Basalt: The Bedrock of the Ocean

Most common rock in Earth's crust (by surface area). It covers ca. 70% of the surface.

Geochemical

• Mafic: Rich in Magnesium and Iron, poor in Silica.
• Extrusive: It forms from lava that erupted onto the surface (or seafloor) and cooled quickly.
• Fine-Grained (Aphanitic): The crystals are too small to see with the naked eye because they didn't have time to grow large.

Types

MORB (Mid-Ocean Ridge Basalt):

• Setting: Divergent boundaries (e.g. the Mid-Atlantic Ridge)
• Source: The Depleted Upper Mantle. This mantle has already been melted before, so its drained of incompatible elements.
• Chemical Signature: Depleted in Light Rare Earth Elements (LREE)
• This is the "standard" basalt (60% Coverage of the Earth)

OIB (Ocean Island Basalt):

• Setting: Hotspots /mantle Plumes
• Source: A Deep Mantle Plume, that is "enriched" or primitive.
• Chemical Signature: Enriched in LREE and incompatibles.
• This proves that the deep mantle is chemically different from the upper mantle.

Question and Answers

What is the definition of Basalt in terms of composition and grain size?::Mafic (Mg/Fe rich) and Fine-grained (Extrusive/Volcanic).

What does the acronym MORB stand for?::Mid-Ocean Ridge Basalt.

What does the acronym OIB stand for?::Ocean Island Basalt.

Geochemically, what is the primary difference between MORB and OIB regarding "Incompatible Elements"?
?
MORB is Depleted in incompatible elements (comes from Depleted Mantle).
OIB is Enriched in incompatible elements (comes from Deep Mantle Plumes).