Post on 16-Dec-2015
The Marine EnvironmentChapter 16
16.1 Shoreline Features
• Shorelines are shaped by the action of waves, tides and currents
• As waves erode coastlines, they create impressive rock formations
• Sometimes, waves drop/deposit loose sediments and build wide, sandy beaches
• Waves move faster in deeper waters than they do in shallow waters
• Wave refractions – difference in wave speed causing straight wave crests to bend when the crest moves into shallow water
Erosional Landforms Rocky headlands – points of land
sticking out into the oceanoCaused by the destructive action of breakersoCan be modified by wave erosionoSea stacks / sea arches form from wave
refraction at the rocky headland
Beaches
Sloping band of sand, pebbles, gravel, or mud at the edge of the sea
Composed of loose sediments deposited and moved about by waves along the shoreline
Beach Composition Depends on source of material
› Hawaii – black sand/tiny grains of minerals from volcanic rocks
› Southern Florida / Bahamas – white / pink sand from fragments of local corals and seashells
› Near mouths of large rivers – sandy sediments washed in by river water, small grains of quartz and feldspar
Size of sediment particles depends on energy of waves striking the coast
Estuaries The area where the
lower end of a freshwater river or stream enters the ocean
Water is blackish in color Water is a mixture of
freshwater and saltwater Nurseries to young of
many different species Examples include the
Chesapeake Bay and the Pamlico Sound
Longshore Currents
Longshore bar – submerged sandbar located in the surf zone
Longshore current – current that flows parallel to the shore
Moves large amounts of sediments Forms when incoming breakers spill over a
Longshore bar
Lon
gsh
ore
Cu
rren
ts
(con
t.)
Movem
ent o
f Sedim
ents
Rip
Curre
nts
Longshore currents move large amounts of sediments along the shore
Fine-grained materials are suspended in turbulent, moving water
Large materials are pushed along the bottom of the current
Some sediments are pushed back and forth on the beach (incoming/outgoing waves)
Longshore transport is generally to the south
Wave action produces rip currents, which flows out to the sea through gaps in the Longshore bar
Return the water spilled into the Longshore trough to the open ocean
Currents can reach speeds of several km/hr One should never swim against rip currents,
but should swim parallel to the shore to get out of the current
DEPO
SIT
ION
AL FE
ATU
RES
O
F SEA
SH
OR
ES
Most seashores are in a constant state of change
Sediments are eroded by large storm waves and are deposited where waves slow down
Sed
imen
ts d
ep
osite
d b
uild
coasta
l lan
dfo
rms
Tom
bolo
Lag
oon
Ridge of sand that forms between mainland and an island › Connects the island to the mainland› When this is present, it makes the island the
“tip” of a peninsula
Shallow, protected body of water behind barrier islands› Essentially coastal lakes› Connected to sea by shallow, restricted
outlets
Dep
ositio
nal F
eatu
res o
f th
e S
hore
line
Sp
itB
arrie
r Islan
ds
Narrow bank of sand that projects into the water from a bend in the coastline› Forms where a shoreline changes direction› Protected from wave action› When growing spit crosses bay, baymouth
bar forms
Long ridges of sand or other sediments› Deposited or shaped by longshore currents
that are separated from mainlands› Can be several km wide / tens of km long› Unstable and temporary
BA
RR
IER
ISLA
ND
S O
F NO
RTH
C
AR
OLIN
A – T
HE O
UTER
B
AN
KS
Protective Structures
Structures are built to prevent beach erosion and destruction of oceanfront properties
Interfere with natural shoreline processes
Exam
ple
s a
nd
Neg
ativ
e E
ffects
of P
rote
ctiv
e S
tructu
res
Seaw
alls
Gro
ins
Built along shore to protect oceanfront properties
Reflect waves back to the beach Protect oceanfront properties Worsens beach erosion
Wall-like structures build into the water perpendicular to the shoreline to trap beach sand
Interrupts natural longshore transport Deprives beaches of sand down the
coast
Exam
ple
s a
nd
Neg
ativ
e E
ffects
of P
rote
ctiv
e S
tructu
res
Jettie
sB
reakw
ate
rs
Walls of concrete built to protect a harbor entrance from drifting sand
Jetties trap sand upshore Prevents sand from reaching beaches
downshore
Built in the water parallel to straight shorelines to provide anchorages for small boats
Affects longshore currents causing them to no longer be able to move its load of sediment
Causes currents to drop and eventually build up to fill anchorage locations
Changes in Sea LevelEarth’s sea level rises and falls over time with
changing of the Earth› Ice Age
Causes sea level to lower as the water freezes into ice caps› End of Ice Age
Causes sea level to rise due to the ice melting› Rise of Earth’s Surface Temperature
Causes seawater to warm up and expand Adds total volume to the sea
› Tectonic Movement (uplifting and sinking) If a coastline sinks, rise in sea level along that coast If a coastline rises, sea level along that coast drops Emergent Coast – region that was formerly underwater
Tends to be relatively straight Characterized by sandy beach ridges located far inland and elevated
marine terraces
16.2 The SeafloorUntil recently, most people had little knowledge of the features of the ocean floor. The topography of the seafloor is surprisingly rough and irregular, with numerous high mountains and deep depressions.
