Marine Unit 2

Describe the structure of the Earth, naming each layer from outermost to innermost.

• Crust — outermost layer, divided into oceanic crust (thin, dense, basaltic) and continental crust (thick, less dense, granitic)

• Mantle — hot, semi-solid rock beneath the crust, moves very slowly

• Core — hot, dense centre made mostly of iron and nickel

What is the difference between oceanic crust and continental crust?

• Oceanic crust is thinner (about 8 km thick), denser, and made of basalt

• Continental crust is thicker (up to 32 km), less dense, and made of granite

• Because oceanic crust is denser, it subducts under continental crust at convergent boundaries

What is the lithosphere?

• The lithosphere is the rigid outermost layer of the Earth

• It is made up of the crust and the upper mantle

• It is approximately 100 km thick

• Tectonic plates are sections of the lithosphere

What is the asthenosphere?

• The asthenosphere sits just below the lithosphere

• It behaves like a plastic — it deforms slowly under pressure rather than breaking

• Tectonic plates float and move on top of the asthenosphere

What drives the movement of tectonic plates?

• Convection currents in the mantle (asthenosphere) drive plate movement

• Molten rock is heated by the core, becomes less dense, and rises

• As it reaches the upper mantle it cools, becomes denser, and sinks back down

• This circular movement drags the plates above it

• Plates move approximately 2–5 cm per year

State four pieces of evidence that support the theory of plate tectonics.

• Jigsaw fit — continental coastlines (especially South America and Africa) fit together like puzzle pieces

• Fossil distribution — identical fossils found on continents now separated by ocean

• Geological matching — rock formations of the same type and age found on different continents

• Paleomagnetic stripes — alternating bands of magnetic polarity on the ocean floor, mirrored on either side of mid-ocean ridges

What are paleomagnetic stripes and what do they tell us?

• As magma rises at mid-ocean ridges and cools, it records the direction of Earth's magnetic field at that time

• Earth's magnetic field reverses direction approximately every 250,000 years

• This creates alternating stripes of normal and reversed polarity on either side of the ridge

• The stripes are mirrored on both sides

• This proves new seafloor is being created and spreading outward from the ridge

What is a convergent plate boundary?

• Plates move toward each other

• One plate subducts under the other

• Usually oceanic crust subducts under continental crust because it is denser

• Considered a destructive boundary because crust is destroyed

What is a divergent plate boundary?

• Plates move away from each other

• Magma rises to fill the gap, creating new crust

• Considered a constructive boundary because new crust is formed

What is a transform plate boundary?

• Plates move antiparallel — sideways past each other

• No crust is created or destroyed

• Considered a conservative boundary

What features form at a convergent boundary and how does each form?

• Ocean trench — oceanic crust subducts under continental crust, forming a deep canyon at the point of subduction

• Volcanoes — subducting crust melts and magma forces up through the crust above and erupts

• Volcanic islands — volcanic material builds up on the ocean floor and rises above sea level

• Tsunamis — sudden plate slippage during subduction displaces the overlying water column

What features form at a divergent boundary and how does each form?

• Mid-ocean ridge — magma rises through the gap between separating plates, cools, and builds up into an underwater mountain chain

• Hydrothermal vents — cold water seeps into cracks near the ridge, gets superheated by magma, and is expelled

• Abyssal plains — older seafloor away from the ridge is flattened by sediment deposition over time

• Volcanoes — magma escapes through the thin crust at the rift zone

What features form at a transform boundary?

• Earthquakes are the main feature

• Plates grinding sideways past each other build up pressure that is suddenly released as seismic energy

• No crust is created or destroyed

• Abyssal plains can also be found near transform boundaries

Explain how an earthquake forms at a plate boundary.

• Plates moving past or toward each other create friction and get stuck

• Pressure and stress builds up over time at the boundary

• Eventually the plates slip suddenly, releasing stored energy

• This energy travels as seismic waves through the lithosphere, causing shaking

• Most earthquakes occur at convergent or transform boundaries

Explain how a tsunami forms.

• A sudden slippage of plates at a subduction zone or transform boundary releases energy into the seabed

• This displaces a large column of water, creating a long-wavelength, high-energy wave

• In the open ocean the wave travels very fast but has a low height

• As it approaches shallow coastal water it slows down

• The wave height increases dramatically as energy is compressed into shallower water

• It reaches the shore as a destructive wave

Describe the conditions found at hydrothermal vents.

• Found at divergent boundaries, in rift zones and mid-ocean ridges

• Located at depths greater than 2000m

• Extremely high pressure due to depth

• Very high temperature water is expelled from the vent

• No sunlight reaches this depth

• Water is acidic and rich in dissolved minerals and toxic metals such as iron, copper, and zinc

• High levels of hydrogen sulfide (H₂S) gas

• Low dissolved oxygen

Explain step by step how a hydrothermal vent chimney forms.

