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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