Oceanic and Continental Crust As learned previously, the Earth has two types of
crust:1. Continental Crust – avg. thickness of 40km2. Oceanic Crust – avg. thickness of 6-7km
Crustal elevation depends on crustal thickness, which is why thicker crust occurs on land and thinner crust occurs deep within ocean basins
Part of the continental section is actually below sea level and the ocean covers parts of the continents. These submerged portions are called continental margins and include the continental shelf, the continental slope and the continental rise.
CO
NTIN
EN
TAL S
HELV
ES
The shallowest part of the continental margin, extending seaward from shore
The average width is 60km, although width varies greatly
Continental Slopes
Sloping regions where the seafloor drops away quickly, beyond the continental shelves
True edge of the continent
Often “cut” by submarine canyons, which are formed from turbidity currents
CO
NTIN
EN
TAL R
ISE
Gently sloping accumulation of sediments dropped off by turbidity currents, forming at the base of the continental slope. This may be several km thick.
Ocean Basins Deeper parts of the seafloor that lie
above thin, basaltic, oceanic crust Beyond the continental margin Represents about 60% of the Earth’s
surface
Top
og
rap
hy o
f Ocean
B
asin
s
Ab
yssa
l Pla
ins
Deep
Sea Tre
nch
es
Smooth parts of the ocean floor 5 or 6 km below sea level Covered with hundreds of meters of muddy
sediment Probably flattest surfaces on Earth
Deepest part of the ocean basins Narrow, elongated depressions in the
seafloor Many lie next to chains of volcanic islands Most located around margins of Pacific
Ocean
Top
og
rap
hy o
f Ocean
B
asin
s
Mid
-Oce
an
Rid
ges
Hyd
roth
erm
al V
en
ts
Most prominent features of ocean basin Run through all ocean basins, total length
of over 65,000km Average height of 1500km, but varies Sites of frequent volcanic eruptions and
earthquake activity
Hole in the seafloor through which fluid heated by magma erupts
Most located along bottom of rifts in mid-ocean ridges
“Black Smoker” or “White Smoker” classification, depending on what is ejected from the vent
Seafloor Volcanoes Mountains are found all throughout the
seafloor Believed to be extinct volcanoes, rather
than mountain ranges Two types:
1. Seamounts Submerged basaltic volcanoes more than 1km
high Many stretched across the Pacific Ocean Basin
2. Guyots Also called “tablemounts” Large, extinct basaltic volcanoes with a flat,
submerged top
Marine Sediments Come from a variety
of sources, most from continents
Includes mud and sand washed into oceans by rivers, as well as volcanic ash blown over the ocean by winds
Typ
es o
f Marin
e
Sed
imen
ts
Man
gan
ese
Nod
ule
sO
oze
Major source of deep-sea sediment (shells and hard parts of marine animals)
Typically accumulate at a rate of a few mm per thousand years
Most are small, consisting of either calcium carbonate or silica
Consist of oxides of manganese, iron, copper, and other valuable metals
Growth rates are incredibly slow Measured in mm per million years