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.

Outlines

00:00

🌋 Formation of Igneous Rocks and Mantle Melting

This paragraph introduces the concept of rocks and their formation from minerals influenced by environmental factors such as pressure, temperature, and chemistry. It delves into the specifics of igneous rocks, which are the first to appear on Earth and are formed from the crystallization of magma. The text explains the distinction between magma and lava and explores the processes of decompression melting and flux melting that cause the mantle to melt, leading to volcanic activity. Decompression melting is described as occurring above mid-ocean ridges and hotspots due to the upward movement of less dense, hot mantle material, which encounters lower pressure and melts. Flux melting, on the other hand, is induced by the addition of water to the mantle rock, particularly at subduction zones, where water released from the subducting lithosphere lowers the melting point of the mantle.

05:03

🔬 Bowen's Reaction Series and Igneous Rock Differentiation

The second paragraph discusses the diversity of igneous rocks despite their common origin from the chemically homogeneous mantle. It introduces Bowen's Reaction Series, developed by geologist N. L. Bowen in 1928, which outlines the sequence of mineral crystallization in a cooling magma. The sequence begins with olivine and ends with quartz and muscovite, with plagioclase present throughout. The crystallization process leads to the enrichment of silica and other elements in the remaining melt as denser minerals like olivine sink. This results in a differentiation of the magma, with the last minerals to crystallize being rich in silica and alkalis. The differentiation process is influenced by the thickness of the crust the magma must pass through, with more differentiated magmas found in areas of thick crust, such as continents. The paragraph contrasts the eruptions of Hawaii, which produces less differentiated, mafic lava, with Yellowstone, where more differentiated, felsic magma leads to explosive eruptions due to its high viscosity.

Keywords

💡Minerals

Minerals are naturally occurring, inorganic substances with a definite chemical composition and crystal structure. They are the building blocks of rocks. In the context of the video, minerals are discussed as they form in response to environmental forcing, such as pressure, temperature, and chemistry. The script mentions that the crystallization of magma and its chemistry are crucial in determining a rock's mineralogy, indicating the importance of minerals in the formation of rocks.

💡Rocks

Rocks are solid, naturally occurring substances composed of minerals or organic material. They are classified into three main types: igneous, sedimentary, and metamorphic, based on their formation process. The video script discusses the formation of rocks, particularly focusing on igneous rocks, which are the first type to appear on Earth after its formation and subsequent cooling.

💡Igneous Rocks

Igneous rocks are formed from the solidification of molten rock material, called magma when underground and lava when erupted onto the surface. The script explains that igneous rocks result from the cooling and crystallization of magma, which was once mantle rock that melted and moved toward the surface. This process is central to the video's discussion on the formation of Earth's early crust.

💡Magma

Magma is molten rock material found beneath the Earth's surface. It is a key component in the formation of igneous rocks. The script describes magma as originating from the mantle, which melts under certain conditions, such as decompression melting or flux melting, leading to volcanic activity. Magma's chemistry plays a significant role in determining the types of minerals that will crystallize as it cools.

💡Decompression Melting

Decompression melting is a process where the mantle melts due to a decrease in pressure, often associated with tectonic activity such as at mid-ocean ridges. The script explains that as mantle rock rises and experiences lower pressure, it melts without a change in temperature, leading to the formation of magma. This process is critical for the creation of new crust at divergent plate boundaries.

💡Flux Melting

Flux melting occurs when the addition of certain substances, like water, lowers the melting point of the mantle, causing it to melt. The script describes how water released from subducting oceanic lithosphere induces melting in the overlying mantle, leading to volcanic activity at subduction zones. This process is essential for understanding the formation of volcanic arcs.

💡Bowen's Reaction Series

Bowen's Reaction Series is a model that describes the sequence in which minerals crystallize from a cooling magma. The script mentions that the first mineral to crystallize is olivine, followed by pyroxene, amphibole, biotite, and finally potassium feldspar and quartz. This sequence is crucial for understanding the chemical evolution of magma and the composition of igneous rocks.

💡Differentiation

Differentiation refers to the process by which a magma changes or becomes different as it cools, leading to the formation of various minerals with distinct chemical compositions. The script explains that as magma cools, minerals with higher melting points crystallize first, leaving a residual melt that becomes enriched in other elements. This process results in the formation of rocks with varying compositions, such as mafic and felsic rocks.

💡Mafic Lava

Mafic lava is a type of lava that is rich in magnesium and iron, typically associated with basaltic composition. The script contrasts mafic lava with felsic magma, noting that Hawaii's volcanoes erupt mafic lava at higher temperatures, around 1300 degrees Celsius, compared to the cooler, more viscous felsic magma at Yellowstone. This distinction highlights the influence of crustal thickness on magma composition and volcanic activity.

