Unit 5: Rock Types and Plate Tectonics

Introduction to different rock types and their characteristics

A rock is a naturally occurring solid aggregate of one or more minerals or mineraloids. There are three main types of rocks, classified by their origin:

1. Igneous Rocks

• Origin: Formed from the cooling and solidification (crystallization) of molten rock (magma or lava).
• Intrusive (or Plutonic) Igneous Rocks:
  • Form when magma (molten rock below the surface) cools slowly inside the Earth's crust.
  • Characteristic: Slow cooling allows large crystals to grow, resulting in a coarse-grained texture (e.g., Granite).
• Extrusive (or Volcanic) Igneous Rocks:
  • Form when lava (molten rock on the surface) cools quickly.
  • Characteristic: Rapid cooling prevents large crystal growth, resulting in a fine-grained texture (e.g., Basalt). If it cools instantly, it forms volcanic glass (e.g., Obsidian).

2. Sedimentary Rocks

• Origin: Formed from the accumulation, compaction, and cementation of sediments on the Earth's surface (usually in water).
• Sediments are pieces of pre-existing rocks (igneous, metamorphic, or other sedimentary rocks) that have been broken down by weathering and erosion.
• Characteristics:
  • Often show layers (stratification or bedding).
  • Are the only rock type that can contain fossils.
• Types of Sedimentary Rocks:
  • Clastic (or Detrital): Made from broken pieces (clasts) of other rocks. (e.g., Sandstone, Shale, Conglomerate).
  • Chemical: Formed when minerals precipitate from a solution (usually water). (e.g., Limestone (in some cases), Rock Salt).
  • Biogenic (or Organic): Formed from the remains of living organisms. (e.g., Limestone (from shells/coral), Coal (from plants)).

3. Metamorphic Rocks

• Origin: Formed when a pre-existing rock (the "parent rock" or "protolith") is changed by intense heat and/or pressure, without melting.
• This "metamorphism" (change in form) causes new minerals to grow and the rock's texture to change.
• Characteristics:
  • Often show foliation, which is a parallel alignment of minerals (like platy mica) that gives the rock a layered or banded appearance.
• Examples of Foliation (increasing metamorphism):
  • Shale (sedimentary parent) → Slate (low-grade) → Phyllite → Schist → Gneiss (high-grade, banded).
• Non-Foliated Rocks:
  • Sandstone (parent) → Quartzite.
  • Limestone (parent) → Marble.

The Rock Cycle

The Rock Cycle is a model that describes how the three rock types are interrelated and how they can be transformed from one type to another. It illustrates that rocks are not permanent but are constantly being changed and recycled by Earth's processes (plate tectonics, weathering, erosion).

Lithospheric plates: continental and oceanic

The Earth's rigid outer layer (the lithosphere) is not one solid piece. It is broken into about a dozen large pieces called tectonic plates (or lithospheric plates). These plates "float" on the semi-molten asthenosphere below.

There are two types of lithosphere (and thus plates):

Property	Oceanic Lithosphere	Continental Lithosphere
Composition	Basaltic (rich in Si, Mg - "SIMA")	Granitic (rich in Si, Al - "SIAL")
Density	Denser (approx. 3.0 g/cm³)	Less Dense (approx. 2.7 g/cm³)
Thickness	Thinner (avg. 7-10 km for crust, ~100km for plate)	Thicker (avg. 35-70 km for crust, ~150km for plate)
Age	Younger (constantly recycled, < 200 million years)	Much Older (can be billions of years old)

Fundamentals of plate tectonics

Plate Tectonics is the unifying theory of geology. It explains that the Earth's plates are in constant, slow motion, driven by convection currents in the mantle. Most of Earth's geologic activity (earthquakes, volcanoes, mountain building) occurs at the boundaries where these plates interact.

Types of Plate Boundaries

1. Divergent Boundary (Constructive):
   • Motion: Plates move apart.
   • Process: Magma rises from the asthenosphere to fill the gap, creating new oceanic crust. This is called sea-floor spreading.
   • Features: Mid-Ocean Ridges (e.g., Mid-Atlantic Ridge), Rift Valleys (on land, e.g., East African Rift).
   • Activity: Shallow earthquakes, minor volcanic activity.
2. Convergent Boundary (Destructive):
   • Motion: Plates move towards each other.
   • Process: The denser plate sinks beneath the less-dense plate in a process called subduction.
     • Oceanic-Continental: Denser oceanic plate subducts under the continental plate. Forms a volcanic mountain range (e.g., The Andes).
     • Oceanic-Oceanic: The older, denser oceanic plate subducts under the younger one. Forms a volcanic island arc (e.g., Japan, The Philippines).
     • Continental-Continental: Neither plate subducts (both are low-density). They collide and crumple, forming huge, non-volcanic mountain ranges (e.g., The Himalayas).
   • Activity: Major, deep earthquakes; explosive volcanoes (except cont-cont).
3. Transform Boundary (Conservative):
   • Motion: Plates slide past each other horizontally.
   • Process: Lithosphere is neither created nor destroyed.
   • Features: Transform faults (e.g., San Andreas Fault in California).
   • Activity: Frequent, powerful, shallow earthquakes; no volcanic activity.

Basic idea about cratons, shield, platforms, mobile-belts, geosynclines

Stable Regions: Cratons

Craton: A craton is the old, stable, interior part of a continent that has been geologically inactive for billions of years. It's the "core" of the continent. Cratons are divided into two parts:

1. Shield: The part of a craton where the ancient (Precambrian) crystalline (igneous/metamorphic) "basement" rocks are exposed at the surface. (e.g., Canadian Shield).
2. Platform: The part of a craton where the ancient basement rock is covered by a thin layer of younger, horizontal sedimentary rocks. (e.g., the Interior Plains of North America).

Active Regions: Mobile Belts & Geosynclines

• Mobile Belt (or Orogenic Belt): A long, narrow region of the Earth's crust that has experienced significant tectonic activity (folding, faulting, mountain building). These are modern or ancient plate boundaries. (e.g., The Himalayan belt).
• Geosyncline: An older, somewhat outdated term for a large-scale depression in the Earth's crust (a trough) that collects a thick sequence of sediments. It was once thought these troughs sank and were later uplifted to form mountains. The modern concept of subduction zones and accretionary wedges in plate tectonics has largely replaced this idea.

Concepts of orogeny and sea floor spreading

Sea Floor Spreading

This is the mechanism that drives divergent plate boundaries. It's the "engine" of plate tectonics.

1. Magma from the mantle rises at a Mid-Ocean Ridge.
2. It erupts as lava, cools, and forms new basaltic oceanic crust.
3. This new crust pushes the older crust away from the ridge on both sides, like a slow-moving conveyor belt.
4. Evidence:
   • Age of the Seafloor: Rocks are youngest at the ridge and get progressively older as you move away from it.
   • Paleomagnetism: The Earth's magnetic field periodically reverses. As lava cools, iron minerals align with the current magnetic field. This creates a "magnetic striping" pattern on the seafloor that is perfectly symmetrical on either side of the ridge.

Orogeny

Definition: An orogeny (or orogenic event) is the process of mountain building, which occurs over millions of years.

Orogenies are a primary result of convergent plate boundaries:

• Andean-type Orogeny: Occurs at an oceanic-continental boundary. The subducting oceanic plate causes magma to rise, forming a volcanic mountain chain on the continent (e.g., The Andes).
• Himalayan-type Orogeny: Occurs at a continental-continental collision. The intense compression and crumpling of the crust forms massive, non-volcanic fold mountains (e.g., The Himalayas).