EARTH: Rock Types, Rock Cycle and Internal Structure and Discontinuties of the Earth

A natural solid mass or compilation of minerals or mineraloid materials is referred to as a rock. It is divided into groups based on the minerals it contains, its chemical form, and how it is produced. One or more minerals constitute rocks. Minerals are inorganic, naturally occurring substances that have a specific chemical form and crystal structure. The most prevalent minerals in rocks are amphibole, feldspar, quartz, and mica.

 

There are three main types of rocks: igneous, sedimentary, and metamorphic.

 

1) IGNEOUS ROCKS: Igneous rocks are formed from the solidification of molten material, either magma (below the Earth's surface) or lava (on the Earth's surface). The cooling and crystallization of molten material lead to the formation of igneous rocks. They can be further classified into two categories:

Ø  Intrusive Igneous Rocks: These rocks form when magma cools and solidifies below the Earth's surface. Due to slower cooling, larger crystals have time to develop. Examples include granite and diorite.

Ø  Extrusive Igneous Rocks: These rocks are created when lava cools and solidifies rapidly on the Earth's surface. The faster-cooling results in smaller crystals or even glassy textures. Examples include basalt and pumice.

 

2) SEDIMENTARY ROCKS: Sedimentary rocks are formed through the accumulation, compaction, and cementation of sediments (particles) derived from pre-existing rocks. These sediments can come from weathering and erosion of igneous, sedimentary, or metamorphic rocks. There are three main types of sedimentary rocks:

Ø  Clastic Sedimentary Rocks: These rocks are composed of fragments or clasts of other rocks. Examples include sandstone, shale, and conglomerate.

Ø  Chemical Sedimentary Rocks: These rocks form from the precipitation of minerals from water. Examples include limestone (from calcium carbonate) and rock salt (from halite).

Ø  Organic Sedimentary Rocks: These rocks are composed of organic material, such as plant remains. An example is coal.

 

3) METAMORPHIC ROCKS: Metamorphic rocks are formed from existing rocks (igneous, sedimentary, or other metamorphic rocks) that undergo changes due to heat, pressure, and chemically active fluids. These changes occur while the rock is still solid. Metamorphic rocks often exhibit distinct textures and mineral arrangements. Examples include marble (from limestone) and schist.

Rock-forming minerals (Source: Geology page)

The three types of rocks are all interconnected in the rock cycle. The rock cycle is a process that describes the continuous formation, transformation, and destruction of rocks. The rock cycle is driven by the Earth's internal and external forces.

 

THE ROCK CYCLE 

 

The rock cycle is a model that describes the continuous formation, transformation, and destruction of rocks. It is a basic concept in geology that helps us to understand the relationships between the three main types of rocks: igneous, sedimentary, and metamorphic. The rock cycle is driven by the Earth's internal and external forces. Internal forces include the heat and pressure generated by the Earth's core and mantle. External forces include the wind, water, ice, and gravity.

 

THE PROCESS OF THE ROCK CYCLE               

 

The steps of the Rock cycle are simply as follows –

 

1. Igneous Rock Formation
2. Weathering and Erosion
3. Sediment Deposition
4. Sedimentary Rock Formation
5. Metamorphism
6. Metamorphic Rock Formation
7. Melting and Magma Formation
8. Solidification and Igneous Rock Formation

FIGURE: Geological Rock Cycle (Credit: Lutgens and Tarbuck)

EXPLANATION

Indeed, the rock cycle can be simplified into the following sequential steps:

 

1. Igneous Rock Formation: Formation begins with the cooling and solidification of magma, either below the Earth's surface or at the surface as lava.
2. Weathering and Erosion: Existing rocks are subjected to weathering, breaking them into smaller particles through processes like wind, water, and temperature changes. Erosion carries these particles away.
3. Sediment Deposition: Particles carried by erosion settle in layers, often at the bottoms of bodies of water, forming sediment layers.
4. Sedimentary Rock Formation: The accumulated sediment layers become compacted and cemented over time, transforming into sedimentary rocks.
5. Metamorphism: Sedimentary or igneous rocks that are buried deep within the Earth's crust undergo changes due to increased heat and pressure.
6. Metamorphic Rock Formation: The rocks undergoing metamorphism experience re-crystallization and mineral rearrangement, leading to the formation of metamorphic rocks.
7. Melting and Magma Formation: Rocks can melt when subjected to extreme heat and pressure, creating molten material known as magma.
8. Solidification and Igneous Rock Formation: The magma cools and solidifies, either underground as intrusive igneous rocks or on the surface as extrusive igneous rocks, thus completing the cycle.