💡Felsic Magma

Felsic magma is rich in silica, sodium, and potassium, and is typically more viscous than mafic magma. The script describes how felsic magma, such as that found at Yellowstone, is cooler and thicker, leading to explosive volcanic eruptions due to its high viscosity. This term is important for understanding the differences in volcanic behavior and the types of rocks produced in various geological settings.

Highlights

Minerals and rocks are formed in response to environmental factors such as pressure, temperature, and starting material chemistry.

Crystallization of magma and its chemistry are key in determining a rock's mineralogy.

There are three main types of rocks: igneous, sedimentary, and metamorphic, each with a distinct formation mechanism.

Igneous rocks were the first to appear on Earth, formed from the crystallization of melted rock.

Magma and lava are terms used for molten rock underground and on the surface, respectively.

Most igneous rocks originate from the mantle, which melts and moves toward the surface under certain conditions.

Decompression melting and flux melting are two processes that cause the mantle to melt in large quantities.

Decompression melting occurs as mantle rock ascends and experiences lower pressure, leading to melting.

Flux melting is induced by the addition of water to mantle rock, lowering its melting point and causing melting at subduction zones.

The crystallization sequence in cooling magma is influenced by the magma's chemistry and mineral melting points.

Bowen's Reaction Series outlines the crystallization order of minerals from a standard magma as it cools.

Differentiation describes the changes in magma composition as it cools, leading to the formation of various minerals.

The amount of differentiation is related to the thickness of the crust the magma moves through to reach the surface.

Cool, felsic magma from Yellowstone and mafic lava from Hawaii demonstrate the impact of crust thickness on volcanic eruptions.

Solid solution and the crystallization of magnesium-rich endmembers before iron-rich ones contribute to the complexity of rock formation.

The crystallization process and the resulting rock types are essential for understanding Earth's geological history and dynamics.

Transcripts

Now that we have a better understanding of  minerals, we are ready to discuss rocks.  

Minerals and rocks form in response to  environmental forcing, with important  

variables being pressure, temperature,  and chemistry of the starting material.  

Here, we will discuss the crystallization of magma  and the importance of its chemistry in determining  

a rock’s mineralogy. There are three types of  rocks: igneous, sedimentary, and metamorphic,  

and these differ in their mechanism of formation.  We will begin our discussion with igneous rocks,  

which were the first type of rocks to appear on  Earth after its formation and subsequent cooling. 

Igneous rocks are formed when a mass of melted  rock crystallizes. There are two terms geologists  

use for molten rock. When it is underground, it  is called magma, up here it is called lava. So,  

where does this magma come from? Most igneous  rocks were once mantle rock that, at some point,  

melted and moved toward the surface, where it  cooled and solidified. You may now be wondering  

why lava isn’t erupting everywhere on Earth. It  turns out that under normal conditions, the mantle  

is perfectly happy as a solid and cannot melt;  and volcanoes are an exception to the rule. So,  

what sort of process is going on here that allows  the mantle to melt? In fact, there are two ways  

to melt the mantle in large quantities:  decompression melting and flux melting.  

Let’s start with decompression melting. This is  exactly what it sounds like, melting the mantle  

by lowering its pressure. Decompression melting is  responsible for the volcanism above both mid-ocean  

ridges and hotspots. Beneath these environments,  hot mantle rock is moving upward due to mantle  

convection. Since hot material is less dense  than cold material, the weight of a column of hot  

material will be less than that of cold material.  Therefore, at a given depth, the pressure is lower  

beneath a mid-ocean ridge or hotspot than  it is beneath a regular piece of crust. 

Let’s follow an ascending piece of mantle rock as  it rises beneath a mid-ocean ridge. At a depth of  

about 30 kilometers, our piece of mantle has  a temperature of 1400°C. Its pressure can be  

calculated using the following equation: P = ρgz,  where ρ is its density, g is acceleration due to  

gravity, and z is the depth below Earth's surface.  Since ρ and g are more or less constant for this  

example, the main variable affecting the pressure  is the depth, which should be fairly intuitive:  

the more mass above a given area, the higher  the pressure will be. The two black arrows  

are representative of the downward pressure.  The upper right section shows a phase diagram  

corresponding to the state of our piece of mantle  in pressure-depth space. The green dashed curve is  

the average geotherm, which shows how temperature  changes with depth. The red solid line is the  

melting point of mantle rock. Notice how the two  curves are nearly parallel, this illustrates the  

requirement for abnormal conditions to get the  mantle to melt. As our mantle rock ascends,  

it experiences lower pressure, since there is  now less mass above it. But notice that the  

temperature did not change, it is still 1400°C.  This is an example of an adiabatic process,  

meaning heat has not been added or removed from  the system. Essentially, the rising motion of  

the mantle occurs at sufficient speed that we can  assume that heat is not lost to its surroundings  

during ascent. Therefore, the key to decompression  melting is the advection of hot material upward,  

to a lower-pressure environment. Finally, at a  pressure of 2 kilobars, the mantle is melting,  

with the purple dashed line signaling the onset of  melting. So long as there is upward-moving mantle,  

our magma factory will persist - this is  decompression melting. The vast majority of  

volcanism on Earth, both past and present,  is due to decompression partial melting. 

