Rocks: Igneous Rocks & Sedimentary Rocks


The Earth’s crust, the outermost layer of our planet, is a diverse and fascinating geological realm, composed primarily of rocks. A rock, in geological terms, refers to any naturally occurring solid aggregate or mineral material that forms an essential part of the Earth’s lithosphere. These rocks are the building blocks of the planet’s crust and hold crucial information about its history and evolution.

A rock can be a singular mineral or a combination of different mineral elements. For instance, silica, a fundamental mineral, consists of silicon and oxygen atoms. When combined in various ways with other minerals, rocks of varying compositions and properties are formed.

The differences between rocks are vast, encompassing factors such as texture, structure, color, permeability, mode of occurrence, and their degree of resistance to denudation, which is the process of erosion and weathering that sculpts the Earth’s surface over time.

Let’s explore the three major types of rocks found in the Earth’s crust:

1. Igneous Rocks: These rocks originate from the cooling and solidification of molten magma or lava. They can be further classified into intrusive igneous rocks (plutonic) and extrusive igneous rocks (volcanic), based on where the cooling and solidification occurred. Intrusive igneous rocks, like granite, form beneath the Earth’s surface, and their slower cooling results in coarse-grained textures. Extrusive igneous rocks, such as basalt, cool rapidly at the surface, leading to fine-grained textures. Igneous rocks often exhibit a range of colors, from dark and dense to light and porous, depending on their mineral content.

2. Sedimentary Rocks: These rocks are formed through the accumulation and cementation of sediments, which can be derived from the breakdown of other rocks, organic materials, or chemical precipitation. Over time, these sediments undergo compaction and lithification to create sedimentary rocks. Examples include sandstone, limestone, and shale. Sedimentary rocks often display distinct layers (strata) and can contain fossils, providing valuable insights into past environments and life forms.

3. Metamorphic Rocks: Metamorphism occurs when pre-existing rocks undergo profound changes due to intense heat, pressure, or chemically active fluids without completely melting. This process alters the mineral composition and structure of the rock. Common metamorphic rocks include marble (derived from limestone) and slate (derived from shale). Metamorphic rocks can have striking textures, like foliation, where minerals align into parallel layers due to pressure during formation.

Each of these rock types reflects the complex geological history and processes that have shaped our planet over millions of years. By studying rocks and their characteristics, geologists can unravel the Earth’s past, gain insights into its present, and even make predictions about its future. Understanding the intricate interplay of these rocks is fundamental to comprehending the dynamic and ever-changing nature of the Earth’s crust.

Characteristics of Igneous rocks

Igneous rocks, one of the three major types of rocks found in the Earth’s crust, exhibit distinctive characteristics that set them apart from sedimentary and metamorphic rocks. These features arise from their formation through the cooling and solidification of magma or lava. Here are some key characteristics of igneous rocks:

1. Crystalline Structure: Igneous rocks are known for their crystalline structure, which means they contain mineral crystals that have grown and arranged in an ordered pattern over time. The size and arrangement of these crystals depend on the cooling rate of the magma or lava. Slow cooling results in larger crystals, forming coarse-grained igneous rocks like granite, while rapid cooling produces fine-grained rocks like basalt with smaller crystals.

2. Absence of Layering: Unlike sedimentary rocks, igneous rocks do not have distinct layers or strata. They form from the cooling and solidification of molten material without undergoing any significant compaction or deposition of sediments. Instead, the mineral crystals in igneous rocks are interlocked in a random or uniform manner, depending on the specific type of igneous rock.

3. Lack of Fossils: Igneous rocks do not contain fossils. Fossils are typically found in sedimentary rocks, as they are formed from the accumulation of organic materials or the remains of plants and animals. Since igneous rocks originate from the cooling of molten material, any organic matter present would be incinerated during the process, leaving no traces of fossils.

4. Hardness and Imperviousness: Igneous rocks are generally hard and durable due to their mineral composition and the cooling process that solidifies them. The minerals in igneous rocks often have strong bonds, making the rocks resistant to physical weathering. As a result, igneous formations can endure harsh environmental conditions and are often found in rugged landscapes.

5. Resistance to Erosion and Weathering: Igneous rocks are renowned for their resistance to erosion and the forces of weathering, which is the breaking down of rocks into smaller particles due to exposure to natural elements like water, wind, and ice. Their hardness, lack of layering, and crystalline structure contribute to their longevity and ability to withstand the relentless forces of nature over long periods of time.

Due to their varied mineral compositions, textures, and origins, igneous rocks can be found in diverse geological settings worldwide. From towering volcanic peaks to vast expanses of volcanic plateaus, these rocks provide valuable insights into the Earth’s history and the dynamic processes that have shaped our planet for millions of years. Their unique characteristics make them a crucial component of the geological puzzle that geologists study to better understand the complex and ever-changing nature of the Earth’s crust.

