Basalt: Basic Igneous Rock

Basalt and Pillow Lava

• Plagioclase is the most common mineral in basalt.
• Basalt is a type of mafic volcanic rock.

Formation of Pillow Lava

• Pillow lava forms in a submarine environment, characterized by rapid cooling and quenching of lava flows.

Types of Basalt

• Alkali basalt forms equidimensional blocks up to about 3 ft in diameter.

Tectonic Setting of MORB

• Mid Oceanic Ridge basalt (MORB) is associated with a divergent tectonic setting.

Orogenic Volcanism and Plutonism

• The Andean subduction zone is a principal subduction zone associated with orogenic volcanism and plutonism.

Mid-Ocean Ridges

• Tensional stresses at mid-ocean ridges are caused by the upwelling of hot mantle rocks and the subsequent divergence of tectonic plates.
• The average rate of spreading at ocean ridges is about 3-5 cm/year.
• The mantle rocks at mid-ocean ridges are prevented from changing from solid to liquid state by the decrease in pressure and increase in temperature.

MORB Characteristics

• Tholeiitic magma is primarily formed at mid-ocean ridges.
• The main characteristic rock texture associated with mid-ocean ridge basalt (MORB) is fine-grained and aphanitic.

Oceanic Crust Formation

• Over 80% of the Earth's oceanic crust has been created by spreading at ocean ridges over the last 200 million years.

Faulting at Mid-Ocean Ridges

• Normal faulting is apparent at mid-ocean ridges due to the stretching of the newly forming ocean crust.

Mantle Rocks and Seafloor Spreading

• When deeper, hotter mantle rocks rise up and approach the surface at mid-ocean ridges, they undergo decompression melting, resulting in the formation of magma.
• Tholeiitic lava is primarily erupted in response to seafloor spreading along the Earth's mid-ocean ridge system.
• The thinning of the crust at mid-ocean ridges, allowing deeper, hotter mantle rocks to rise up and approach the surface, is caused by the divergent plate motion and the resulting extension of the crust.