Earth’s ultralow-velocity zones

Earth has ultralow-velocity zones (ULVZs) deep underground, where sound waves travel slower through.

One of them under Iceland is horizontal cigar shaped, not vertical.

ultralow velocity zones ULVZwith sophisticated computer models that use the waves from large earthquakes to create CT scan–like tomographic pictures of Earth’s interior ... The picture gets cloudier in the lower mantle, where the ULVZs live. The regions get their name from the way that earthquake waves travel so much more slowly through them.

One way to explain that speed drop would be if they were partially molten. Another camp has held that the speed drops can be explained if ULVZs are made of a dense, different type of rock, perhaps enriched with iron, and chemically distinct from the rest of the mantle.
A molten puddle deep under Iceland may reveal where volcanoes get their lava | Science Mag

Anything else that could explain this phenomena?

Earth ultralow-velocity zones ULVZthe scene underneath Iceland provides a better picture. That’s because earthquake waves pass underneath the region from different directions and can be picked up by sensors on opposite sides of the world, unlike the Pacific islands.

Using earthquake waves picked up by arrays of sensors in the United States and China, her team better identified the position and shape of the ULVZ. They found its shape was a stubby cylinder - like a pillbox - 800 kilometers across and 15 kilometers tall, more or less directly under the plume that feeds Iceland’s volcanoes

... More powerful computers will allow her to use more of the high-frequency content of earthquake waves, the part that is best at illuminating shallow structures like ULVZs.
A molten puddle deep under Iceland may reveal where volcanoes get their lava | Science Mag

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