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

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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

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Most seashores are in a constant state of change

Sediments are eroded by large storm waves and are deposited where waves slow down

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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

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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

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Protective Structures

Structures are built to prevent beach erosion and destruction of oceanfront properties

Interfere with natural shoreline processes

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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

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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.

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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

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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

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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

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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

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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