This morning, in that !terrible! right-wing rag, the Daily Mail, I was intrigued to read this:

How Hawaii and Iceland were born: Stunning 'CT scans' of the Earth's interior reveal vast plumes under the ocean feeding volcanic hotspots

• Connects plumes of hot rock rising with surface hotspots 
• Hotspots go on to generate volcanic island chains like Hawaii and Iceland

which featured wonderful pictures like this one, though they left off the scale bar at the left.

Looking to the source, I found a press release CT scan of Earth links mantle plumes with volcanic hotspots; Simulations Run at NERSC (University of California, Berkeley) Show How Seismic Waves Travel Through Mantle

So that, if I read correctly,  d ln Vs in the picture above refers to the difference in the natural logarithm of the velocity of shear waves through the magma, though the scale should range from –2% to +2%.  Without the % sign it would imply that our Earth is in a funny old state indeed.

We are familiar with volcanoes and earthquakes occurring where tectonic plates are grinding against each other, especially round the Pacific “ring of fire”.  But here is an explanation for volcanic islands that appear in the middle of the ocean, such as Hawaii and Tahiti in the Pacific, and Iceland and Las Canarias in the Atlantic.

On further picture, from an article published two years ago New model of Earth's interior reveals clues to hotspot volcanoes which to my mind at least illustrates those plumes of hot rock rising.

The second and third linked articles should furnish enough detail in the first instance for an interested reader.