Glossary of terms - U

ultramafic

Igneous rocks with a very low silica content, generally 18% or more magnesium, high iron, low potassium, and are composed of usually greater than 90% mafic minerals (dark colored, high magnesium and iron content). The mantle of the Earth is composed of ultramafic rock.

umbel

Branches or rays arising from a common point, about equal length or entending to the same plane, resembling framework of an umbrella.

undulate

Having a wavy or wave-like appearance or form.

unipinnate

solitary compound leaf with a row of leaflets arranged along each side of a common rachis.

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