Unit 2: Surface Processes and the Hydrosphere

Lesson 7: Physical Oceanography(Heath Earth Science – Pg. 308-352)

Today’s Objectives Describe features and processes

associated with physical oceanography, including: Identify techniques used to study the ocean and

ocean floor Diagram a typical ocean floor including

continental margins, mid-ocean ridge, rift, trench, abyssal plain, and seamounts

Relate the formation of turbidity currents to submarine canyons

Identify the physical properties of sea water (temperature, density, etc)

Explain the general pattern of major currents in oceans

How do we study the ocean floor? During the last century, we have learned a lot

about the ocean floor: Deep sea mountain ranges Underwater volcanoes and faults Trenches Type/age/composition of rock/sediment

But……the ocean floor is very deep, and is under enormous pressure…how do we learn all of this?

Three methods: 1) Remote Sensing 2) Sediment Sampling 3) Direct Observations

1) Remote Sensing Early days, depth was measured using a weight

on a line This took a very long time

Today, ships have a device called a precision depth recorder to find the distance to the ocean floor

Sends a sound signal through the water to the seafloor Length of time needed for signal to reach the bottom and

echo back to ship measures the depth of the water Traces a continuous profile of area which ship is sailing

over This method can make accurate maps of the seafloor

Variations of this method can even detect different layers of sediment on the seafloor

Satellites in space use same technology to map at much higher speeds than boats

1) Remote Sensing

2) Sediment Sampling Echo sounding provides data about the shape of

the seafloor, but not much about what materials are present

Sampling is a method used to collect sediment from the seafloor for observations

A) A scoop called a dredge is dragged along the seafloor to collect rock samples on seafloor without sediment

B) When sediment samples are needed, a corer is used Corers are devices used to collect long, cylindrical

samples of sediment from the seafloor They are shaped like a hollow tube with an open end, and

are lowered from a ship into the water Several types of corers are used which can take samples

up to 1500 meters long! (pg. 324)

2) Sediment Sampling

Dredge

Gravity Corer

3) Direct Observations Sometimes, we can make direct observations

from inside mini-submarines Alvin is an example of such a submarine One drawback is much time required to

descend/ascend too/from surface Another option is deep-towed vehicles

Argo is an example of a deep-towed vehicle “fly” above the seafloor as they are towed along

by a research ship at the surface Cameras onboard Argo send photographs back to

the research ship through cables

3) Direct Observations

Deep-tow vehicle

Mini-submarine

Practice

Topic QuestionsPg. 325, #1-3

The Ocean Floor The ocean floor is divided into two major regions:

1) the continental margins 2) the ocean basins

The continental margins themselves are further divided up into three more sub-regions: 1a) continental shelf 1b) continental slope 1c) continental rise

In addition to these regions are several other prominent features of the ocean floor, such as the: seamount, abyssal fan, trench, rift, abyssal plain, abyssal hills, guyots, coral atolls and mid-ocean ridge

1) Continental Margins

1a) Continental Shelf Underwater extension of the continent Extremely flat Extend from shoreline of the continent to the

shelf edge, (boundary between the shelf and continental slope)

Shelf edge marks location where sea depth increases rapidly

On average, the shelf edge is 130 meters deep Characteristics depend on type of continental

margin: Active margin: narrow shelf, bordered by ocean

trench, shoreline is rugged with coastal mountains Passive margin: broad shelf, up to 300 km wide,

no trench or mountains, bordered by coastal plain

1b) Continental Slope Begin at the shelf edge, where water depth

starts to increase rapidly Boundary is clear and abrupt Seafloor is no longer level, but begins to

slope toward the deep ocean at an average angle of 4 degrees

Generally about 200 km wide, and descend to 3 km deep

Change from continental to oceanic crust often occurs beneath the continental slope Active margin: slope ends in a deep-sea trench Passive margin: slope ends in a wide band of

sediment, the continental rise

Submarine Canyons Sometimes, gigantic canyons, called

submarine canyons cut into the continental shelf and slope

These canyons can be larger than the grand canyon!

Two possible causes for these canyons: 1) Continuation of a continental river valley

that has been drowned by rising sea level 2) Cut by powerful currents called turbidity

currents A turbidity current is a massive underwater

landslide caused by either an earthquake or gravity “turbid” means muddy

Turbidity Currents

• Turbidity currents build up great fan-shaped deposits at the mouths of many submarine canyons

• These features are called submarine fans, or abyssal fans

1) Continental Margin 1c) Continental Rise:

The gently sloping region between the continental slope and the ocean basin

Formed by deposition of masses of sediment several km’s thick

Sediment originates from the land, brought to the region by turbidity currents and gravity flows

Not found at active margins – deep-sea trenches that occur there trap sediments

Found only at passive margins – may reach 1000 km wide

Practice

Topic QuestionsPg. 329, #4-7

2) Ocean Basin The ocean basin is the second major region

of the ocean floor There are several features that are found in

the ocean basin, the most expansive of which are the abyssal plains

Abyssal plains range in depth from 3000-6000 meters, and are extremely flat (flattest areas of Earth’s surface)

Composed of sediments over 1 km thick in some areas

Most of this material came from the continents via turbidity currents

Abyssal Plain

Ocean Basin Features Abyssal hills are another part of the ocean

basin They are small, rolling hills that occur in groups

next to continental margins and oceanic ridge systems

In the Atlantic ocean, abyssal hills follow the mid-Atlantic ridge on either side

Seamounts are cone-shaped mountain peaks that rise high above the deep ocean floor May occur alone but more commonly found in

clusters, or rows, often near plate boundaries Volcanic in origin The Hawaiian Islands are actually a group of

seamounts that are tall enough to rise above the surface

Ocean Basin Features

Ocean Basin Features Some seamounts have a flat top, and these are

called guyots (gee-oh) Guyots started as seamounts, but their tops reached sea-

level and were worn down by wave action Sinking oceanic crust lowered the guyots below the

surface Coral atolls are also results of crustal sinking

Imagine a seamount rises above sea-level, and a coral reef grows around the island

The crust begins to sink, causing the island to sink The coral continues to grow in a ring around the sinking

island forming a ring of coral called a coral atoll

Ocean Basin Features

Ocean Basin Features Trenches are long, deep, steep sided

depressions formed along active continental margins at subduction zones (when one plate is forced beneath another plate) Usually occur along coasts where a dense,

oceanic plate is subducting below a less dense, continental plate

Ocean Basin Features Mid-ocean ridges are long mountain ranges

that form along divergent (two plates moving apart) plate boundaries on the ocean floor

Formed by magma pouring out of the rift between the two plates

The rift is located down the center of the ridge Mid-Atlantic ridge runs the entire length of

the Atlantic ocean

Ocean Basin Features

Practice

Topic QuestionsPg. 335, #8-11