Plate Tectonics

Chapter 17

Great Idea:The entire earth is still changing, due to the slow

convection of soft, hot rocks deep within the planet.

Chapter Outline

• The Dynamic Earth

• Plate Tectonics: A Unifying View of Earth

• Another Look at Volcanoes and Earthquakes

The Dynamic Earth

• Small-scale changes– Construction site

• Erosion by rain

• Large-scale changes– Volcanoes – Earthquakes – Erosion

The Case of the Disappearing Mountains

• Erosion– Few hundred million years– Mountains continually forming

• Earth’s surface is not static

A map of the world’s continents reveals the similar shapes of coastlines on the two sides of the Atlantic Ocean.

Map of continents

The Movement of the Continents

• F. Bacon– Continents like a puzzle

• Wegener– Continental Drift

• Continents in

motion

• Current Evidence– Ocean Floors

– Magnetic Reversals

– Rock Ages

The German scientist A. Wegener (1880 - 1930) postulated that a supercontinent once existed — and later broke apart — called

Pangea.

Once the motion of continents was determined, scientists could predict how the Earth’s surface might have looked in the past. More than 200 million years ago, the present-day continents were joined together as the ancient continent Pangaea.  

Plate Reconstructions

Ocean Floors

• Mapping– Ocean floor dynamic

• Canyons, mountains

• Mid-Atlantic Ridge– Earthquakes,

volcanoes,

lava flows

Magnetic Reversals

• Earth’s magnetic field– Changes periodically

• Magnetite– Crystals in lava align to magnetic field

• Paleomagnetism• Seafloor Spreading

– New rock comes to surface

Measures of tectonic rates: magnetic:

Magnetic stripes that parallel ocean ridges must form as new magma wells up from the fissure and pushes out to the sides. In this cross-sectional view, older rocks lie farther from the ridge. (The “lithosphere” includes the uppermost mantle and all of the crust.)

Spreading Ridge magmatism (Iceland)

Measurements in the late 1950s and early 1960s revealed magnetic stripes running nearly parallel to the Vancouver province and Washington state coastlines.

Magnetism of the sea floor

Rock Ages

• Radioactive Isotopes– Rocks near Mid-Atlantic Ridge younger– Rocks farther away older

New Support for the Theory

• Measuring motion of continents

• Radio Astronomy– Measured arrival of radio waves– Repeated over several years

• North America and Europe– Separating at 5 cm per year

Plate Tectonics: A Unifying View of Earth

Plate Tectonics

• Plate Tectonics– Large-scale surface features– Related phenomena

• Tectonic plates– Rigid, moving sheet of rock– Crust and upper mantle– Continental

• 100 km thick• Lower density (granite)

– Oceanic• 8-10 km thick• Dense rock (basalt)

• Earth’s surface– ¼ continent, ¾ water

The major plates of the Earth with their directions of motion shown by arrows.

Plate motions

The Convecting Mantle

• Mantle convection– Motion driven by Earth’s interior heat energy

• Sources of energy– Gravitational potential energy– Decay of radioactive elements

• Movement– Heat moves to cooler regions– Convection cells in mantle– Very slow

• 200 million years for one cycle

In the 1960’s, Harry Hess, Robert Deitz and other geologists and oceanographers, put forward the hypothesis of ‘seafloor spreading.’ The key data for this hypothesis were the topographic maps of the ocean floors….

Seafloor Spreading:

The ‘Ring of Fire’

Earth’s lithosphere is a mosaic of 13 major plates:

Plate boundaries may be convergent, divergent, and transform….

Earth’s lithosphere is a mosaic of 13 major plates:

Plate boundaries may be convergent, divergent, and transform….

Divergent, oceanic boundaries:

Mid-Atlantic Ridge, Iceland

Divergent, continental boundaries:

A satellite photograph of a portion of Africa’s Great Rift Valley. The narrow body of water defines a divergent plate boundary where new plate material is being created and plates are moving out to either side.

The ‘Afar triangle’

Gulfs formed between rifted continental fragments:

Convergent Oceanic boundary:

Convergent Ocean-Continent boundary:

Continent-Continent collision boundary:

Transform Plate Boundary

• Two plates move past each other– NOT smooth– Earthquakes as a result of movement

Transform, or ‘Strike Slip’ boundaries:

The principal layers, which differ in chemical composition and physical properties, are the core, the mantle, the crust, and the atmosphere (not shown). When looked at in detail, each of these layers is itself composed of smaller layers.

The layers of Earth

Volcanoes and Earthquakes-Evidence of Earth’s Inner Forces

• Volcano– Magma breaks

through surface

• Earthquake– Rocks breaks along

fault– Energy transmitted as

wave– Richter scale

A cross-section of a volcano reveals a magma chamber, which stores molten rock, and a system of pipes, cracks, and vents that lead to the surface. The terms in the orange area refer to the kinds of rock formations resulting from cooled magma. Xenoliths are the original rocks encased in this cooled magma.

Volcanic settings

A divergent plate boundary defines a line along which new plate material is formed from volcanic rock.

Divergent Plate Boundaries

Subduction magmatism

Kauai, currently the oldest, is between 3 and 5.5 million years old, while Hawaii, the youngest, is less than 0.8 million years old.

Hawaiian example of plate motion

Another Look at Volcanoes and Earthquakes

• Plates and Volcanism– Divergent Plate

Boundaries– Convergent Plate

Boundaries• Subduction zones

– Hotspots• Source stationary, plates

move• Chain of volcanoes

• Earthquakes– At plate boundaries or

elsewhere

Seismology: Exploring Earth’s Interior with Earthquakes

• Seismology– Study of sound vibrations within earth– Used to determine earth’s inner structure

• Seismic waves– Compressional or longitudinal– Transverse or shear waves

Seismology: Exploring Earth’s Interior with Earthquakes

How do we know that the Earth’s tectonic plates move (continental drift)?

a) Observation of the Earth’s rivers

b) Direct measurements, observation of the rifts in the ocean floors, magnetic reversals

c) Calculations by Einstein

d) Erosion of mountains

A Clicker Question.

The Geological History of North America

• Northeastern Canada and Greenland– Several billion years old

• Western US– Terranes

• Added to continent over time

• Appalachian Mountains– Formed 450-300 million years ago– Continental-continental convergence zone

• Rocky Mountains– 60 million years ago– Warping, folding and fracturing of continent

• The Colorado Plateau– Gentle uplift

• The Sierra Nevada– Molten rock pushed up sediments