Marine ScienceChapter 2 – The Sea Floor

FOCUS QUESTIONS

I. What are the major physical features of the Earth’s surface?

II. What explanation have scientists developed for the existence and distribution of these features?

III. How do these features affect the Earth’s oceans?

Our Fledgling Planet

• Earth – approximately 4.5 billion years old• Formed from an aggregation of dust clouds

and gas particles that condensed into solid matter.

• Intense heating melted many of these heavier elements in the core and mantle of the planet.

• The uppermost of layers cooled and formed the Earth’s crust.

The Structure of the Earth Materials have settled in the planet

according to their density.

Lithosphere (crust – very thin)Asthenosphere (upper mantle – fluid)

Core (mixtures of iron – inner is solid, outer is molten)

Mesosphere (mid to lower mantle – solid but very hot)

d = m/V

All Crust is not the Same

• Continental crust is composed of granite.• Oceanic crust is composed of basalt.• Continental crust is:

– lighter– thicker (20-50 km vs. 5 km for ocean)– less dense – geologically older

… than Oceanic crust.See Table 2.2 on page 24

Hypothesis of Continental Drift

• Developed by Alfred Wegener in 1915.• Proposed that present continents had drifted

apart after the breakup of a single supercontinent, Pangaea.

• Pangaea animation

Theory of Plate TectonicsPlate tectonics – the study of the processes by which the

lithosphere moves laterally across the asthenosphere.

This hypothesis is supported by:• presence of the mid-ocean ridge• comparative fossil data from different continents• apparent puzzle-piece shape of different continents• magnetic reversal and sediment patterns on the sea floor• distribution of geological features on the Earth

Plate Tectonics in Action

• Plates “float” on the asthenosphere.

There are areas where plates spread apart…

…and areas where plates collide.

Mid-ocean Ridges/ Rifts Subduction zones/ Trenches

Types of Plate Boundaries

http://pubs.usgs.gov/publications/text/Vigil.html

Oceanic-Continental Boundaries• Heavy oceanic crust slides

under continent.• Called subduction.• Trench forms.• Causes explosive

earthquakes.• Molten lithosphere seeps

up through continental crust and forms volcanoes.

• Oceanic Subduction Animation

Oceanic-Oceanic Boundaries• One plate will slide

under the other.• Trench forms.• Slow-flowing

volcanoes form – sometimes break the

ocean surface (island arcs).

– Examples:• Aleutian Islands, AK• Mariana Islands

Continental-Continental Boundaries

• Two continents smash against one another with force and become “welded” together.

• Crust buckles forming mountain ranges.

• No trenches or volcanoes.

India Collision Animation

Californians Get Closer Together• In addition the Pacific

Plate (which contains the city of Los Angeles) and the North American Plate (which contains San Francisco) share a shear boundary.

• These two plates slide past each other such that these cities get 4 inches closer each year.

Major Features of the Sea Floor

World Distribution of Volcanoes & Earthquakes

2002-2003 Annual Seismic Activity at Mt. St. Helens, WA

The Mid-Ocean Ridge• A continuous chain of submarine

volcanic mountains that circles the globe.

• Displaced at regular intervals by transform faults.

• Occasionally breaks the surface at places like Iceland and the Azores.

• Examples:– Mid-Atlantic Ridge– East Pacific Rise

Above: Mid-Atlantic Ridge Left: Iceland –showing ridge

Above: The Glomar Challenger was the first research vessel specifically designed in the late 1960s for the purpose of drilling into and taking core samples from the deep ocean floor.

Rift Valleys• A gap or depression

resulting in the center of a mid-ocean ridge or on land.– African Rift Valley

Features of Mid-Ocean Rifts

• Hydrothermal vents – deep water hot-springs that dissolve minerals in the Earth’s crust (mainly sulfides)

• Black smokers – chimney-like structures that build up deposits of minerals and “smoke” a cloud of mineral particles.

Comparative Fossil/Sediment Data

Geologists noted the similarities between the fossils and sediment deposits found on opposite sides of the Atlantic Ocean.

Magnetic Reversals

• Rocks contain magnetic particles that align with the Earth’s magnetic field.

• During periods called magnetic reversals, the Earth’s north pole switches to the opposite side of the globe as it is now.

