Basalt Tetrahedron | GATE/NET IIT-JAM | GeologyConcepts.com

Summary

In this video, the speaker explains the basalt tetrahedron, a tool for classifying basalt based on its mineral composition. Basalt, a fine-grained volcanic rock, is mainly composed of calcic plagioclase feldspar and pyroxene. The video covers how basalt forms through partial melting of mantle peridotite, with variations in composition leading to types like tholeiitic and alkaline basalts. The tetrahedron diagram illustrates different basalt categories based on silica saturation, such as quartz-thiolite, olivine-thiolite, and alkali basalts. Additionally, the speaker introduces the thermal divide concept, which differentiates alkaline and sub-alkaline basalts, and promotes online IIT-JEE classes.

Takeaways

• 😀 Basalt is a fine-grained volcanic rock primarily found in the oceanic crust and formed by the partial melting of peridotite in the Earth's mantle.
• 😀 The basalt tetrahedron model classifies basalts based on four essential minerals: olivine, quartz, plagioclase feldspar, and clinopyroxene.
• 😀 Basalt is composed of **calcic plagioclase feldspar** and **pyroxene**, with **quartz** being either absent or in small quantities.
• 😀 Tholeiitic basalt is the most common form of basalt found on Earth's oceanic crust, with **tholeiitic** being the primary type at the surface.
• 😀 Basalts are classified into different types: **tholeiitic**, **alkaline**, and **sub-alkaline** basalts, depending on their mineral content.
• 😀 The **basalt tetrahedron** is divided into three main regions: **quartz-saturated**, **olivine-tholeiite**, and **silica-under-saturated**.
• 😀 The **quartz-saturated region** of the tetrahedron contains basalts that are silica-rich, primarily composed of **clinopyroxene**, **orthopyroxene**, and **quartz**.
• 😀 The **olivine-tholeiite region** includes basalt that is silica-saturated, with olivine being a key mineral present.
• 😀 The **silica-under-saturated region** features basalts like **alkaline basalts** and **nephilinites**, where quartz is minimal or absent.
• 😀 The **thermal divide** in the basalt tetrahedron separates **alkaline** and **tholeiitic** basalt series, and this divide is observed only at low pressures.
• 😀 The video also promotes free demo classes for IIT JEE preparation, encouraging viewers to register via the provided link.

Q & A

What is basalt, and where is it commonly found on Earth?

Basalt is a fine-grained volcanic rock primarily found in the oceanic crust, especially in the upper layers. It forms from the partial melting of peridotite or mantle rocks.

What are the main minerals that make up basalt?

Basalt is mainly composed of calcic plagioclase feldspar, pyroxene (usually clinopyroxene), and occasionally quartz. The exact mineral composition can vary.

What are xenoliths, and why are they important in the study of basalt?

Xenoliths are fragments of rock from deeper parts of the Earth's crust or mantle that are trapped within basalt. They provide evidence of deep crustal or mantle processes.

How does basalt form on the Earth's surface?

Basalt forms as a result of partial melting of mantle rocks, such as peridotite. When the molten material rises to the Earth's surface, it solidifies into basalt.

What is the relationship between basalt and gabbro?

Basalt is the volcanic, fine-grained equivalent of gabbro, which is a coarse-grained, intrusive rock. Both share similar mineral compositions.

What is tholeiitic basalt, and where is it found?

Tholeiitic basalt is a type of basalt that is more depleted in minerals and is commonly found on the Earth's surface, especially in oceanic crust or shallow subsurface areas.

What are the two categories of basalt based on their mineral composition?

The two categories of basalt are tholeiitic (found in the shallow subsurface) and alkaline (found in deeper regions). Tholeiitic basalt is silica-depleted, while alkaline basalt is silica-enriched.

What is the significance of the basalt tetrahedron in classification?

The basalt tetrahedron is a graphical representation used to classify basalt based on four key minerals: nephilim, olivine, quartz, and clinopyroxene. It shows the relationships and divides basalts into different types, such as silica-saturated and silica-undersaturated basalts.

What does the critical plane of silica saturation in the basalt tetrahedron represent?

The critical plane of silica saturation separates the silica-rich region (quartz tholite) from the silica-undersaturated region. Basalts to the right of this plane are quartz-rich, while those to the left are silica-undersaturated.

What is the role of nephilim in the basalt tetrahedron classification?

Nephilim is a key mineral in the basalt tetrahedron that represents an alkali or silica-undersaturated region. It is typically found in basalts with high alkaline content and is not naturally associated with quartz.