Test 2: Lesson 2.3

why do volcanic arcs form along subduction zones?

-as descending slabs heat up, wet ocean crust is dehydrated (water is liberated) -water moves up into the mantle above the descending slab -this water causes the mantle to melt, and magma rises to the surface and erupts (forms volcanoes)

volcanic arcs

-row of volcanoes in the overriding plate, that parallels the subduction zone -called "arc" because subduction zones are always curved

accretionary prism/wedge

-seafloor sediment is scraped off subducting plate, and transferred onto overlying plates -creates a thick mount (sediment at the bottom is buried deeply enough to be metamorphosed

4 features of all subduction zones

-trench -volcanic arc -accretionary prism -deep, large earthquakes

subduction zones

-where an oceanic plate dives back into mantle -oceanic lithosphere is destroyed -balanced by the creation of oceanic lithosphere at MORs

Key points on plate tectonics

1. lithosphere is broken up into 12 major and many minor plates 2. plates move relative to one another 3. plate boundaries don't necessarily correspond to edge of continent 4. most rock deformation occurs at plate boundaries

subducting plates have an average dip of what?

45 degrees

Earthquakes occur all along the subducting slab, to depths up to _________

600 km (below that, it's too hot)

plates

A section of the lithosphere that slowly moves over the asthenosphere, carrying pieces of continental and oceanic crust.

Wadati-Benioff zone

A sloping band of seismicity defined by intermediate- and deep-focus earthquakes that occur in the downgoing slab of a convergent plate boundary. -inclined area underground where subduction-caused EQs occur

p-waves

A type of seismic wave that compresses and expands the ground. -travels through liquid

s-waves

A type of seismic wave that moves the ground up and down or side to side -cannot travel through liquids

convergent plate boundaries

Areas where plates move toward each other and collide, causing uplift.

active continental margin

At some point the continental margin becomes active. A subduction zone forms and the deformation process begins.

ocean trench

Deep valley in the ocean floor that forms along a subduction zone

passive continental margin

Margins that consist of a continental shelf, continental slope, and continental rise. They are not associated with plate boundaries and therefore experience little volcanism and few earthquakes.

Theory of plate tectonics

The theory that pieces of Earth's outer layer are in constant motion -"grand unifying theory" -Explains about many geological events (earthquakes, faulting/rifting, distribution of resources, etc)

Which of the following is considered a PASSIVE MARGIN? West coast of Africa West coast of North America West coast of South America Mid-Atlantic Ridge

West coast of Africa

accretionary prisms/wedges can occasionally be pushed where?

above sea level

Why does mantle rock melt in subduction zones? a. The down-going oceanic slab transfers heat to the mantle rock, causing the mantle rock to melt b. The down-going oceanic slab transfers water to the mantle rock, causing the mantle rock to melt c. The down-going slab increased pressure on the mantle rock, causing the mantle rock to melt d. Trick question - mantle rock does not melt in a subduction zone

b. The down-going oceanic slab transfers water to the mantle rock, causing the mantle rock to melt

the continental lithosphere is too _________ to subduct. If it moves into a subduction zone, what happens?

buoyant; subduction stops

If part of a plate made of oceanic lithosphere converges with part of a different plate made of continental lithosphere, what will happen? a. The continental lithosphere will subduct beneath the oceanic lithosphere b. A continental collision zone will develop c. The oceanic lithosphere will subduct beneath the continental lithosphere d. This is an impossible situation. These two types of lithosphere can never converge

c. The oceanic lithosphere will subduct beneath the continental lithosphere

What does the accretionary wedge consist of? a. Igneous rock created by melting the mantle in subduction zones b. Oceanic crust c. Volcanoes from the volcanic arc d. Sediments scraped of the subducting slab. They may undergo metamorphism as they become deeply buried in the wedge

d. Sediments scraped of the subducting slab. They may undergo metamorphism as they become deeply buried in the wedge

why does magma rise and erupt after it forms by rock melting deep underground?

density--liquid rock is less dense than solid rock

types of plate boundaries

divergent, convergent, transform

The Wadati-Benioff zone marks ________ _________ between plates

frictional contact

the amount of oceanic plate consumption at subduction zones worldwide, averaged over time is equal to the amount of what?

oceanic crust produced by seafloor spreadding at MORs

________ waves travel faster through cold rock than hot rock

p-waves

we can use ________ _________ to image the cold subducting slabs underground

seismic waves

what happens when ocean-ocean or ocean-continent plates converge? what about continent-continent?

subduction; collision

plates move and glide over what?

the weak asthenosphere

Oceanic lithosphere is destroyed at ______________. This process is called _________________.

trenches; subduction

oceanic sediments and crust contain __________

water

what happens to subducted plates?

we think they pile up at the core-mantle boundary (slab graveyard)