Plate Tectonics Slides

Rangeland Soils Overview

  • Soils are crucial to life on Earth.

  • Without productive soils, life would perish.

Plate Tectonics and Earthen Material

  • Definition of Plate Tectonics:

    • Theory that Earth’s outer shell is divided into several plates that glide over the mantle.

    • Lithosphere: The strong outer layer includes the crust above the mantle.

    • The crust is the thin outermost region dominated by crystalline rock.

Earth's Crust and Lithosphere

  • The Earth's crust is the outermost solid layer of the planet.

  • Thickness of the crust varies:

    • Oceanic Crust: ~5–10 km (3–6 miles)

    • Continental Crust: ~30–70 km (18–43 miles)

    • Average Thickness: ~35 km (~22 miles) on continents.

  • Lithosphere consists of:

    • Crust + Rigid uppermost portion of the mantle.

    • Characteristics: Mechanically strong, broken into large tectonic plates.

Subduction and Plate Boundaries

Subduction

  • Definition: Occurs when converging plate boundaries force one plate to move down into the mantle.

  • Plate boundaries are involved in:

    • Spreading

    • Converging

    • Transforming

Types of Plate Boundaries

1. Divergent Boundaries (Spreading)

  • Plates move apart.

  • Magma rises to form new oceanic crust.

  • Example: Mid-Atlantic Ridge.

2. Convergent Boundaries (Colliding)

  • Plates move toward each other.

  • Subduction: One plate may be forced beneath another, generating volcanic arcs and mountain ranges.

3. Transform Boundaries

  • Plates slide horizontally past one another.

  • Example: San Andreas Fault in California.

Diastrophism

  • Definition: The forces of pressure that shape Earth’s surface.

  • Types of Diastrophism include:

    • Folding

    • Caused when tectonic forces cause rock layers to bend.

    • Faulting

    • Tilted Strata

Types of Deformation

  1. Folds: Bending of rock without breaking, including tilting.

  2. Faults: Fracture of rock with displacement.

  3. Joints: Fracture of rock without displacement.

    • Joints affect resistance to erosion by weakening the rock.

Types of Folding

  • Anticline: Upward-arching fold (convex upward).

  • Syncline: Downward-arching fold (concave upward).

  • Monocline: A step-like fold in otherwise horizontal strata.

Faulting Types

  • Normal Fault: Caused by extensional stress; hanging wall moves down relative to the footwall. The uplifted block is called a horst, and the down-dropped block is called a graben.

  • Reverse (Thrust) Fault: Caused by compressional stress; hanging wall moves up.

  • Strike-Slip Fault: Caused by shear stress; blocks move horizontally past each other.

Diastrophism - Tilted Strata

  • Definition: Tilted strata are rock layers originally deposited horizontally but slanted due to tectonic forces.

  • Causes of tilting:

    • Folding

    • Faulting

    • Uplift on one side of a fault block.

Joints

Definition

  • Cracks or fractures in rock with no movement/displacement.

Formation Causes

  1. Tensional Stress: Rocks pulled apart.

  2. Cooling and Contraction: Common in igneous rocks (e.g., columnar joints in basalt at the Giant's Causeway).

  3. Unloading (Pressure Release): Rock expands when overlying rock is removed by erosion.

Vulnerability to Weathering

  • Joints weaken rocks and make them vulnerable to:

    • Weathering

    • Erosion

    • Water infiltration

    • Freeze-thaw action.

Summary of Diastrophism Effects

Type of Deformation

Rock Breaks?

Rock Moves?

Example

Folding

No

No

Anticline, Syncline

Faulting

Yes

Yes

Horst, Graben, Normal Fault

Joints

Yes

No

Columnar joints

Forces in Diastrophism

  • Three things happen due to stress:

  1. Tension: Pulling apart – stretching.

  2. Compression: Pushing together – shortening.

  3. Shear: Twisting laterally – sliding.

Volcanism and its Relation to Tectonics

  • Volcanism occurs when tectonic forces result in the formation of new crust involving molten rock (magma).

