Origin of Igneous Rocks

Summary

This script delves into the formation of rocks, focusing on igneous rocks and their crystallization from magma. It explains two primary melting mechanisms: decompression and flux melting, which are key to volcanic activity. The crystallization sequence, as outlined by Bowen's Reaction Series, reveals how the chemistry of magma influences the mineral composition of igneous rocks. The process of differentiation is highlighted, showing how magma evolves as it cools, with examples of how it affects volcanic eruptions in different geological settings.

Takeaways

• 🌋 Minerals and rocks are formed in response to environmental factors such as pressure, temperature, and the chemical composition of the starting material.
• 🔥 Igneous rocks are the first type of rocks to appear on Earth and are formed from the crystallization of melted rock, known as magma when underground and lava when erupted.
• 🌎 Magma originates from the mantle, which can melt and move towards the surface under certain conditions, such as decompression melting and flux melting.
• 📉 Decompression melting occurs when mantle rock ascends and experiences lower pressure, leading to melting, which is responsible for volcanism at mid-ocean ridges and hotspots.
• 💧 Flux melting is induced by the addition of water to mantle rock, lowering its melting point and causing melting, which is common at subduction zones.
• 📊 Bowen's Reaction Series explains the crystallization sequence in a cooling magma, starting with olivine and ending with quartz, muscovite, and alkali feldspar.
• 🔬 The crystallization process in magma leads to a change in the composition of the remaining melt, enriching it in silica, calcium, alkalis, and aluminum as it cools.
• 📚 Differentiation describes how magma changes as it cools, with the amount of differentiation related to the thickness of the crust it moves through.
• 🌍 The location of volcanic eruptions and the type of magma they produce are influenced by the thickness of the crust, with thicker continental crust leading to more differentiated, felsic magmas.
• 🏔️ The comparison between Hawaii and Yellowstone illustrates the impact of crust thickness on volcanic activity, with Hawaii's thinner oceanic crust producing less viscous, mafic lava.
• 📈 The crystallization of minerals from magma is influenced by the relative melting points and chemical compositions of the minerals, with magnesium-rich endmembers crystallizing before iron-rich ones.

Q & A

What are the three types of rocks mentioned in the script?

The three types of rocks mentioned are igneous, sedimentary, and metamorphic, which differ in their mechanism of formation.

What is the difference between magma and lava according to the script?

Magma is molten rock when it is underground, while lava is the term used for molten rock when it reaches the surface of the Earth.

Why doesn't lava erupt everywhere on Earth?

Lava doesn't erupt everywhere because under normal conditions, the mantle is solid and cannot melt; volcanoes are an exception due to specific melting processes.

What are the two processes that can cause the mantle to melt in large quantities?

The two processes are decompression melting and flux melting.

How does decompression melting occur?

Decompression melting occurs when mantle rock moves upward due to convection and experiences lower pressure, which leads to melting, especially above mid-ocean ridges and hotspots.

What is the equation used to calculate pressure in the context of the script?

The equation used to calculate pressure is P = ρgz, where ρ is density, g is acceleration due to gravity, and z is the depth below Earth's surface.

What is Bowen's Reaction Series and why is it significant?

Bowen's Reaction Series is a model that describes the crystallization order of minerals in a cooling magma. It's significant because it explains the sequence in which minerals crystallize and how the composition of the remaining melt changes.

How does the crystallization of olivine affect the composition of the remaining magma?

As olivine crystallizes, it depletes the magma of MgO and enriches it in silica, calcium, alkalis, and aluminum because olivine has a smaller fraction of silica compared to the original melt.

What is the role of solid solution in the crystallization process?

Solid solution complicates the crystallization process by causing minerals to crystallize in a sequence where magnesium-rich endmembers form before iron-rich ones, and calcium-rich before sodium-rich ones.

Why do some volcanoes erupt with more differentiated magmas?

Volcanoes erupt with more differentiated magmas if the magma has moved through a thicker crust, which allows for more extensive cooling and crystallization, as seen in the comparison between Hawaii and Yellowstone.

What is the difference between the magma erupted at the Hawaii hotspot and Yellowstone hotspot?

The Hawaii hotspot erupts basaltic, mafic lava at higher temperatures, while Yellowstone erupts cooler, more viscous, felsic magma due to its location beneath thick continental crust.