Plate Tectonics

notes from professor

Earth is a rocky planet

Solid rock and liquid rock (magma/lava)

What is magma?

Liquid rock below Earth’s surface

What is lava?

Magma that reaches Earth’s surface

What is a volcanic eruption?

When magma reaches Earth’s surface

What does every volcano add?

Rock to Earth’s surface

What are Earth’s three main layers?

Core, mantle, crust

What property differentiates Earth’s layers?

Density differences

What is the composition of the core?

Iron and nickel

What is the inner core?

Solid iron due to high pressure

What is the outer core?

Liquid iron that creates Earth’s magnetic field

Which layer is least dense?

The crust

What are the two types of crust?

Oceanic crust and continental crust

Describe oceanic crust.

Denser

Describe continental crust.

Less dense

Why does ocean water collect on oceanic crust?

Because it is lower and denser

What is the lithosphere?

Crust plus uppermost mantle acting as tectonic plates

Where do most earthquakes and volcanoes occur?

At plate boundaries

How do tectonic plates move?

In different directions and at different rates

What are the three types of plate boundary motion?

Divergent, subduction, transform

What happens at divergent boundaries?

Oceans open and plates move apart

What happens at convergent boundaries?

Oceans close and plates collide

What role do volcanoes and subduction play?

Volcanoes add rock and subduction recycles rock into the interior

How are mountains on land commonly formed?

At convergent ocean-continent or continent-continent boundaries

How are volcanic islands commonly formed?

At convergent ocean-ocean boundaries or hot spots

What characterizes divergent boundaries?

Opening oceans

What is decompression melting?

Melting of hot rock as pressure decreases near the surface

What is a mid-ocean ridge?

Underwater mountain chain formed at divergent seafloor boundaries

What happens to seafloor rock age at divergent ridges?

Youngest near ridge

What forms when continents diverge?

Rift valleys with shallow earthquakes and volcanoes

What can form if continental rifting continues?

A long linear sea

What occurs at convergent ocean-continent boundaries?

Oceanic plate subducts beneath continental plate

What feature forms at subduction zones?

Deep ocean trench

What types of earthquakes occur at subduction zones?

Shallow

How do volcanoes form at ocean-continent convergence?

Flux melting from water and friction of subducting plate

What occurs at convergent ocean-ocean boundaries?

Denser oceanic plate subducts forming trenches and volcanic island arcs

What characterizes island arcs?

Arc-shaped chains of similar-aged active volcanoes

What occurs at convergent continent-continent boundaries?

Ocean closes and very high mountains form

Why are volcanoes few at continent-continent convergence?

Extreme pressure limits magma formation

What characterizes transform boundaries?

Shallow earthquakes and no volcanoes

Where are transform faults commonly found?

Between offset divergent boundaries

What is the hot spot (mantle plume) theory?

Volcanoes form above fixed rising plumes of hot mantle rock

Can hot spots occur at plate boundaries?

Yes

How do hot spot island chains form?

Plate moves over fixed plume creating progressively older islands

Which hot spot islands remain volcanically active?

Those closest to the plume

What happens to volcanic islands as they move away from a hot spot?

They cool

What are two ways to create islands?

Ocean-ocean convergence or hot spots

How do island ages differ at hot spots?

Youngest near the hot spot and older farther away

How is rate of plate motion calculated?

Distance divided by time in cm/year

What is the typical range of plate motion rates?

About 1–14 cm per year

How is direction of plate motion determined using hot spots?

Arrow from youngest volcano toward oldest rock

How do rocks respond to stress at plate boundaries?

They fold or fracture

What is an anticline?

Fold with oldest rock at the center

What is a syncline?

Fold with youngest rock at the center

What is a fault?

Fracture where rocks have moved causing earthquakes

What are the three main types of faults?

Normal, reverse, strike-slip

Which stress causes normal faults?

Tensional stress at divergent boundaries

Which stress causes reverse faults?

Compressional stress at convergent boundaries

Which stress causes strike-slip faults?

Shear stress at transform boundaries

What is the focus (hypocenter) of an earthquake?

Point within Earth where slip occurs

What is the epicenter?

Point on Earth’s surface directly above the focus

What are seismic waves?

Energy released during fault slip

What are P waves?

Fast compressional waves that travel through solids and liquids

What are S waves?

Slower transverse waves that travel only through solids

Which seismic waves arrive first?

P waves

Which seismic waves cause more structural damage?

S waves

What are surface waves?

Slow waves traveling along Earth’s surface and most destructive