Flux melting involves the addition of water  to mantle rock, which lowers its melting  

point. This is responsible for the volcanism at  subduction zones. Here, the subducting oceanic  

lithosphere releases water into the mantle  above it, which induces melting. It is not  

actual liquid water that is transported to the  mantle, but hydroxide anions that are released  

from the high-pressure breakdown of hydrous  minerals, like serpentine, or Mg3Si2O5(OH)4.  

This chemically bound water is released  around the same depth everywhere on Earth,  

about 100 kilometers, which explains why volcanoes  occur in a line, or arc at subduction zones. 

Since most igneous rocks are derived from  the mantle, which is relatively chemically  

homogeneous, we might expect igneous rocks to also  be this way, but they aren’t. To understand this,  

we must first discuss the crystallization process,  and more importantly, crystallization sequence. In  

a cooling magma, crystallization order is  dependent upon the chemistry of the magma  

and the relative melting points of the stable  minerals. Since most magmas are derived from  

melting the same stuff, the mantle, we can ignore  the influence of chemistry for these purposes.  

In 1928, pioneering geologist N. L.  Bowen created an experimental model  

for the crystallization order of a standard  magma as it cools and chemically evolves. In  

Bowen’s Reaction Series, the first mineral to  crystallize is olivine, followed by pyroxene,  

then amphibole, then biotite, and finally  potassium feldspar, muscovite, and quartz.  

Plagioclase can exist at all stages of Bowen’s  Reaction Series, as it is the most abundant  

mineral in the crust. We may recall that the first  mineral to crystallize, olivine, an orthosilicate,  

has the smallest amount of silica of all the  silicate minerals, while the last group contains  

potassium feldspar and quartz, which are framework  silicates, the minerals with the most silica. This  

has important implications for the composition of  a cooling melt. Let’s consider an average basalt,  

having 50% SiO2, 15% Al2O3, 20% MgO, 10% CaO, and  5% alkalis. Now, let’s start to crystallize out  

forsterite, the magnesium endmember of olivine,  which is composed of 57% MgO and 43% SiO2. What  

is going to happen? Well, the olivine crystals,  which are more dense, will sink to the bottom  

of the magma chamber, becoming isolated from  the upward-moving magma. Since olivine has a  

smaller fraction of silica in comparison to the  melt, the melt will become enriched in silica  

as olivine crystallizes. In fact, the melt  will become enriched in everything but MgO,  

which gets depleted making olivine, since olivine  has a higher fraction of MgO than the melt. The  

result is a melt that becomes enriched in silica,  calcium, alkalis, and aluminum as it cools and  

moves upward. This process is then repeated for  pyroxene, amphibole, and biotite, which depletes  

the melt in everything but silica, aluminum, and  the alkalis, which become concentrated in the  

cooling melt. This results in the last minerals to  form being those with silicon, aluminum, sodium,  

and potassium in their chemistry, such as  quartz, muscovite, and alkali feldspar. 

Solid solution further complicates things. For a  general rule of thumb, magnesium-rich endmembers  

crystallize before iron-rich endmembers,  and calcium-rich endmembers form before  

sodium-rich endmembers. This explains the  right side of Bowen’s Reaction Series;  

since anorthite has a higher melting point than  albite, the first plagioclase that crystallizes  

will be calcium-rich and the last will be more  sodium-rich. The same thing happens with olivine,  

pyroxene, and amphibole; the first minerals  to form will be magnesium-rich, with crystals  

becoming progressively iron-enriched during  cooling. These processes are together referred  

to as differentiation, which describes how a  magma changes, or becomes different, as it cools. 

The amount of differentiation a magma  undergoes is generally related to the  

amount of crust that it had to move through to  make it near the surface. Therefore, the coolest  

and most differentiated magmas commonly erupt  in areas where the crust is relatively thick,  

such as on continents. This is easy to see when  comparing Hawaii and Yellowstone, as they are both  

hotspots. The volcanoes of Hawaii erupt basaltic,  or mafic lava at around 1300 degrees Celsius,  

whereas Yellowstone erupts cool, felsic magma  that is so thick and viscous, that it clogs up the  

volcano, causing extremely explosive eruptions.  What is the difference? The Yellowstone hotspot  

is beneath thick continental crust, while the  Hawaii hotspot is beneath thin, oceanic crust. We  

have more to discuss regarding igneous rocks, so  let’s move forward and learn more about them now.