Oceanography - Chapter 3

Morphology of the Earth

• The more dense material which is heaviest is pulled by gravity to the center of the Earth

• Earth is divided into layers based on density—this process is called density stratification

What are the three layers of the Earth?

The crust, mantle and core

How much of Earth is composed of continental crust?

22%

How much of Earth is composed of oceanic crust?

78%

What is oceanic crust made of?

Basalt, the most common rock on Earth

What is continental crust made of?

Andesite and granite (lower density rocks)

List the layers of the Earth in order

Crust, mantle, outer core, inner core

What elements make up the inner core?

Iron, nickel and sulfur

Lithosphere

The solid, outer part of Earth. Includes the brittle upper portion of the mantle and the crust, the outermost layers of Earth's structure. Up to 100 km thick

Asthenosphere

The upper portion of the Earth's mantle that flows like molten plastic despite being solid. Known for its ability to deform plastically under stress. Extends to a depth of 700 km

Mesosphere

The lower portion of Earth’s mantle. The gases that comprise this layer continue to become denser as one descends. Extends to the core. This layer is hotter than the asthenosphere, but great pressure at this depth prevents it from flowing

Outer core

A fluid layer about 2,260 km (1,400 mi) thick, composed of mostly iron and nickel that lies above Earth's solid inner core and below its mantle

Inner core

The innermost geologic layer of the planet Earth. It is primarily a solid ball with a radius of about 1,220 km. It is about as hot as the temperature of the sun (5,430°C)

Continental drift

A hypothesis stated that the continents had once been joined to form a single super-continent. It was proposed that all landmasses were one big super-continent, called Pangaea (meaning “all earth”), and began to break apart 200 million years ago

Alfred Wegener

The scientist who proposed Continental Drift theory, but was unfortunately turned down by his colleagues at first

How did Pangaea break apart?

• Heat from the lower crust and mantle accumulated along certain lines in the Earth

• Pangaea began to warp, stretch and crack

• Magma rose up through cracks

• Laurasia and Gondwana Land began to separate as the crack widened

• Water from Panthalassa flowed into the rifts

Panthalassa

The vast super-ocean that encompassed planet Earth and surrounded the super-continent Pangaea

Evidence for Continental Drift Theory

• The continents appeared to fit together as the shapes lined up

• Fossils of the same species found in one continent (ex. Africa) were found in other continents (ex. South America)

• Areas of erosion due to separation of glaciers

• Rock evidence for continental drift exists in the form of several mountain belts that end at one coastline, only to reappear on a landmass across the ocean

• Signs of glaciers in regions that are now tropical

• Signs of tropical plants in regions that are now polar

How did scientists find evidence for internal layers of the Earth?

• Scientists measured seismic waves

• Time, location of arrival, changes in frequency and strength reveal details about nature of interior

• Scientists used a seismometer

• Data from earthquake near Anchorage Alaska, 1964

Laurasia

A super-continent of the Northern Hemisphere made up of the landmasses that currently correspond to North America, Greenland, Europe, and Asia (except India). One of the two super-continents that made up Pangaea

Gondwana

This was a large landmass, sometimes referred to as a super-continent. The remnants make up around two-thirds of today's continental area, including South America, Africa, Antarctica, Australia, Zealandia, Arabia, and the Indian subcontinent. One of the super-continents that made up Pangaea

Fault

a fracture in a rock mass along which movement has occurred

Fracture

a crack or breakage

Magma

Liquid, molten rock. If it erupts onto the earth’s surface, it may also be called lava

Seafloor spreading theory

A theory suggested first by Harry Hess (early 1960’s)

Continental drift

Theory developed by Alfred Wegener (1911)

Plate tectonics theory

A combination of both continental drifting and seafloor spreading

Ocean ridge

Mountain systems often found in the center of deep ocean basins

Rift valley

A linear shaped lowland between several highlands or mountain ranges produced by the action of a geologic rift

Subduction

The sideways and downward movement of the edge of a plate of the earth's crust into the mantle beneath another plate

Mid-Atlantic Ridge

An extensive submarine mountain system that runs down the length of the Atlantic Ocean. In its center, also running down its length, is the Central Rift Valley

Seafloor spreading

A process that occurs at mid-ocean ridges, where new oceanic crust is formed via magma from within the Earth rising and spreading outwards. New seafloor remains close to the ridge while older seafloor is pushed away from the ridge

What are the seven major tectonic plates of Earth?

