Geological Structures and Rock Classifications
Folds: Bending of Rock Layers
Folds are wave-like undulations in rock strata. They are most common in sedimentary rocks.
Origin and Mechanism of Folds
Folds originate primarily due to Compressional Stress. When rocks are pushed together, they shorten and thicken.
Ductile Deformation: Folds form deep within the crust where high temperature and pressure allow rocks to bend without breaking.
Tectonic Forces: Major fold belts (like the Himalayas) originate from the collision of tectonic plates.
Classification of Folds
Anticline: An upward-curving fold (arch) where the oldest rocks are at the core.
Syncline: A downward-curving fold (trough) where the youngest rocks are at the center.
Symmetrical Fold: Both limbs dip at the same angle, and the axial plane is vertical.
Asymmetrical Fold: Limbs dip at different angles due to unequal pressure.
Overturned Fold: Extreme pressure pushes one limb over the other; both limbs dip in the same direction.
Recumbent Fold: The axial plane is horizontal, making the fold appear to be “lying down.”
Faults: Breaking of Rock Layers
A fault is a fracture in the Earth’s crust along which significant displacement (movement) has occurred.
Origin and Mechanism of Faults
Faults originate when stress exceeds the elastic limit of the rock, causing it to snap (brittle failure).
Tensional Stress: Pulling apart (stretching).
Compressional Stress: Pushing together (shortening).
Shear Stress: Sliding past each other.
Classification of Faults
Normal Fault: Caused by tensional stress. The hanging wall moves down relative to the footwall (common in rift valleys).
Reverse Fault: Caused by compressional stress. The hanging wall moves up relative to the footwall.
Thrust Fault: A low-angle reverse fault (dip < 45°).
Strike-Slip Fault: Caused by shear stress. Blocks move horizontally past each other.
Example: San Andreas Fault.
Oblique-Slip Fault: A combination of both vertical (dip-slip) and horizontal (strike-slip) movement.
Volcanoes: Definition and Origin
A volcano is a vent or fissure in the Earth’s crust through which molten rock (magma), volcanic gases, and pyroclastic debris erupt.
Origin: Most volcanoes occur at tectonic plate boundaries (divergent or convergent) or over mantle hot spots (e.g., Hawaii).
Magma vs. Lava: Magma is molten rock stored underground; it becomes lava once it breaks the surface.
Classification of Volcanoes
Volcanoes are typically classified in two ways for exams: by their eruptive history and by their physical form.
Based on Periodicity and Activity
Active: Erupts frequently or has erupted in recent history (e.g., Mt. Etna, Italy).
Dormant: “Sleeping” volcanoes that haven’t erupted for a long time but are expected to again (e.g., Mt. Fuji, Japan).
Extinct: “Dead” volcanoes with no magma supply; unlikely to ever erupt again (e.g., Mt. Kenya).
Earthquakes: Definition and Classification
Definition: An earthquake is the sudden shaking of the ground caused by the release of energy in the Earth’s lithosphere, which creates seismic waves. This usually occurs when rocks under stress suddenly break along a fault.
Classification of Earthquakes
Earthquakes are classified based on their origin (cause) and their focal depth.
Classification by Cause
Tectonic Earthquakes: The most common type (~90%). They occur when the Earth’s crustal plates move against each other, building up strain until the rocks snap.
Volcanic Earthquakes: Caused by the movement of magma or gas pressure within a volcano before or during an eruption.
Collapse Earthquakes: Occur in areas of intense mining where the roofs of underground mines or caverns collapse.
Explosion Earthquakes: Result from the detonation of nuclear or chemical devices underground.
Rocks: Origin and Classification
Definition: A rock is a naturally occurring, solid aggregate of one or more minerals. The Earth’s outer solid layer, the lithosphere, is made entirely of rocks.
The Rock Cycle and Origin of Rocks
The origin of all rocks is a continuous process known as the Rock Cycle. It explains how rocks are created, destroyed, and transformed over millions of years.
Primary Source: Magma (molten rock) is the starting point for all rocks.
Transformation: Tectonic movement, weathering, and heat/pressure drive the transitions between different rock types.
Classification of Rocks
Rocks are classified into three major families based on their mode of formation:
Igneous Rocks (Primary Rocks)
Formed by the cooling and solidification of molten magma or lava.
Origin: From the Greek ignis (fire). They are called primary rocks because all other rocks are ultimately derived from them.
Classification:
Intrusive (Plutonic): Magma cools slowly deep inside the crust, forming large crystals. Example: Granite.
Extrusive (Volcanic): Lava cools rapidly on the Earth’s surface, forming small crystals or glass. Example: Basalt, Obsidian.
Chemical Classification: Acidic (high silica, e.g., Granite) vs. Basic (low silica, e.g., Basalt).
Sedimentary Rocks (Secondary Rocks)
Formed by the accumulation, compaction, and cementation of organic and mineral particles (Lithification).
Origin: Formed from the “settling” of sediments transported by water, wind, or ice.
Classification:
Mechanically Formed: Formed from fragments of pre-existing rocks. Example: Sandstone, Shale.
Organically Formed: Formed from remains of plants or animals. Example: Coal, Chalk.
Chemically Formed: Formed by the precipitation of minerals from water. Example: Rock Salt, Gypsum.
Feature: These are the only rocks that contain fossils.
Metamorphic Rocks (Tertiary Rocks)
Formed when existing rocks are subjected to intense heat and pressure without melting.
Origin: The term “metamorphic” means “change of form.”
Classification:
Foliated: Have a layered or banded appearance due to pressure. Example: Gneiss, Schist, Slate.
Non-Foliated: Do not have a layered texture. Example: Marble (from Limestone), Quartzite (from Sandstone).