ocean basins- chapter 15

Where did water come from?

Native volatiles from the molten early Earth and Cometary water

Why do volcanoes still spew water vapor?

Volatiles recycled with tectonic activity

what are Native volatiles from the molten early Earth?

Gases from initial coalescence

what is cometary water?

The moon-forming impact would have caused net loss of volatiles

what are three crustal differences?

•Oceanic crust is denser than continental crust

•Rides lower – isostatically

•Thinner – 4 to 7 kilometers (km) versus 20 to 40 for continental crust

What is true about the ocean basins size?

•Ocean basins cover 71% of Earth

•2 to 11 km in depth (approximately 5 km average)

•1.4 billion km³ water

•No basins, water would be 2 km deep everywhere

rock dredge

open mouth steel net

coring

a pipe is used to collect a mud core

sea-floor drilling

like as used for oil drilling

submersibles

both manned and robotic

echo sounding

used to map seafloor topography (SONAR)

seismic profiler

like sonar, but higher-energy waves penetrate sea-floor and reflect data back

magnetometer

towed behind ships to record magnetic fields of rock

microwave radar

used to map sea-surface, surface mimics sea-floor topography

mid-ocean ridges:

•After World War One – early echo-sounding technology showed topography

•After World War Two – more detailed maps made for naval use

mid-ocean ridge system

continuous mountain chain connected across the globe, about 80,000km long

rift valley

•1 to 2 km wide split along the ridge’s center where new crust forms

•Normal faults and earthquakes in valley

transform faults

hundreds of fractures crossing perpendicular to the ridge

black-smokers

sulfur-laden jets of hot water occurring near ridges

Chemosynthesis

bacteria produce hydrogen sulfide in the black-smokers

what is happening in this picture?

A cross-sectional view of the central rift valley in the Mid-Oceanic Ridge. As the plates separate, blocks of rock drop down along the fractures to form the rift  valley, bounded by normal faults. Movements across these faults cause earthquakes.

what is happening in this picture?

The seafloor sinks as it grows older. At the Mid-Oceanic Ridge, new lithosphere is buoyant because it is hot and of low density. As it moves away from the ridge, the lithosphere cools, thickens, and becomes denser, causing it to sink. On average, the seafloor lies at a depth of about 4 kilometers, relative to 2 to 3 kilometers at the Mid-Ocean Ridge.

fast spreading of the seafloor causes a _____ seafloor

higher

higher seafloor means _____ volume for water

less

how does the seafloor spread?

•Continent and basin ratios affect sea-level

•Tectonics can cause continents to move, shift, come together, and become relatively higher or lower

what is happening in this picture?

• (A) Slow seafloor spreading creates a narrow, low-volume Mid-Oceanic Ridge that displaces less seawater and lowers sea level.

• (B) Rapid seafloor spreading creates a wide, high-volume ridge that displaces more seawater and raises sea level.

trench

narrow, often deep, depression along a subduction zone

island arc

form above the subducting plate boundary

accreted terrane

when a plate with an island arc subducts and causes the arc to become part of the continental crust

what is happening in this picture?

An oceanic trench forms at a convergent boundary between two oceanic plates. One of the plates sinks and heats, generating magma that rises to form a chain of volcanic islands called an island arc.

seamounts

submarine mountain that rises 1km above sea floor

oceanic island

a seamount that rises above the ocean surface

guyot

a worn down, sunken, oceanic island

atoll

circular coral reef surrounding a central lagoon

what is happening in this photo?

The Hawaiian Islands and Emperor Seamounts sink as they move away from the mantle plume currently located under the volcanically active Island of Hawaii.

what is happening in this photo?

• (A) A volcanic island rises above sea level.

  • (B) Wave energy erodes a flat top on a sinking island to form a guyot.

terrigenous sediment

sand, salt, clay erodded and transported from continents

pelagic sediments

mix of fine clays and biogenic debris

abyssal plain

flat expanses of pelagic sediments

Early thought was that old oceans would have thick sediments everywhere, but what was discovered later?

•Discovered in 1947 that mud was thinner closer to ridges

pillow basalt

•rounded basalt structure caused by magma oozing out into the cool ocean

•1 to 2km thick

what does this picture show?

The three layers of oceanic crust. The uppermost layer consists of sediment. The middle layer consists of pillow basalt. The deepest layer is made of vertical dikes of basalt that merge downward into gabbro. Below this lowermost layer of oceanic crust is the upper mantle.

passive continental

continental and oceanic crust joined

continental shelf

shallow, gently sloping surface

sediment laden

isostatically depressed

carbonate platforms

limestone beds, occur in warm areas with little sediment

what is happening in this picture?

A passive continental margin consists of a broad continental shelf, a slope, and a rise formed by accumulation of sediment eroded from the continent.

continental slope

steep edge of the shelf where oceanic and continental crust meet

continental rise

slope shallows out and meets the deep ocean floor

submarine canyon

deep valley eroded into the continental shelf

abyssal fan

sediment fan often found at end of submarine canyon

Turbidity currents

•form when loose, wet sediments tumble downslope

•A rapid and extensive submarine landslide

active continental margins

form at subduction zones

•Once plate (oceanic) is actively descending beneath the other (continental)

•Trench associated with descending slab

• 4 degrees to 5 degrees near top, steepens to 15 degrees or more at depth

what is happening in this picture?

Along most active continental margins, an oceanic plate sinks beneath a continent, forming an oceanic trench. The continental shelf is narrow, the slope is steep, and the continental rise is small to nonexistent.