Transform fault

A transform fault or transform boundary is a special kind of crack in the Earth's surface where the movement is mostly side-to-side. These faults usually happen where pieces of the Earth’s outer layer, called plates, meet. The motion along a transform fault ends suddenly where it connects to another place where plates meet, like another transform fault, a ridge where plates are moving apart, or a zone where one plate slides under another.

Most transform faults are found in the deep ocean. They help make up the spaces between areas where the seafloor is spreading apart. This spreading doesn’t always happen straight, so the faults create a zigzag pattern. While many of these faults are hidden under the ocean, some famous ones are on land, like the San Andreas Fault in California and the North Anatolian Fault in Turkey. These faults are important because they show how the Earth’s surface moves and changes over time.

Nomenclature

Transform boundaries are also called conservative plate boundaries. This is because they do not add or remove any part of the Earth's surface layers. The movement along these boundaries is mostly side-to-side.

Background

Geophysicist and geologist John Tuzo Wilson noticed that the offsets of oceanic ridges by faults do not follow the usual pattern. These faults, called transform faults, move in the opposite way compared to what we might expect. The distance between the ridges does not change during earthquakes because the ridges are spreading centers. This idea was proven true by studying the movement on these fault planes, showing the slip goes in the opposite direction than we might think.

Difference between transform and transcurrent faults

Transform faults and transcurrent faults both move sides to sides, but they are different. Transform faults always end where they meet another plate boundary, like another transform fault, a spreading ridge, or a subduction zone. Transcurrent faults might just stop without meeting another fault. Also, transform faults are special because they form a plate boundary, while transcurrent faults do not.

Mechanics

Faults are places where the Earth's rocks move and change shape due to forces pushing, pulling, or sliding them. Transform faults are special kinds of faults where rocks mainly slide past each other sideways. They help move pieces of the Earth's crust between places where new crust is made or where old crust is being pushed down. These faults can also make rocks split apart in areas where the crust is stretching.

Transform faults and divergent boundaries

Transform faults are often found connecting parts of divergent boundaries, like mid-oceanic ridges. These ridges are where new seafloor is made as hot rock from deep inside the Earth comes up. As the new seafloor moves away from the ridges, older parts of the seafloor slide slowly toward the continents. Even though these ridge segments are close, the way they move makes parts of the seafloor push past each other sideways. This side-to-side movement is where transform faults are active.

Unlike other types of faults, transform faults keep the ridges in fixed places while new seafloor moves away from them. We can see evidence of this movement in special patterns on the ocean floor. Studies suggest that transform faults form when parts of the mid-oceanic ridges slowly bend and stretch over time, eventually breaking to create these faults. Rocks found near these faults support the idea that new seafloor is created at the ridges, which helps us understand how the Earth's plates move.

Types

Geologist Tuzo Wilson studied transform faults and found they must connect to other faults or plate boundaries on both ends. Because of this, transform faults can grow longer, stay the same length, or even shrink.

When a transform fault links certain types of boundaries, it can grow longer. In other cases, it stays the same length. This can happen for different reasons, like when two ridges move outward at the same time, or when new seafloor is swallowed up by a subduction zone. Sometimes, transform faults can even get shorter over time, especially when certain plates move under each other.

Transform faults can also be either sinistral or dextral, depending on the direction the blocks move past each other.

Examples

The biggest examples of these faults are found in the Atlantic Ocean between South America and Africa. These places, called the St. Paul, Romanche, Chain, and Ascension fracture zones, have deep and clear faults and ridges. Other places include the East Pacific Ridge in the southeastern Pacific Ocean, which connects to the San Andreas Fault to the north.

These faults aren’t just in the ocean; many are on the edges of continents. The best example is the San Andreas Fault on the Pacific coast of the United States. It connects the East Pacific Rise off the coast of Mexico to the Mendocino triple junction near the Northwestern United States. This fault formed not too long ago, during a time called the Oligocene Period, between 34 million and 24 million years ago.

In New Zealand, the South Island’s Alpine Fault is a transform fault for much of its length. Another example is the Húsavík‐Flatey fault in Iceland, which is mostly under the water but can be studied on land near the town of Húsavík.

Other examples include:

• The Dead Sea Transform Fault in the Middle East
• The Chaman Fault in Pakistan
• The North Anatolian Fault in Turkey
• The Queen Charlotte Fault in North America
• The Sagaing Fault in Myanmar