Download Fig. 15-26, p.370

§
Chapter 15: Ocean Basins
§ Tubeworms, tiny crabs and other sea life
use heat, minerals and chemical energy
near hot water vents on the deep sea floor
to survive without sunlight.
§
§
“Bolide” impacts
may have brought
volatiles to the
inner planets,
which eventually
formed our
atmosphere,
oceans and
foundation for life
on Earth.
§
See page 376 for
an explanation of
this drawing.
Fig. 15-1, p.354
The Earth’s Oceans
§
Oceans cover about 71% of the Earth’s surface. The seafloor is
about 5 km deep in the central part of ocean basins. Oceans basins
are continually changing. What is happening with the Pacific and
the Atlantic ocean basins? (one is closing while the other is
enlarging)…Why does water eventually end up in the oceans? (read
on the density of oceanic crust, Page 377).
§ Oceans affect global climate and the biosphere in many ways:
§ They reflect and store solar heat different than rocks/soil
(oceans are generally warmer in the winter and cooler in the
summer than adjacent land).
§ Most precipitation is from evaporation from oceans.
§ Ocean currents transport heat toward poles.
§ Plate tectonics alters basins which alters currents and affect
climate.
§ Halley’s Comet. The early Solar System was
crowded with comets, meteoroids and asteroids.
Bolide impacts may have imported volatiles such
as CO2, water vapor, ammonia, simple organic
molecules and other volatiles.
Fig. 15-2, p.354
§ Schematic cross section of the continents and
ocean basins. Vertical axis shows elevation
relative to sea level. Horizontal axis shows the
relative areas of the types of topography (e.g.,
mountains and ocean floor).
Fig. 15-3, p.355
§ Studying the seafloor:
Oceanographers extract sediment
from a core retrieved from the
seafloor.
Fig. 15-4, p.356
§ Alvin is a
research
submarine
capable of
diving to
the sea
floor.
Scientists
on board
control
robot arms
to collect
sea-floor
rocks and
sediments.
Fig. 15-5, p.357
§ Mapping the topography of the sea floor
with an echo sounder. A sound wave
bounces from the sea floor and back up to
the ship, where its travel time is recorded.
Fig. 15-6a, p.357
§ A seismic
profiler records
both the sea
floor
topography and
the layering of
sea floor
sediments and
rocks.
Fig. 15-6b, p.357
§ Features of the
Sea Floor
Fig. 15-7, p.358
§ The Mid-Oceanic Ridge System (MORS) and other
features of the sea floor show there is as much
topography here as on the continents. MORS is a
continuous submarine mountain chain that
encircles the globe; it rises 2-3 km above the sea
floor, and is Earth’s largest mtn chain (covering
20% of its surface).
§ Divergent plate boundaries, or spreading
centers, coincide exactly with the MORS in
the world’s oceans.
Fig. 15-7b, p.358
§ The sea floor sinks as it grows older. At the MORS,
new lithosphere is buoyant because it is hot and of
low density. It ages, cools, thickens and becomes
denser as it moves away from the ridge and sinks.
The central part of the sea floor lies at a depth of
about 5 km.
Fig. 15-8, p.360
§ A cross-section
view of the
central rift
valley of the
MORS. As the
plates
separate,
blocks of rock
drop down
along the
fractures to
form the rift
valley. The
moving blocks
cause
earthquakes
along “normal”
faults.
Fig. 15-9, p.360
§ Transform faults offset segments of the
MOR. Adjacent segments of the ridge may
be separated by steep cliffs 3 km high.
Note the flat abyssal plain far from the
ridge.
Fig. 15-10, p.361
§ The MORS can cause
a rise and fall in
global sea level (if
they didn’t exist, sea
level would fall 400
meters). Slow
spreading (above)
creates a narrow,
low-volume ridge that
displaces less sea
water and lowers SL.
§
Rapid sea floor
spreading (right)
creates high-volume
ridge, displacing
more sea water and
raises SL.
Fig. 15-11, p.361
§ Life on
the MidOceanic
Ridge.
§ Black
smoker
to right
(see
page
384).
