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 km  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 text to 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 km 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 Convergence Colliding 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 Convergence Subduction 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