and small

1. What type of stress characterizes transform plate boundaries?

shear

A transform fault is a special type of _____ f ault, which are characterized by _____ displacement.

strike-slip; horizontal

At trench-trench transform boundarie s, the direction of _____ is changed.

subduction

transform faults on the continents are important because

they are areas of high seismic activity

They have a characteristic sheared shape similar

to what you may have seen with your modeling clay.