• Believed to be related to movements in the Earth’s molten core.

Magnetic Anomalies

• When molten rock comes to the surface, the magnetic particles are free to align with the Earth’s magnetic pole.

• When the rock cools, the particles are fixed in the magnetic orientation at the time.

Magnetic Anomalies

• Geologists have thus discovered a pattern of magnetic banding parallel to the mid-ocean ridge.

Geological “Hot Spots”• Occur in over 100

places on the globe• Areas where hot

magma sporadically forces its way through the lithosphere to erupt in volcanic activity.

• Examples: – Hawaiian Islands– Yellowstone National

Park

Hot Spot Formation

Although this model has been contested, it describes a stationary magma source over which plates slide.

Seafloor Topography

• Continental shelf• Continental edge /

Shelf break• Continental slope• Abyssal plain• Ocean ridge and

rise systems• Trenches• Seamount

Global Seafloor Topography

Seafloor Topography

Geological Provinces of the Ocean FloorA. Continental shelf – The shallow gently sloping section of the

continental margin that extends from the shore to the point where the slope gets steeper.

B. Shelf break – The section of the continental shelf where the slope abruptly becomes steeper, usually at a depth of 120 to 200 m (400 to 600 ft).

C. Continental Slope – The steeper, seaward section of the continental margin.

D. Continental Rise –The gently sloping area at the base of the continental slope.

E. Abyssal Plain – The nearly flat region of the deep-sea floor.F. Guyot – A flat topped- seamountG. Trench – A narrow deep depression in the sea floor.H. Seamount – A submarine volcano in the abyssal plain

Continental Margins

• Active Margin – a continental margin that is colliding with another plate and as a result is geologically active

• Passive Margin – a continental margin that is located at the trailing edge of a continent and as a result shows little geological activity

Further Evidence of Earth’s Changing Nature

• Sediments• Sea Level Changes• Greenhouse Effect

Origin of the Atmosphere

• Originally free oxygen not present in the atmosphere.

• 600 mya – O2 produced by single-celled photosynthetic organisms (cyanobacteria)

• Stromatolites

Photo credit

http://www.discoverwest.com.au/australia-images/l741b.jpg

Stromatolites – Shark Bay, WA

http://www.ldeo.columbia.edu/~small/personal/Images/Stromatolites.jpg

http://www.discoverwest.com.au/album/northwest/north_west_map.gif

Photo credits

Results of an O2 Rich Atmosphere

• Aerobic organisms began to out-compete anaerobes.

• Excess oxygen begins to react in the high atmosphere with ultraviolet radiation. This creates ozone (O3)– prevents harmful UV rays from penetrating to the earth. – Allows organisms to colonize the land.

• Also, Earth ideally settled in an orbit where molecular water can exist in all three states – particularly the liquid state!

Sediments

• Lithogenous sediment – results from the physical and chemical weathering of rocks on land.– Carried by run-off and rivers to the ocean.

• Biogenous sediment – skeletons and shells of marine organisms that have fallen to the ocean floor.– Diatoms, radiolarians, foraminiferans, and

coccolithophorids

Biogenous sediments

• Calcareous ooze – skeletons composed of calcium carbonate (CaCO3)

• Siliceous ooze – skeletons composed of silica (SiO2).

Sea Level Change

• Interglacials – warm periods during which freshwater melts and sea level rises.

• Ice ages – cold periods when large amounts of water are stored in ice caps and glaciers and sea level drops.

Greenhouse Effect

• Global temperatures and the rate of glacial melting are rapidly increasing.–Still uncertainty as to whether this is a

natural phenomenon or whether it has been accelerated by humans.

References• Castro, P. & J. Huber (2005) Marine Biology, 5th ed.

McGraw-Hill Higher Education, Boston, MA.• Watson, J.M. (1999) “Understanding plate motions.”

USGS. Retrieved on September 27, 2004 from http://pubs.usgs.gov/publications/text/understanding.html

• Watson, J.M. (1999) “Understanding plate motions.” USGS. Retrieved on September 27, 2004 from http://pubs.usgs.gov/publications/text/hotspots.html

• http://www.ngdc.noaa.gov/mgg/fliers/97mgg03.html

Global projection of Earth showing tectonic boundaries and areas of active seismic and vulcan activity.