 

This cyclical process highlights how various geological forces interact to continuously transform rocks from one type to another over millions of years. The rock cycle is a way of understanding how the Earth's crust is constantly changing. It helps us to understand the relationships between the different types of rocks and the processes that form them. The rock cycle is also important for understanding the history of the Earth and the resources that we can find in the rocks.

 

Internal Structure and Composition of the Earth

 

Introduction: The Earth's interior structure is composed of several distinct layers, each with its own physical and compositional characteristics. These layers are divided based on their properties and behavior, which are inferred from seismic waves, mineralogy, and geophysical observations. The Earth can be divided into three main layers: the crust, mantle, and core.

Artistic depiction of the Earth's structure (Image via Victoria Museum)

EARTH’S INTERNAL STRUCTURE

Even though researchers have learned some interesting things, the deepest that scientists have managed to drill into the Earth is only about 12 kilometers (around 7.5 miles). However, the Earth's center is thousands of kilometers away. This means that scientists can't directly study the Earth's interior. Instead, they have to use clues and information from other sources to imagine what it's like inside. A lot of what we know about how the Earth is put together comes from watching how seismic waves travel through the Earth after earthquakes happen. Think of these waves like ripples in a pond. Cooler parts of the Earth make these waves move faster because they are more solid, while hotter areas slow them down. Parts that are heavy absorb these waves, and different changes in heaviness make the waves bounce back or change direction. These details have helped scientists figure out what the Earth's insides might look like and create maps of its basic structure, sort of like how explorers in Jules Verne's stories uncover new places. The next part of this discussion will explain the different layers that make up the Earth's structure.

 

THE MAJOR LAYERS 

 

1. CRUST:

 

Depth: The continental crust varies in thickness but generally ranges from about 30 to 50 kilometers (18 to 31 miles) beneath continents. The oceanic crust is thinner, typically around 5 to 10 kilometers (3 to 6 miles) beneath oceans.

Structure: The crust is the Earth's outermost layer and is divided into two types: continental and oceanic. It consists of rocks, minerals, and sediments that make up the Earth's surface.

Density: The average density of the continental crust is about 2.7 to 2.8 g/cm³.

The oceanic crust has a higher density, averaging around 2.9 to 3.0 g/cm³ due to the presence of denser minerals like pyroxene and olivine.

 

Mineral Composition: 

Continental Crust: Composed mainly of feldspar minerals (plagioclase and orthoclase), quartz, mica (biotite and muscovite), and various accessory minerals. As a result, continental crust is often called SIAL, short for silica and aluminium.

Oceanic Crust: Composed primarily of pyroxene minerals (mainly agate), plagioclase feldspar, and olivine, thus, oceanic crust is often called SIMA, which is short for silica and magnesium.

 

2. MANTLE:

 

Depth: The upper mantle extends from the base of the crust to a depth of about 670 kilometers (416 miles). The lower mantle extends from about 670 kilometers (416 miles) to approximately 2,900 kilometers (1,800 miles).

Structure: The mantle is solid but can flow slowly over geological timescales due to its semi-plastic behavior in the asthenosphere. It is divided into the upper mantle (which includes the asthenosphere) and the lower mantle.

Density: The density of the upper mantle ranges from about 3.3 to 4.4 g/cm³.

The density of the lower mantle increases with depth and ranges from about 4.4 to 5.6 g/cm³

 

Mineral Composition: 

Upper Mantle: Dominated by minerals like olivine, pyroxene, and garnet.

Lower Mantle: Likely contains high-pressure forms of minerals like perovskite (MgSiO₃) and magnesiowüstite (MgO + FeO).

 

3. CORE:

 

I) Outer Core:

 

Depth: The outer core extends from about 2,900 kilometers (1,800 miles) to around 5,150 kilometers (3,200 miles).

Structure: The outer core is composed of liquid iron and nickel due to the high temperatures and pressures at these depths.