• Cold, dense ocean water seeps down into thin cracks in the oceanic crust near a rift zone

• As it passes close to magma it heats up to extremely high temperatures

• It dissolves minerals including copper, sulfur, zinc, and iron

• The superheated water is under pressure and is forced back up through fissures in the crust

• As it exits, it meets the cold ocean water and cools rapidly

• The dissolved minerals become less soluble as the water cools and precipitate out of solution

• These minerals solidify and build up layer by layer to form the chimney structure

State the key properties of water released from a hydrothermal vent.

• The water is under very high pressure

• It is extremely hot, sometimes exceeding 400°C

• It is rich in dissolved nutrients and minerals

• When released it forms a hydrothermal vent plume

• The plume spreads warm, mineral-rich water into the surrounding ocean

Why can the effects of a hydrothermal vent plume be detected far from the vent?

• The warm, buoyant plume water rises above the vent

• It carries dissolved minerals, heat, and chemical signatures into the surrounding water

• Ocean currents spread this water horizontally away from the vent

• Scientists can detect changes in temperature, chemistry, and turbidity well away from the vent site

What is the difference between weathering and erosion?

• Weathering is the breaking down of rocks due to exposure to atmosphere, water, or biological organisms

• Weathering does not involve movement — the rock breaks down in place

• Erosion is the movement or transport of weathered material to a new location

• Erosion requires a transporting agent such as water, wind, ice, or gravity

Describe chemical weathering and give an example.

• Chemical weathering changes the chemical composition of a rock through reactions with water or oxygen

• The rock is broken down chemically rather than physically

• Example: water dissolves calcium ions as it passes over limestone, gradually breaking it down

• Example: oxygen reacts with iron minerals in rock causing oxidation (rusting) which weakens the rock

Describe physical weathering and give an example.

• Physical weathering breaks rocks into smaller fragments without changing their chemical composition

• The rock is broken mechanically

• Example: waves crash against a cliff, breaking off fragments through repeated impact

• Example: temperature changes cause rock to expand and contract repeatedly, eventually cracking and splitting

Describe organic (biological) weathering and give an example.

• Organic weathering involves living organisms breaking down rock physically or chemically

• Example: tree roots grow into cracks in rock and expand over time, forcing the rock apart

• Example: lichens attach to rock surfaces and release acids that chemically break down the surface

Describe the four types of erosion.

• Ice erosion — glaciers crush, grind, and break rocks as they move and carry rock fragments to new locations

• Water erosion — rivers and runoff carry sediment downstream toward the ocean

• Wind erosion — wind picks up loose sediment particles and transports them to new locations

• Gravity erosion — weathered rock and sediment move downhill under gravity, such as in landslides and rockfalls

Define sedimentation and explain how water speed and particle size affect it.

• Sedimentation is the deposition of suspended particles when a transporting agent slows down

• Faster water can carry larger, heavier particles

• As water slows, it can no longer carry large particles and they are deposited first

• Smaller particles can be carried further and remain suspended longer

• The very smallest particles such as silt and clay stay suspended in slow-moving water and cause turbidity

Define turbidity.

• Turbidity is the cloudiness of water caused by suspended particles

• Caused by fine silt and clay particles that remain suspended rather than sinking

• Common in estuaries and muddy shores where wave action is limited

Define the littoral zone.

• The littoral zone is the intertidal region on a shoreline

• It is the area between the highest spring tide mark and the lowest spring tide mark

• It is alternately covered and exposed by the tide

• Also known as the intertidal zone

State the five examples of littoral zone environments.

• Rocky shores

• Sandy shores

• Muddy shores

• Estuaries

• Deltas

Describe the morphology and formation of a rocky shore.

• Rocky substrate with varying and often steep slopes

• Made of resistant rock such as granite or igneous stone

• Subject to constant wave action and erosion

• Erosion rate is slow but greater than sedimentation rate

• Very little sediment accumulates because wave energy removes it

• Larger rocks tend to be found offshore where wave energy is highest, smaller fragments inshore

Describe the morphology and formation of a sandy shore.

• Gradual, gentle slope

• Formed where sedimentation rate exceeds erosion rate

• Sand is constantly moved by wave action and wind

• Sandy particles are medium-sized — small enough to be transported but large enough to settle when energy drops

• Exposed to wave action but not enough to prevent sediment build-up

Describe the morphology and formation of a muddy shore.

• Very little to no slope — almost completely flat

• Found in protected, sheltered regions with little wave action or erosion

• Sedimentation rate greatly exceeds erosion rate

• Finest particles (silt and clay) settle here because the water is calm enough

• Creates mudflats with a thick, fine substrate

• Low oxygen levels in the mud due to high organic content and bacterial decomposition

Describe the morphology and formation of an estuary.

