12.2

Features of Plate Tectonics

OVERVIEW

There is lots of evidence that the Earth’s interior is NOT simply a solid ball of rock: Earthquakes Volcanoes Mountains

When the Earth cooled, light elements/materials (ex. silicon and oxygen) floated to the top forming the crust. Earth is made up of 4 layers with distinct characteristics.

I. TECTONIC PLATES

The outer layer of Earth is made up of several large, rigid but mobile chunks of rock Known as plates or tectonic plates

Lithosphere: formed by tectonic plates in the crust and uppermost mantle Ranges in thickness from 65-100km 12 major plates and several smaller ones

There are 2 types of plates: Oceanic Plates – contain dense rock basalt Continental Plates – contain large amount

of granite

II. CROSS-SECTION OF EARTH

Use page 519 in your textto complete the chart in your notes!

III. PLATE MOTION

Asthenosphere: found below the lithosphere – a partly molten layer in the upper mantle Varied temperature throughout layer due to

radioactive elements (ex. uranium) and radioactive decay

Heated particles have more kinetic energy causes a convection current as the hotter/less dense material rises then cools and sinks

Mantle convection hypothesized as a leading factor in plate movement.

Rising currents of magma will eventually reach the Earth’s surface at spreading centres

Oceanic Ridge (or spreading ridge): a spreading centre that occurs in the ocean

Rift Valley: a spreading centre that occurs on land less common

Ridge Push: (1) Magma reaches the surface and cools, creating “new rock”; (2) new rock pushes older material away, forcing plates away from the ridge

Subduction: a dense ocean plate hits a continental plate heavy ocean plate will dive deep under the lighter continental plate Thought to contribute to plate motion

Subduction Zones: areas generally experience large earthquakes and volcanic eruptions

Slab Pull: as the edge of a tectonic plate subducts deep into the mantle, it pulls the rest of the plate with it

Convection currents, ridge push, and slab pull helps keep tectonic plates in motion!

IV. PLATE INTERACTIONS

Plate Boundary: a region where 2 tectonic plates are in contact

Plate interaction is important in the geological and biological of Earth. There are 3 main types of plate interactions: Divergence (moving apart) Convergence (moving together) Transform (sliding by)

The way that plates interact depends on 2 factors: Type of plate Direction of plates movement (relative to

one another)

DIVERGENT PLATE BOUNDARIES

Divergent Plate Boundaries: areas where tectonic plates are spreading apart

Diverging Plates: plates that are spreading apart

Example: the Mid-Atlantic Ridge System of spreading ridges separating the

Americas from Africa, Europe, and Asia About 16 000km long (largest mountain range

with greatest amount of volcanic activity) Sea-floor spreading causes plates to separate

CONVERGENT PLATE BOUNDARIES

Convergent Plate Boundary: occurs where tectonic plates collide

Converging Plates: plates that collide

There are 3 types of converging plates: Oceanic-Continental Plate Convergence Oceanic-Oceanic Plate Convergence Continental-Continental Plate

Convergence

1. Oceanic-Continental Plate Convergence When a dense oceanic plate collides with a

continental plate oceanic plate forced to slide under the continental plate

Trench: a deep underwater valley formed where plates make contact

Large pieces fall off and melt, crystallizing into large rock masses below the continental plate

Magma can force its way up , creating cone-shaped volcanoes Ex. off the coast of North America (Juan de Fuca

Plate and North American Plate) Cascadian subduction zone

Volcanic Belt: a long chain of volcanoes Collision creates mountain ranges as

continental rock crumples and folds (ex. Coast and Cascade Mountains)

If plates remain stuck in place, pressure builds and can result in earthquakes

2. Oceanic-Oceanic Plate ConvergenceColliding causes one plate to be denser than

the other the denser one slides deep into the mantle

May produce a long chain of volcanic islands called a volcanic island arc

Ex. islands of Japan, Indonesia, and Aleutian Islands Regions experience earthquakes

3. Continental-Continental Plate ConvergenceSubduction doesn’t happen due to similar

densities Edges will fold and crumple, forming

mountain ranges Ex. Himalayas caused by Indian continent

hitting Asian continent

TRANSFORM PLATE BOUNDARIES

Transform Plate Boundaries: usually near ocean ridges where convection currents cause tectonic plates to slide past each other No volcanoes or mountains form Earthquakes and faults can result

Faults: breaks in rock layers due to movement on sides

Transform Fault: a fault that occur at a transform plate boundary Ex. San Andreas Fault (California) due to

oceanic Pacific Plate sliding past continental North American Plate