Unit Three:

Plate Tectonics and

Volcanoes

Earth Science

Big Idea: Plate tectonics is the central organizing theory of geology and is part of the explanation of every phenomena and process observable in the geosphere. Plate tectonics influence phenomena in the atmosphere, hydrosphere, and biosphere.

Explain how plate tectonics accounts for the features and processes that occur on or near the Earth’s surface.

Explain why tectonic plates move. Distinguish types of plate boundaries, and explain the features and actions that occur at each.

I CAN…

Continental drift theory began in the early 20th century

By 1968, Plate Tectonics had become the newest acceptable theory.

Plate Tectonics

1915 Alfred Wegener (German scientist) The Origin of Continents and Oceans Proposed theory Supercontinent called Pangaea (all land) Drifted into current locations

Continental Drift History

South America and Africa

Distribution of fossils, rock structure, and ancient climates

Evidence for Continental Drift

Fossil Evidence

Continental Margins

Ancient Climates

Rock Types and Structures

Alfred Wegener would die before plate tectonics would become the accepted norm.

The main problem with his continental drift theory was that he could not explain the mechanism that caused the plates to move.

Evidence for Plate tectonics◦ Mapping of the ocean floor◦ Lithosphere vs. Asthenosphere◦ Plates in motion

7 major plates◦ Move with ocean floor◦ Unequal heating within the Earth causes

movement Conveyer belt

Moving lithospheric plates grind slowly past each other, causing earthquakes, volcanoes, and turning rocks into mountains.

Deformation of the plates usually takes place a the boundaries.

There are 3 distinct types of boundaries.◦ Divergent boundaries◦ Convergent boundaries◦ Transform boundaries

Plate Boundaries

Divergent Boundaries

Plates spreading apart Fractures are filled with molten rock from

the asthenosphere New sea-floor is created Sea-floor spreading (mid-ocean ridges) Example: Mid-Atlantic Ridge

◦ Youngest rocks on Earth◦ On land created Rifts

East African Rift valley Red Sea (creating new ocean?) Iceland (part of Mid-Atlantic Ridge)

Divergent Boundaries

Mid Ocean Ridges

Oceanic crust is subducted and destroyed at subduction zones

Creates ocean trenches Three types

◦ Oceanic-oceanic◦ Oceanic-continental◦ Continental-continental

Convergent Boundaries

Oceanic-oceanic boundaries

One slab of oceanic crust descends below another.

Creates a trench Creates island arcs of volcanic islands. NOT Hawaii. Examples: Aleutian Islands, Japan, &

Philippines Most occur in the Pacific ocean

Oceanic-oceanic

Oceanic-Continental

Oceanic and Continental crust coming together

Oceanic slab descends because it is denser Partial melting occurs and magma begins to

make its way to the surface Creates chains of volcanic mountains on the

land Examples: Cascades in Pacific NW, and the

Andes mountains.

Oceanic-Continental

Continental-continental

Neither plate subducts because they are both lighter and less dense.

The rock “folds” up on itself Creates mountain ranges Examples: Himalayas, Alps, Appalachians,

and Urals

Continental-continental

Transform Boundary

Plates grind past each other No destruction of lithosphere No creation of lithosphere All horizontal displacement Called fracture zones Examples: San Andreas fault

California WILL NOT fall into the ocean.

Transform Boundaries

Why we won’t lose Cali

Paleomagnetism Polar Wandering Magnetic Reversals and Seafloor spreading Earthquake Patterns Ocean Drilling Hot Spots

Evidence for Plate Tectonics

Some rocks contain minerals that act as fossil compasses.

When they cool, they align with north and south poles at the time.

Current magnetic field becomes recorded in rock.

Rocks formed thousands of years ago “remember” the location of the magnetic poles at the time of their formation.

Do not all currently point toward north/south

Paleomagnetism

During the past 500 million years the poles have wandered.

However, the poles are relatively stable and there is proof in rocks that lava flows near Hawaii have been near the poles.

Either poles moved or continents. Continents much more likely to more larger

distances than magnetic poles.

Polar Wandering

Occasionally the north and south poles reverse.

A rock that solidifies during a current episode of polarity has normal polarity, while a rock that has opposite polarity is said to have reverse polarity.

The ocean floor shows a pattern of many magnetic reversals over millions of years.

Magnetic Reversals and Seafloor Spreading

Magnetic Reversals and Seafloor Spreading

Earthquakes line up with plate boundaries and plate descending locations.

Earthquake Patterns

Deep Sea Drilling Project Older rocks are found farther away from the

ocean ridges and younger rocks are found nearer to the ridge.

Remember new ocean floor is cooled at the ridges.