Composition: Primarily composed of liquid iron and nickel, with smaller amounts of sulfur and other light elements.

Density: The density of the outer core is estimated to be around 9.9 to 12.2 g/cm³.

 

II) Inner Core:

 

Depth: The inner core extends from around 5,150 kilometers (3,200 miles) to the Earth's center at approximately 6,371 kilometers (3,959 miles).

Structure: The inner core is solid due to the immense pressure, despite the extremely high temperatures.

Composition: Primarily composed of solid iron and nickel, possibly with some alloying elements like sulphur, silicon, and carbon.

Density: The density of the inner core is estimated to be around 12.8 to 13.1 g/cm³.

Each layer's depth, structure, mineral composition, and density contribute to its unique behaviour and role in the Earth's overall dynamics. Understanding these characteristics helps us comprehend the planet's geological processes, heat distribution, seismic activity, and magnetic field generation.

Earth's Interior Structure (Credit: Alan F. Arbogast)

TABLE: Interior structure of the Earth in short

Layer	Depth Range	Structure	Mineral Composition	Density (g/cm³)
Crust
Continental	Variable (30-50 km)	Solid, outermost layer	Feldspar (plagioclase, orthoclase), quartz, mica	2.7 - 2.8
Oceanic	5-10 km	Solid, outermost layer	Pyroxene (agate), plagioclase feldspar, olivine	2.9 - 3.0
Mantle
Upper	0-670 km	Solid, semi-plastic nature (Asthenosphere)	Olivine, pyroxene, garnet	3.3 - 4.4
Lower	670-2,900 km	Solid, rigid behaviour	High-pressure forms of minerals (e.g., spinel)	4.4 - 5.6
Outer Core
Outer	2,900-5,150 km	Liquid	Iron, nickel, sulphur, and other light elements	9.9 - 12.2
Inner Core
Inner	5,150-6,371 km	Solid, despite high temperatures	Solid iron, nickel, and possibly alloying elements	12.8 - 13.1

  

The Earth's interior structure plays a crucial role in shaping the planet's geological processes, magnetic field generation, and overall behaviour. Seismic studies and other geophysical observations continue to provide insights into the properties and dynamics of these layers.

 

The Discontinuity between the Layers

 

Discontinuities within the Earth's internal structure are boundaries where abrupt changes occur in properties such as density, composition, and seismic wave behaviour. These boundaries provide valuable insights into the transitions between different layers of the Earth. There are several significant discontinuities within the Earth's interior:

1. Mohorovicic Discontinuity (Moho):

Location: Between the Earth's crust and the underlying mantle.

Depth: On average, around 5 to 10 kilometers beneath ocean floors and 20 to 70 kilometers beneath continents.

Significance: Marks the boundary between the solid, outermost layer (crust) and the solid but more ductile mantle beneath.

 

2. Conrad discontinuity:

This is a boundary within the crust that separates the upper crust from the lower crust. It is located about 10-35 kilometers (6-22 miles) below the surface of the Earth. The Conrad discontinuity is marked by a decrease in the speed of seismic waves.

 

3. Repetti discontinuity:

This is a boundary within the mantle that separates the upper mantle from the lower mantle. It is located about 410-520 kilometers (255-323 miles) below the surface of the Earth. The Repetti discontinuity is marked by a sudden increase in the speed of seismic waves.

 

4. Gutenberg Discontinuity:

Location: Between the Earth's mantle and outer core.

Depth: Approximately 2,900 kilometers below the Earth's surface.

Significance: Marks the transition from the solid rock of the mantle to the liquid outer core. Seismic S-waves cannot pass through the outer core, indicating its liquid state.

 

5. Lehmann Discontinuity:

Location: Between the Earth's outer core and inner core.

Depth: Approximately 5,150 kilometers below the Earth's surface.

Significance: Represents the boundary between the liquid outer core and the solid inner core. The Lehmann Discontinuity causes the reflection of seismic waves.

 

These discontinuities are crucial markers that help scientists understand the Earth's internal structure, its layer transitions, and the unique characteristics of each layer. The study of seismic waves and their behaviour as they pass through these boundaries has provided a wealth of information about the Earth's composition, state, and dynamics.