• A sheltered area where fresh water from a river meets salt water from the sea, creating brackish water

• Shallow slope with a muddy substrate

• Protected from strong wave action and erosion

• High sedimentation rate — fine particles settle in the calm, sheltered water

• High turbidity due to mixing of fresh and salt water

• Examples include bays, lagoons, sloughs, sounds, and wetlands

Describe the morphology and formation of a delta.

• Forms at the mouth of a river where it meets the sea, on a broad continental shelf

• As the river reaches the sea it slows down dramatically

• Slower water can no longer carry sediment, which is deposited in large quantities

• Over time sediment builds up into landmasses and sandbars that extend into the sea

• More exposed to tidal action than an estuary

• Examples: Mississippi Delta and Nile Delta

What are tides and what causes them?

• Tides are the regular rise and fall of sea level

• Caused by the gravitational pull of the moon and, to a lesser extent, the sun

• Most locations experience two high tides and two low tides per day (semi-diurnal) approximately every 12.5 hours

• Some locations experience one high and one low per day (diurnal)

Define tidal range.

• Tidal range is the difference in height between high tide and low tide

• A large tidal range means a big difference between high and low tide levels

• A small tidal range means high and low tide levels are close together

What are spring tides and when do they occur?

• Spring tides occur during new moon and full moon phases

• The sun, Earth, and moon align in a straight line

• Their gravitational pulls combine, producing a stronger overall pull

• This results in a larger tidal range — higher high tides and lower low tides

What are neap tides and when do they occur?

• Neap tides occur during quarter moon phases

• The sun and moon are perpendicular to each other

• Their gravitational pulls partially oppose each other

• This produces a weaker overall gravitational pull and a smaller tidal range

State four factors that affect tidal range.

• Gravitational pull — spring tides produce a stronger pull and larger range than neap tides

• Coastline features — narrow channels concentrate water and increase tidal range (e.g. Bay of Fundy, Canada)

• Wind and air pressure — low air pressure allows water to swell upward; high winds push water toward shore causing tidal surges

• Size and volume of the water body — smaller enclosed bodies of water have a smaller tidal range

Define surface currents and state what drives them.

• Surface currents are continuous, steady movements of water in the upper ocean

• Driven primarily by wind, tidal influence, and the Coriolis effect

• They form large circular patterns called gyres

• They are predictable because they follow global wind patterns caused by Earth's rotation

Define deep ocean currents and what drives them.

• Deep ocean currents are driven by density differences caused by temperature and salinity

• This is called thermohaline circulation

• Cold, salty water is denser and sinks, driving deep circulation

• This is part of the global conveyor belt that moves water around the entire ocean over hundreds of years

Explain the Coriolis effect and its impact on ocean currents.

• The Coriolis effect is the deflection of moving water and wind caused by Earth's rotation

• In the Northern Hemisphere, currents are deflected to the right, forming clockwise gyres

• In the Southern Hemisphere, currents are deflected to the left, forming counter-clockwise gyres

• The effect is stronger further from the equator and negligible at the equator itself

Describe upwelling and explain why it is important.

• Upwelling is the movement of cold, nutrient-rich water from the deep ocean toward the surface

• Driven by winds blowing surface water away from a coast or by underwater topography such as seamounts

• Cold deep water rises to replace the surface water that was moved

• Important because it brings nutrients up into the photic zone

• This increases productivity and supports large food webs

Describe downwelling and explain what drives it.

• Downwelling is the movement of surface water downward into the deep ocean

• Driven by density — cold, salty surface water becomes dense enough to sink

• Carries dissolved oxygen from the surface down into deep water

• Occurs in polar regions where surface water cools and sea ice formation increases salinity

Describe normal conditions in the Pacific (non-El Niño).

• Trade winds blow from east to west across the Pacific

• These winds push warm surface water toward Australia and Asia

• Upwelling occurs along the coast of South America as cold, nutrient-rich water rises to replace displaced surface water

• High productivity off South America supports large food webs

• Warm water near Australia and Asia causes evaporation, bringing rainfall to those regions

Describe El Niño conditions and their effects.

• Trade winds slow down or stop

• Warm water that was pushed west stays near the coast of South America

• Upwelling is suppressed — no cold, nutrient-rich water reaches the surface

• Productivity decreases, food webs shrink, and fish populations decline

• Drought occurs in Indonesia and Australia

• Heavy rainfall and flooding occur in Peru and South America

• Stronger and more frequent hurricanes develop in the warmer Pacific waters

Describe La Niña conditions and how they compare to El Niño. A:

• La Niña often follows El Niño

• Trade winds become stronger than normal, blowing from east to west

• Even more warm water is pushed toward Australia and Asia

• Strong upwelling off South America — cold, nutrient-rich water rises

• Higher productivity and larger food webs off South America

• Drought conditions in South America, heavy rainfall in Australia and Indonesia

• La Niña is essentially the opposite pattern to El Niño