Oldest ocean sediment: 180 myo Oldest continental sediment: 3.9 byo

Ocean Drilling

Mapping of seafloor revealed seamounts or a chain of volcanic structures

Most studied from Hawaii to Aleutian trench. Old volcanoes increase in age away from

the Big Island of Hawaii. Scientists believe there is a rising plume of

mantle beneath the island of Hawaii and the Pacific plate is being pulled over it creating a path of islands and seamounts.

Hot Spots

Kauai is the oldest of the large Hawaiian islands.

It’s volcanoes are extinct. Only Mauna Loa and Kilauea remain active

on the Island of Hawaii. The chain of islands and seamounts follow

the path of the plate.

Hot Spots

Hot Spots

Hot Spots

Hawaii and Seamount Chain

Igneous Activity

What determines a volcano’s strength? What is erupted? What are the types of volcanoes? What other features are formed? What does magma/lava do in the ground? What does magma/lava do at plate

boundaries?

Nature of Volcanic Eruptions

Magma’s composition Magma’s temperature What dissolved gasses are present Viscosity of magma

Viscosity: resistance to flow. Syrup is more viscous than water.

Factors affecting Eruptions

Warmer magmas flow more easily Cooler magmas have difficultly flowing

Temperature of Magma

Magma’s viscosity is directly related to it’s silica content

More silica, more viscosity

Magma Composition

Dissolved gasses give the force behind volcanic eruptions

Mostly water vapor and carbon dioxide As gasses near the surface, it is like opening

a pop bottle: gasses expand and rush out When the gasses push out, they can force

magma into the air causing fountains of lava

These fountains are usually harmless

Dissolved Gasses

Highly viscous magmas (think thick, difficult flowing) impede gasses escaping and gasses collect in pockets and bubbles

The pockets and bubbles increase until the pressure causes huge explosions

Mt St Helens Mt Pinatubo

Dissolved gasses

Types of Lava Rock Gas

What is erupted?

Pahoehoe◦ Resemble ropes or braids◦ Top cools and bottom continues to move

underneath Aa

◦ Rough jagged blocks◦ Tumbles over itself creating blocks and chunks

Pillow◦ Erupts underwater◦ Cools quickly◦ Looks soft and poofy (but watch out, its hot!)

Lavas

Amounts of gasses in lava are estimated 1-5% Mostly water vapor Carbon dioxide 5% sulfer (stinky)

Volcanoes are a natural source of air pollution and can cause serious problems◦ Acid rain

Gasses

“Fire fragments” Named for their size

◦ Ash – fine glassy fragments (welded tuff, pumice)◦ Cinders – pea sized particles◦ Lapilli – little stones, walnut size fragments◦ Blocks – larger than lapilli◦ Bombs – same as blocks only they are ejected,

flying through the air

Pyroclasts

Parts of Volcanoes

Craters and Calderas are the same thing, the difference is that Calderas exceed 1km in diameter.

Shield Composite (Stratovolcano) Cinder cone

Types of Volcanoes

Fluid lavas are extruded, and the magmas spread easily

Volcano takes the shape of a broad, slightly domed structure

Mauna Loa is one of 5 shield volcanoes that make up the island of Hawaii.

Kilauea is also a shield volcano

Shield Volcanoes

Shield Volcano

Most beautiful volcanoes Large, symmetrical, layers of lava and

pyroclasts Mt Fuji, Mt. St. Helens, Mt. Rainier, Mt.

Shasta Can alternate between quiet and explosive

eruptions Steep summit, gentle slopes Most violent

Composite (Stratovolcano)

Composite Volcano

Built from ejected lava fragments Steep angle Small Usually occur in groups

Cinder Cones

Cinder Cone

Crater or caldera 1.    Steep walled depression at summit 2.    Caldera – a crater that exceeds one km

in diameter

Volcanic landforms

1.    From silica-rich magma2.    Consists of ash and pumice fragments3.    Material is propelled from the vent at high speed4.    E.g., Yellowstone plateau

Pyroclastic flow

Volcanic material extruded from fractures E.g., Colombia Plateau

Fissure eruption and lava plateau

1.    Resistant vent left standing after erosion

2.    E.g., Ship Rock New Mexico

Volcanic Neck

Magma placed at depth Underground igneous body is called a

pluton Plutons are classified according to:

◦ Shape ◦ Orientation (direction in the ground)

Intrusive Igneous activity

Tabular (sheet-like) Massive

Shape

  Discordant – cuts across sedimentary beds Concordant – parallel to sedimentary beds

Orientation (with respect to surrounding rock)

Dike, a tabular, discordant pluton Sill, a tabular, concordant pluton (e.g.,

Palisades sill, NY) Laccolith

◦ Forms the same way as sill◦ Lens shaped mass◦ Arches overlying strata upward

Batholith◦ Largest intrusive body◦ Surface exposure 100+ square km (smaller bodies are

termed stocks)◦ Frequently form the cores of mountains

Types of Intrusive Igneous Bodies

Dike

Sill

Laccolith

Batholith