The Sea Floor Chapter 2

The Water Planet   Habitats are shaped by geological processes  o Form of coastlines  o Depth  o Type of bottom – sandy, muddy, rocky

Case in Point Satellite images of Aceh Province, Indonesia before (left) and after (right) the December 2004 tsunami destroyed coastal villages. Note areas denuded of vegetation along the coast.

The Water Planet   Earth has lots of liquid water  Most other planets have little water  Oceans play major role in weather  Without water life would not be possible  71% of the earth is covered by water  2/3 of the land on earth is found in the Northern Hemisphere

5 Major Ocean Basins  Pacific – deepest and largest (as big as the others combined)  Atlantic – 2 nd Largest  Indian - similar depths to Atlantic  Arctic – smallest, shallowest  Southern – 4 th Largest – not universally recognize as an ocean

The World’s Oceans

World Ocean  The oceans are interconnected – not separate  The connections allow sea water, materials, and some organisms to move from one to another

Big Bang Earth is about 4.6 billion years old  Big Bang Theory - 15 billion years ago  Materials settled according to their density

Density Density is mass of a given volume  Which has more volume, a pound of lead or a pound of feathers?

Density  Denser materials tend to sink  Densest material flowed to the center of the earth  Lighter material floated on the surface.  Light material cooled and formed a thin crust

Core  Innermost layer – core o Mostly iron alloys o Solid inner, Liquid outer core O High pressure o 4000 degrees Celsius

Mantle – outside core * solid  very hot  flows like liquid, but much slower  Crust  Outermost  Very thin  Like rigid skin floating on the mantle

Minerals  Oceanic crust – minerals called basalt  Continental – granite  Oceanic crust is denser  Oldest oceanic rocks are less than 200 million years old * Continental rocks – up to 3.8 billion years

Continental Drift  1620 Francis Bacon  Noted the continents fit together like pieces of a puzzle  Coal deposits and other geological formations match up on opposite sides of the Atlantic  Fossil evidence also supports this idea

Continental Drift  Alfred Wegener – proposed “continental drift”  proposed that all the continents were once joined in a single “supercontinent”  Pangea  Started breaking up 180 million years ago

Plate Tectonics  What causes the continents to drift? A process called plate tectonics

Mid Ocean Ridge  Sonar surveys revealed a mid ocean ridge system  Continuous chain of volcanic submarine mountains that circle the earth  Largest geological system on the planet  At regular intervals – transform faults  Sometimes the ridges are so high they break the surface like iceland  Mid-Atlantic Ridge – runs down the middle of the Atlantic Ocean

Ridges

Ridges & Trenches  Great deal of activity around the ridges  Earthquakes around the ridges  Volcanoes at the trenches  Scientists found that the farther you get from the ridge, the thicker the sediments get and the older the rocks

Earth’s Magnetic Field  The earth’s magnetic field reverses a few times every million years  During the shift, a compass would point to the magnetic “north” – that is what is now the south pole.  Cause of the reversals is unknown, but thought to be related to changes in movement of material in the outer core

Magnetic Anomolies  Many rocks are magnetic. If they are in the molten outer core, they are free to point to magnetic north  When the rocks cool, they stay in the same position  Geologists found a pattern of magnetic stripes or bands in the mid ocean ridge system.  Symmetrical around the ridge – mirror images  The magnetic bands are called “magnetic anomalies”

A – 5 mya B – 3 mya C- present day

What did that show?  Shows the sea floor was not formed all at once, but in strips along the mid-ocean ridge

Creation of the Sea Floor  Magnetic anomalies together with other evidence led to the understanding of plate tectonics  At the ridges, large pieces are separating  As the pieces separate, they form a crack or “rift”   This releases pressure and some material rises through the rift  The ascending material pushes up and forms a ridge  The process is called sea-floor spreading

Sea Floor Spreading and Plate Tectonics  Lithosphere – “Rock Sphere”  Broken into plates called lithospheric plates

Plates  The plates are about 100 km thick   Plates contain oceanic crust, continental crust or both  The Mid-Ocean Ridges form the edges of many of the plates

How Fast Do They Move?  The plates move at between 2 and 18 cm per year  Human fingernails grow at 6 cm per year

Subduction  As lithosphere is created at the ridges, it must be destroyed somewhere else  It is destroyed at the trenches  Trenches are formed when 2 plates collide and one sinks below the other  The downward movement is called subduction  Trenches are sometimes called “subduction zones”

??? Oceanic plate always descends into the mantle when colliding with a continental plate. Why?

Answer Because the continental plate is less dense

A Record in the Marine Sediments  2 major types of sediments are found in the sea.  Lithogenous sediment - Physical and chemical breakdown or weathering of rock  Biogenous Sediment  Skeletons and shells of marine organisms  Calcium carbonate – calcareous ooze  Silica – siliceous ooze

A Record in the Marine Sediments  Most sediment is in the form of microfossils  Tell what kind of organisms lived there in the past  Clues to environment – cold, warm for example

Guyot Shelf Break

Deep Ocean Floor  Abyssal plain – flat, 3000 to 5000 feet deep  Dotted with seamounts – volcanoes  Flat topped seamounts – guyots  Hydrothermal vents – areas where heated seawater are forced up through the crust

Hydrothermal Vents  Water is up to 350º C (660º F)  Mainly sulfides are dissolved in the water and form black smokers  Black smokers are chimney like structures that build up as minerals solidify  Rich marine life