Oceanography Chapter 2: Plate Tectonics and the Ocean Floor

What is a volcanic arc?

a row of volcanoes that forms on the overriding plate near a subduction zone

Where is the youngest ocean floor found?

along the crest of mid-ocean ridges

Continental transform faults

at least one continental plate slides past another plate

Why do hotspots result in chains of volcanoes instead of a single volcanic peak?

because the plate is moving over the hotspot

Mid-Ocean Ridge transform faults

connect two oceanic ridge segment Perpendicular fracture zones causes a discontinuous pattern

Which of the following features are found at an oceanic-continental convergent plate boundary?

continental arc mountains trench

Magnetic stripes

creation of new ocean crust over geologic time

Where do mantle plumes originate?

deep in the mantle, near the core

Ocean crust, formed at _______________, is dense.

divergent margins

Plates move apart at __________ boundaries, move together at __________ boundaries, and move side by side at __________ boundaries.

divergent/convergent/transform

Harry Hess

envisioned new sea floor being created at the mid-ocean ridge and spreads outward

How often does the Earth's magnetic field switch polarity?

every 100,000 to several million years

Which geologic features are associated with divergent boundaries?

mid-ocean ridges and continental rift valleys

Complete this statement: Divergence is to moving away as convergence is to _______.

moving toward

Which geologic features are associated with convergent boundaries?

ocean trenches and volcanoes

Where would you expect to see alternating bands of rock with different magnetic polarities?

on both sides of the mid-ocean ridge

Transform

plates horizontally grind against one another

Divergent

plates move away from each other

Convergent

plates move towards each other

When flow reverses, polarity of magnetic field ___________

reverses

Transform boundaries are classified under what type of fault?

strike-slip

As ocean crust ages and cools, its great density relative to the continents results in ______________ as plates _______________.

subduction, converge

The other kind of plate margins, ______________, are parallel to the current motion of the plates.

transforms

Convergent boundaries

• Old crust sinks back into Earth at subduction zones along convergent plate boundaries - One plate sinks into the asthenosphere below another plate

The rise of plate tectonics

1. Earthquakes are concentrated at what is currently known as plate boundaries 2. Age of ocean crust identified as YOUNG 3. Seafloor features became better known 4. Seafloor spreading

Evidence for continental drift

1. Fit of coastlines of South America and Africa 2. Similarities of fossils found across separated continents 3. Mountains line up in Northern Hemisphere 4. Ancient glacier

Supporting Plate Tectonics

1. Locations of earthquakes ("Ring of Fire") 2. Age structure of the seafloor and continents 3. Seafloor spreading 4. Paleomagnetism 5. Hotspot tracks

Paleomagnetism: Earth's "fossil" magnetic field

1. Magnetic particles (magnetite) emerge in magma at mid-oceanic ridges 2. Particles orient themselves in direction of magnetic field 3. As magma cools, magnetite Particles lock in place

Oceanic-Continent

1. Oceanic plate slides under continental plate 2. A deep trench forms along the subduction zone 3. Lighter continental crust is pushed up and forms a continental arc Ex. Andes Mountain, South America Mount St. Helen's, Cascades, WA Rocks are folded and raised along coastline forming mountains Coasts are relatively straight with narrow continental shelves and steep continental slopes

Three Types of Convergent Boundaries

1. Oceanic-Continent 2. Oceanic-Oceanic 3. Continent-Continent

Divergent plate boundaries - rifts

1. Plates pull apart creating a dropped zone called a rift 2. Crust widens and thins, valleys and volcanos form 3. As spreading continues, a long, narrow sea can form when ocean water slowly floods the valley 4. Eventually, an expansive ocean basin and ridge are created

Continent- Continent

1. Two continental plates collide and smash together 2. Continental crust is deformed and uplifted, creating mountains

Oceanic-Oceanic

1. Under the ocean, one plate slides underneath another 2. A deep trench forms along the contact 3. Magma wells up along the subduction zone creating volcanoes, which can form volcanic island arcs Ex. Japan Trench

Pangaea (All Earth)

200 MYA- split into two major landmasses: Laurasia and Gondwana 135 MYA- Gondwana splits further 65 MYA- NA and Eurasia separate; SA separates from Africa 50-40 MYA- Aust. Separates from Antarctica

San Andreas fault

800 Miles Long • 1857- Fort Tejon • 1906- San Francisco • 1989- Loma Prieta

How are normal polarity and reverse polarity of Earth's magnetic field different?

A compass points toward the North Pole during normal polarity and to the South Pole during reverse polarity.

In general, what is our best evidence that the orientation of Earth's magnetic field has changed over time?

A record of Earth's magnetic field is recorded in oceanic rocks, which show a clear pattern of changes in Earth's magnetic polarity.

Which of the following mountain ranges are examples of continental arcs?

Andes and Cascades

Which type of plate boundary is most closely associated with the formation of new ocean floor?

Divergent

Ocean basins form at

Divergent Plate Boundaries

Plate boundaries

Divergent, Convergent, Transform

What would happen to Earth if ocean floor were created at divergent boundaries at a faster rate than it is destroyed at convergent boundaries?

Earth would increase in volume.

Which of the following statements about Earth's magnetic field is most accurate?

Earth's magnetic field switches polarity at irregular time intervals.

Liquid iron in outer core conducts both:

Electricity & Convective flow

Which geographic example below was created by a hotspot?

Hawaii

Which geographic examples below result(s) from ocean-ocean convergence?

Marianas Trench Aleutian Islands Japan

Divergent Boundaries

Mid Ocean Ridges have basaltic flows or pillow lavas - Distinctive bulbous shape resulting from underwater eruptions

Plate movements tracked by hotspots

Mid-plate volcanos ("hot spots") are produced as they move over continuous upwelling from the hot mantle - Ex: volcanic island chains like Hawaii Image courtesy Jacques Descloitres, MODIS Land Rapid Response Team at NASA GSFC - Heat source = stationary "plume" rising from lower mantle - Narrow region that is hotter than the surrounding mantle - Allows the material to rise to the surface - melt when it reaches the lithosphere - erupt as a volcano

Do fracture zones near transform boundaries play a role in plate motion?

No; fracture zones play no role in plate motion.

Hotspot

Plumes are found in continental areas too

The theory of tectonic plates

Rigid lithospheric plates move across Earth's surface Plate movement rate = 5 cm/yr Driving force = mantle convection

What is the relationship between the crust and the lithosphere?

The crust is part of the lithosphere.

What happens when an oceanic plate converges with a continental plate?

The oceanic plate is subducted.

What happens when an oceanic plate meets another oceanic plate?

The older, colder oceanic plate is subducted.

What happens when a continental plate meets another continental plate?

The two plates are forced upward, and a large mountain range is formed.

What role do transform boundaries play?

Transform boundaries connect other segments of plate boundaries.

What is the relationship between transform boundaries and mid-ocean ridges?

Transform boundaries connect segments of mid-ocean ridges.