Mode of Formation

They are formed by the cooling and solidification of molten magma, ejected from beneath the earth’s crust. As the magma moves towards the surface, it comes in contact with lower temperatures, hence cooling and solidifying to form igneous rock.

There are two major types of Igneous rock. These are:

  • Plutonic (Intrusive) Igneous rocks: These are Igneous rocks formed inside the earth’s crust as a result of the cooling and solidification of the molten magma inside the crust i.e. before it gets to the surface. This results to rocks with large crystals. Examples include granite, gabbro, and diorite.
  • Volcanic (Extrusive) Igneous rocks: These rocks form outside the earth’s crust due to the cooling and solidification of molten magma on the surface of the earth. This results to rocks with small crystals. Examples include basalt.

Characteristics of Sedimentary Rocks

Sedimentary rocks, one of the three main types of rocks found in the Earth’s crust, possess a distinct set of characteristics that make them unique and easily recognizable. These rocks are formed through the process of sedimentation, which involves the accumulation and consolidation of various materials over time. Here are the key characteristics of sedimentary rocks:

1. Layered Structure: One of the most prominent features of sedimentary rocks is their occurrence in layers or strata. These layers are a result of successive deposition of sediments, such as sand, mud, clay, or organic matter, over extended periods. Each layer represents a snapshot of the environmental conditions prevailing during its formation, making sedimentary rocks valuable archives of Earth’s history.

2. Varied Texture: Sedimentary rocks can exhibit a wide range of textures, depending on the size and shape of the constituent particles and the nature of the sedimentation process. Coarse-grained sedimentary rocks, like conglomerate, contain larger particles, whereas fine-grained ones, such as shale, consist of tiny particles like clay and silt. The texture can also vary from soft and crumbly, as seen in some sandstones, to hard and compacted, like in limestone.

3. Non-Crystalline Nature: Unlike igneous rocks, sedimentary rocks are not crystalline. Instead of forming directly from molten material, they are composed of sediments that have undergone compaction and cementation. These sediments often retain their original characteristics, such as rounded edges or organic remains, providing valuable clues about their origin and environmental history.

4. Fossil Content: Sedimentary rocks have a unique capacity to preserve fossils, making them crucial resources for paleontologists and geologists. The layers can encapsulate the remains of ancient plants, animals, and other organisms, providing insights into past life forms and the conditions under which they lived. Fossils found in sedimentary rocks help reconstruct the Earth’s biodiversity and its evolution over geological time scales.

5. Erosion Susceptibility: Unlike igneous and metamorphic rocks, sedimentary rocks are not resistant to erosion. Over time, the forces of weathering and external agents like wind, water, and ice can wear away the softer layers of sedimentary rocks, leading to the formation of valleys, canyons, and sedimentary basins. This continuous process of erosion plays a vital role in shaping landscapes and redistributing sediments across the Earth’s surface.

Sedimentary rocks offer valuable insights into Earth’s past environments, climate changes, and geological processes. Their unique characteristics, such as layering, diverse textures, fossil preservation, and susceptibility to erosion, contribute to their importance as records of our planet’s geological history. By studying these rocks, geologists can unravel the mysteries of the past, trace the evolution of life on Earth, and better understand the dynamic forces that have shaped the planet’s surface over millions of years.

Mode of Formation

Sedimentary rocks are formed from sediments deposited by water, wind, or ice. These sediments are deposited in layers or strata one on top of another, and after a long period of time, they become hardened by compression to form Sedimentary rocks. Sedimentary rocks are therefore said to be stratified or layered.

There are three main types of Sedimentary rocks. This classification is based on their mode of formation.

  • Mechanically formed Sedimentary rocks: These are formed from sediments of other rocks that have accumulated over a period of time. Examples include sandstone, breccia, shale, clay, and conglomerates.
  • Organically formed Sedimentary rocks: These are rocks formed from the remains of living organisms eg plants and animals. Those formed from plant remains are known as CARBONACEOUS Rocks e.g. Coal, Peat, Lignite, Petroleum, etc while those formed from animal remains are known as CALCAREOUS Rocks eg Limestone, and Chalk.
  • Chemically formed Sedimentary rocks: These are rocks precipitated chemically from solutions. Examples include Potash, Sodium Chloride(Common Salt), Nitrate, Gypsum, and Dolomite.

Read also:

The Earth: Internal Structure, Crust, Mantle

Structure of The Earth

Distances and Local Time

Latitude & Longitude

The Earth

Leave a Comment

Your email address will not be published. Required fields are marked *

Get Fully Funded Scholarships

Free Visa, Free Scholarship Abroad

           Click Here to Apply