    • This heat results in the creation of igneous rock.

  • Stress in the crust can:

    1. Deform the Crust:

    • Folding

    • Faulting

    • Jointing

    1. Create pathways for Magma:

    • Cracks (joints)

    • Fault zones

    • Rifts.

Cascade Mountain Range

  • Volcanic origin due to converging with the Pacific Plate.

  • Volcanoes include:

    • Mt. Baker

    • Glacier Peak

    • Mt. Rainier

    • Mt. St. Helens

    • Crater Lake

    • Three Sisters.

Different Volcanoes and Rock Composition

  • Rocks influence soils and ecosystems.

  • Composition of magma affects:

    • Types of igneous rocks developed

    • Manifestation of volcanic activity.

Types of Igneous Rock Formation

Origin of Igneous Rocks

  • Igneous rocks form through the cooling and solidification of molten material:

    • Magma: Molten rock beneath the Earth’s surface.

    • Lava: Molten rock that has reached the surface.

Differences between Magma and Lava

  • Magma:

    • Beneath Earth's surface, contains dissolved gases (H₂O, CO₂, SO₂).

    • Crystallizes slowly if underground; forms intrusive rocks.

  • Lava:

    • Reaches the surface, rapidly loses pressure and gases, cools quickly; forms extrusive rocks.

  • Influence on Soil Development:

    • Lava's fine-grained texture contributes to rapid chemical weathering, forming new soil parent material.

Composition of Magma

  • Forms through:

    • Decompression melting at divergent boundaries

    • Flux melting at subduction zones

    • Heat transfer melting.

  • Magma Composition Factors:

    • Source rock composition

    • Partial melting processes

    • Crystallization history

    • Assimilation of surrounding rock.

Characteristics of Intrusive and Extrusive Rocks

1. Intrusive (Plutonic) Igneous Rocks

  • Form from slowly cooled magma beneath the Earth’s surface.

  • Characteristics:

    • Coarse-grained texture (large, visible mineral crystals).

    • Examples: Granite (felsic), Diorite (intermediate), Gabbro (mafic).

2. Extrusive (Volcanic) Igneous Rocks

  • Form from rapidly cooled lava at or above the Earth's surface.

  • Characteristics:

    • Fine-grained or glassy texture; may contain vesicles (gas bubbles).

    • Examples: Basalt (mafic), Rhyolite (felsic).

Bowen's Reaction Series

  • Model developed to explain the crystallization order of silicate minerals from cooling magma.

  • Core Principles:

    • Minerals crystallize at specific temperature ranges.

    • High-temperature minerals form first, influencing magma evolution.

Significance of the Series

  • Explains mineral crystallization at different temperatures, indicating why various rocks have specific mineral compositions.

Mafic vs. Felsic Magma

Mafic Magma

  • High in magnesium and iron, low in silica; typical rock: Basalt.

  • Characteristics:

    • 45–55% SiO₂

    • Higher temperature (1100–1250°C).

Felsic Magma

  • High in silica, aluminum, potassium, and sodium; typical rock: Granite.

  • Characteristics:

    • 65% SiO₂

    • Lower temperature (650–800°C).

Silica's Role in Magma Behavior

  • Silica (SiO₂) controls:

    1. Viscosity: High silica = thick, sticky lava; low silica = thin, runny lava.

    2. Eruption Type: High silica = explosive eruptions; low silica = gentler flows.

    3. Soil Fertility: Low silica = more nutrient release; high silica = less nutrient cation availability.

Polymerization of Silica

  • Silicon–oxygen tetrahedra can exist independently or share bonds, leading to:

    • Chains

    • Sheets

    • 3D frameworks.

Influence on Soil Chemistry

  • Polymerization affects:

    • Crystallization pathways for minerals.

    • Weathering rates and nutrient release in soils.

Conclusion

  • The relationship between magma chemistry, mineral composition, and soil fertility is essential for understanding ecosystem productivity.

  • Different magma types result in varying rock compositions, ultimately leading to diverse soil types and ecosystem health.