This article fell out of some internal (infernal?) discussion about technical terms used here at VH. Seemed like a good idea to at least thrash through a definition and attempt to put it into perspective. It is by no means a comprehensive treatment of the subject. Rather it is an introduction to a large and complex field.
Encyclopedia.com, A Dictionary of Earth Sciences, published by Oxford University Press in 1999 defines a low-velocity zone as the following:
low-velocity zone (LVZ) The zone within the upper mantle beneath the oceans within which seismic P-waves are slowed and S-waves are slowed and partially absorbed. The top of the zone is some 40–60 km deep near the oceanic spreading ridges, and this depth increases to 120–160 km beneath the older oceanic crust. The bottom of the zone is poorly defined, but in the region of 250–300 km in depth. Beneath the continents, a restricted low-velocity zone occurs beneath crust areas subjected to orogenesis during the last 600 million years or so, but is not found beneath cratonic (see CRATON) areas. It is attributed to the presence of a 0.1% fluid phase and commonly ascribed to the partial melting of mantle rocks at these depths. It is often considered coincident with the asthenosphere, but probably this is valid only for oceanic areas.
This source is as good as any, as the various definitions are quite similar. http://www.encyclopedia.com/science/dictionaries-thesauruses-pictures-and-press-releases/low-velocity-zone
This is quite a mouthful. So what does it really mean? The important part in our discussion is in the underlined sentence, second from the last in the definition. In short, in the mantle, this is where the magma is. The same term is used for pockets of melt in the crust and for the same reason. The more melt we have, the closer that melt is to the surface, the more likely it is to reach the surface in some sort of eruption.
The definition also implies that we can use various seismic waves to see through the earth and observe differences based on how quickly they arrive.
To put this in perspective, a digression into imaging might be in order.
Imaging in the Electromagnetic Spectrum
The Electromagnetic spectrum encompasses everything from radio waves to gamma rays. It describes the behavior of basic packets of light – photons. Photons travel in waves and wave equations are used to describe how they behave. Human (and animal) eyes are designed to use visible light. Differences in the wavelengths give us colors with reds being longer and violets being shorter. Some animals can see a bit farther into the reds (infrared) and a bit farther into the violets (ultraviolet) than we can.
As you work your way into shorter wavelengths, you work your way through x-rays and gamma rays. As you work your way into longer wavelengths, you work your way through microwaves and radio waves. Some very low frequency radio waves are hundreds of km long.
Astronomers observe in various wavelengths to see different things in the surrounding universe. Taking a look in the x-ray range shows very high energy objects like neutron stars, magnetars and black holes. Taking a look in the microwave range with radar allows them to see through clouds of dust, particulates, and droplets of liquid. This is how they get surface pictures of cloudy planets like Venus and Saturn’s moon Titan.
High and low energy observations are combined to create images of objects and regions that would not otherwise be observable.
Imaging with Sound
It is also possible to image things with sound. Sea-going mammals like whales and dolphins have used this technique since they returned to the sea. Bats use sonar (a series of clicks) while flying to find food and one another while flying at night. Humans also use sonar to operate militarily undersea. Sonar is also used to find fish.
Medicine uses sound to create sonograms of organs and growing infants. These allow non-invasive observation of things going on inside the human body. A sound is introduced from the exterior, it bounces off whatever is inside, is received through a microphone and processed through equipment that turns it into a picture (moving or otherwise).
Oil, natural gas and other drilling companies use a similar techniques to generate pictures of layered rock under the surface of the earth. Sometimes the energy is input with small explosions. Sometimes vehicles which thump the earth are used. When the layers are made of the right materials, in the correct shape, and return a signal the right way this is interpreted as a higher or lower possibility of finding whatever the intended drilling target is.
Imaging with Earthquakes
These techniques were applied to energy waves created by earthquakes. An earthquake creates three types of waves – P (Primary), S (Secondary) and surface waves. Primary waves are compressional, secondary waves are shear waves, and surface waves are similar to what a rock does when it enters the surface of the water. The P and S waves are faster than the surface waves and tend to be the first notification of an earthquake. We here in Anchorage rode out our 7.1 Magnitude quake last year. It announced itself with what felt like the house buzzing. That buzzing was quickly followed by a massive shaking as the surface wave arrived.
Using the same techniques as the oil and natural gas exploration uses, other geologists have figured out how to construct images of things deep within the earth based on how these waves travel, how long they travel, how they are bent as they hit differing types of materials, and how they are attenuated (gradual loss of intensity, energy loss in travel) between their source and where they are received.
Over time, the scientists created a model of the interior of the earth. They created a model of what was happening at plate boundaries. They created a model of what is below regions of volcanic activity. One of the things discovered was the presence of what came to be described as low-velocity zones that appeared to be regions of more melted rock (magma) than the surrounding rock. There is an ongoing argument about the action of water to enhance melting. Shallower zones were typically associated with areas of volcanic activity (past and present).
IRIS has an interactive presentation of P and S waves rolling through the earth from the 2004 Sumatra earthquake.
They also have an extended description of the waves.
All this takes us back to the definition. There is a global low velocity zone that starts some 40 – 60 km at the surface and extends some 250 – 300 km deep. Under the continents, it starts some 120 – 160 km. It is this region that appears to be source for magmas that make their way toward the surface.
There are also shallower low-velocity zones. They are much smaller than the continental regions. Some are associated with hot spots like Iceland, Yellowstone and Hawaii. There are shallow low-velocity zones in areas of volcanic activity. The Alitplano is one such region. Typically, the closer to the surface the zone, the more eruptible magma or crystal mush is available for eruption after the next injection of hot basalt from the depths. Some of these are quite large. Some are not. Some will erupt. Some will cool into batholiths.
I am not an expert and write as much to figure out how things work as anything else.