A type of tholeiitic basalt (see tholeiite), erupted from mid-ocean-ridge constructive-plate margins; it is one of the most abundant of all rocks and covers much of the Earth’s surface. It is characterized by very low concentrations of K₂O and TiO₂; low iron, P₂O₅, Ba, Rb, Sr, Pb, Th, U, and Zr; and high CaO. When the concentration of each rare-earth element in the basalt is divided by its mean concentration in chondrite meteorites (a standard for comparison), this type of basalt shows a progressive lowering of the ratios for the light rare-earth elements (LREEs), compared to the ratios for the heavy rare-earth elements (HREEs). MORBs are said to be LREE depleted, a reflection of the chemically depleted nature of the mantle source regions from which they are derived. Since leaving their source region in the mantle, these basalts, often termed low-potassium tholeiites, have not been contaminated by passing through any continental crust and therefore retain the chemical signature of the mantle from which they were derived. MORBs thus provide an insight into the composition of the sub-oceanic mantle.