Fig. 15-12, p.362
Fig. 15-13, p.362
§ Oceanic
Trenches and
Island Arcs:
An oceanic
trench forms
at a
convergent
boundary
between two
oceanic
plates. One
plate sinks,
generating
magma that
rises to form a
chain of
volcanic
islands called
Fig. 15-14, p.362
§ Onekotan is one of many volcanic islands in the
Kuril Island arc that formed along the Kuril trench
in the western Pacific. The deepest place on Earth
is the Mariana trench of the sw Pacific, where the
ocean floor sinks to about 11 km below SL.
§ Island arcs
eventually
migrate toward a
continent and
becomes part of
it (to buoyant to
sink). This is a
way continents
can grow by
accreted
terranes.
Fig. 15-16, p.363
Fig. 15-16a, p.363
Fig. 15-16b, p.363
Fig. 15-16c, p.363
§ The
accreted
terranes of
western
North
America
are microcontinents
and island
arcs from
the Pacific
Ocean that
were added
to the
continent.
Fig. 15-17, p.364
Seamounts, Oceanic Islands and Atolls
§ Seamount: submarine mtn that rises 1
km or more above the ocean floor.
§ Oceanic Island: is a seamount that
rises above sea level.
-both are volcanoes commonly made of
basalt formed at a “hot spot” above a
mantle plume. As the plate overrides the
hot spot, the seamount becomes inactive.
The Hawaiian Island-Emperor Seamount
Chain is an example (15.18). As the
seamounts move away they erode into a
flat-topped “guyot” and sink.
§ The Hawaiian
Island-Emperor
Seamount
Chain becomes
older in a
direction going
away from the
island of
Hawaii. In 1015 million
years the
island of
Hawaii may
sink and
become
eroded.
Fig. 15-18, p.365
§ The Hawaiian Islands and Emperor
Seamounts sink as they move away from
the mantle plume.
Fig. 15-19, p.365
§ Formation of a guyot.
Fig. 15-20, p.366
Fig. 15-20a, p.366
Fig. 15-20b, p.365
§ A fringing reef grows along the shore of a
young volcanic island. As the island sinks,
the reef continues to grow upward to form
a barrier reef that encircles the island.
Finally, the island sinks below sea level and
the reef forms a circular atoll.
Fig. 15-22a, p.367
Fig. 15-22b, p.367
Fig. 15-22c, p.367
§ The Tetiaroa Atoll in French Polynesia
formed by the process described in Figure
15.22. Over time, storm waves wash coral
sands on top of the reef and vegetation
grows on the sand.
Fig. 15-21, p.366
§ Sediments and rocks of
the sea floor.
§ The three layers of
oceanic crust are layer
1: sediment (Terrigenous
and Pelagic); layer 2:
pillow basalt and layer 3:
upper mantle (basalt
dikes and gabbro).
§ The oceanic crust is 4-7
km thick (1-2 km of
pillow basalts and 3-5
km of dikes/gabbro).
Fig. 15-23, p.368
§ Terrigenous sediment
is sand, silt and clay
eroded from the
continents and carried
to the deep sea floor by
gravity (rivers,
landslides) and
submarine currents.
§ Pelagic sediment
collects even on the
deep sea floor far from
continents (clay and
remains of tiny plants
and animals). It
accumulates at a rate of
2-10 mm/1000 yrs. Near
the MOR there is
virtually none (why?).
Fig. 15-24, p.368
§ Pillow lavas…how do they form?
Fig. 15-25, p.368
§ Continental
margins.
Fig. 15-26, p.370
§ Continental crust fractured as Pangea
began to rift.
Fig. 15-26a, p.370
§ Faulting and erosion thinned the crust as it
separated. Rising basaltic magma formed
new oceanic crust in the rift zone.
Fig. 15-26b, p.370
§ Sediment eroded from the continents
formed broad continental shelves on the
passive margins of North America and
Africa.
Fig. 15-26c, p.370
§ A passive continental margin consists of a broad continental
shelf, slope and rise formed by the accumulation of sediment
eroded from the continent.
§ Submarine canyons are deep valleys from the edge of a
continent to the rise (where abyssal fans may form) and
occur where large rivers enter the sea. Sediments from
rivers create turbidity currents that can travel as speed
greater than 100 km/hr for up to 700 km.
Fig. 15-27, p.371
Fig. 15-28, p.371
§ At an active continental margin an oceanic
plate sinks beneath a continent, forming an
oceanic trench. The continental shelf is
narrow, the slope is steep and no rise
Fig. 15-29, p.372
exists.
p.374