The internal structure of the Earth consists of three main parts: the crust, mantle and core. The division between crust and mantle is known as the Moho, and marks a significant change in chemical composition. Much effort has focused on trying to understand the region around this boundary — the so-called Moho transition zone — to work out how the Earth's lower crust was created. All of the data point to the existence of gabbroic-melt accumulations in this region (gabbro is the subsurface equivalent of basalt lava). These structures had never been imaged, but in this week's issue of Nature, Mladen Nedimović and colleagues1 reveal the first detailed snapshots.

By mapping the propagation of elastic waves through the Earth's interior, and noting abrupt changes in seismic velocities at specific depths, subdivisions of the planet's structure can be identified. Nedimović et al. studied seismic data collected along the Juan de Fuca ridge, a region of volcanic activity where oceanic crust is forming, in the northeast Pacific, off the west coast of Washington and Oregon states. In these data, the Moho discontinuity appears primarily as a single reflector, although the strength of the reflection varies; in places, the reflection is strikingly sharp and strong.

What do the reflection images reveal? Images with a single reflection imply a sharp boundary between layers of significantly different physical properties. On the other hand, areas with weak or multiple Moho reflections might be indicative of a broad Moho transition zone with multiple layers of gabbro lenses. By combining such seismic profiles with other geophysical investigations, the authors infer the existence of magma bodies or 'melt lenses' beneath portions of the Moho. These magma chambers have a maximum diameter of several kilometres and can be up to 200 metres thick.

It is the first time that these frozen subcrustal magma structures have been 'seen'. Their existence lends support to the notion that the lower crust is formed from different magma chambers than the upper crust.