• The Pacific Plate

• The North American Plate

• The Eurasian Plate

• The African Plate

• The Antarctic Plate

• The Indo-Australian Plate

• The South American Plate

How much do tectonic plates move each year?

2 inches per year (2 in/year)

Divergent boundaries

(Also called spreading centers.) Boundaries between two plates in which the plates are moving apart

Divergent boundary: (Continent-continent crust) Continental rifts

When spreading centers develop within a continent, the landmass may split into two or more smaller segments, forming a rift

Convergent boundary

Boundaries between two plates in which the plates are pushing against each other

Subduction zone

Area where one oceanic plate is forced down into the mantle beneath a second plate

Divergent boundary: Oceanic-oceanic

New oceanic crust and a rift is formed at the spreading center

Ex. Mid-Atlantic Ridge

Convergent boundary: Oceanic-continental

• Denser oceanic slab sinks into the asthenosphere

• Pockets of magma develop and rise

• Continental volcanic arcs form in part by volcanic activity caused by the subduction of oceanic lithosphere beneath a continent

  • Forms trench and earthquakes at subduction zone

• Examples include the Andes, Cascades, and Sierra Nevadas

Convergent boundary: Oceanic-oceanic

• Two oceanic slabs converge and one descends beneath the other; Older cooler plate slips beneath, Subducted crust melts, magma rises

  • Forms deep trench and strong earthquakes, tsunamis

• This kind of boundary often forms volcanoes on the ocean floor

• Volcanic island arcs form as volcanoes emerge from the sea, Ex. Earthquake in Anchorage Alaska, 1964; Japan, 2011

• Examples include the Aleutian, Mariana, and Tonga Islands

Convergent boundary: Continental-continental

• When subducting plates contain continental material, two continents collide.

• Neither mass is subducted

• Plate edges compress, fold, uplift

• This kind of boundary can produce new mountain ranges, such as the Himalayas.

Transform Fault Boundaries

Margins where two plates slide side by side against each other in opposite directions. (Ex. San Andreas Fault) Plates grind past each other without destroying the lithosphere.

•  Most join two segments of a mid-ocean ridge

• At the time of formation, they roughly parallel the direction of plate movement

• They aid the movement of oceanic crustal material

What are the eight pieces of evidence used to confirm the plate tectonics theory?

• Paleomagnetism

• Cores of ocean floor

• Filming of seafloor cracks

• Hotspots, atolls, guyots

• Sediment age & distribution

• Oceanic ridges

• Heat flow

• Biological evidence

Paleomagnetism

The natural magnetism found in rock bodies; this permanent magnetization acquired by rock can be used to determine the location of the magnetic poles at the time the rock became magnetized

Normal polarity

When rocks show the same magnetism as the present magnetism field

Reverse polarity

When rocks show the opposite magnetism as the present magnetism field

Hot spots

A concentration of heat in the mantle capable of producing magma, which rises to Earth’s surface

Atoll

A ring-shaped coral reef, island, or series of islets. Surrounds a body of water called a lagoon. Sometimes protect a central island

Guyots

(Also called table mounts.) A chain of odd, flat-topped, submerged volcanic mountains

• Volcanoes were carried away from spreading center, became inactive and were submerged

• Wave action shaves tops flat

• Can form from atolls when submerge too low too fast for coral to survive

Seamounts

Have a similar origin to guyots, but retain their pointed shape because they never reached the surface

Age and Distribution of Sediments

• Core sampling and dating of ocean floor near spreading centers and subduction zones show younger ocean floor near spreading center and older near subduction zones

• Consistent with plate tectonics idea that ocean basins are continuously created and destroyed

Oceanic Ridges

• Volcanic nature of ridge islands like Iceland

• Shape of longitudinal rifts

• Sinking of seabed as new oceanic crust cools and travels outward

• Distribution of transform faults and fracture zones along oceanic ridges

• All support plate tectonics theory

Heat Flow

• Heat escapes rapidly from ocean bottom at mid-ocean ridges suggesting hot new crust

• Slowly in vicinity of trenches suggesting older crust

• Predicted by plate tectonics

• Plateaus, isolated segments of seafloor, ocean ridges, ancient island arcs and parts of continental crust that collect on the face of a continent are called terraces

Biological Evidence

• Distribution of fossils of the same identical intricate organisms on different continents is extremely unlikely unless they were once attached

• Sea turtles’ navigational skills have evolved along with plate movement so they could successfully migrate to breeding places