Chapter 2 The Sea Floor

Geologic Processes Sculpt shorelines Determine water depth Control whether bottom is muddy, sandy, or rocky Create new islands and undersea mountains Determine the nature of marine habitats

Ocean Basins World Ocean: all oceans –71 % of the planet Pacific Ocean –Deepest and largest –64 mil sq miles –Avg depth= 13,741 ft –Mariana Trench = 36,163ft Atlantic Ocean –33 mil sq miles –Avg depth= 12,258 ft Indian Ocean –28 mil sq miles –Avg depth= 12,704 ft Arctic Ocean –3.7 mil sq miles –Avg depth =4,364ft

History of Earth Big Bang ~13.7 billion years ago Dust particles collided to form larger planets Earth ~4.5 billion years ago Materials settled according to density Densest= center Lightest = thin crust Atmosphere and oceans formed

Internal Structure Core Innermost layer Mostly iron Pressure is 1 million times more than the surface Over 7,200 degrees F Solid inner core Liquid outer core Produce magnetic field Mantle Outside Earth’s core Solid, but hot Flows slowly Crust Outermost Floats on mantle

Continental and Oceanic Crusts Oceanic Sea floor Basalt: mineral that makes up sea floor –Dark in color Denser than continental –Floats lower in mantle Thinner Oldest= less than 200 million years old Continental Land rocks Granite: different than basalt –Lighter in color 3.8 billion years old

Evidence of Continental Drift Sir Francis Bacon (1620) Coasts in Atlantic ocean fit together like a puzzle Coal deposits match Fossils similar Alfred Wegner (1912) Continental drift: continents were once joined in a single “supercontinent” –Pangaea –180 million years ago Couldn’t explain HOW

Theory of Plate Tectonics The main features of plate tectonics are: –The Earth’s surface is covered by a series of crustal plates –The ocean floors are constantly moving; spreading in the center and sinking at the edges and being regenerated. –Convection currents beneath the plates assist movement –Heat from the mantle drives these currents

Evidence of Plate Techtonics Mid-Ocean Ridge System Continuous chain of submarine volcanic mountains that encircles the globe –Largest geologic feature on earth Earthquakes clustered at ridge Volcanoes near trenches Rock near ridge is young Further away is older and consists of more sediment

Runs down the center of the Atlantic –Forms a Y in the Indian Ocean main section of ridge in Eastern Pacific Faults East Pacific Rise Faults: cracks that displace the ridge Transform faults: in earth’s crust High submarine mountains form islands Mid-Atlantic Ridge

Trenches System of deep depressions in the sea floor –Common in the Pacific

Earth’s Magnetism Magnetic field reverses about every 700,000 years –5,000 years to occur –Movement of material in outer core Magnetic Anomalies

Creation of Sea Floor

Sea-Floor Spreading

Changing Oceans

Lithogenous sediments – derived from the break-down of rocks (weathering) Biogenous sediments – derived from the skeletons and shells of marine organisms Marine Sediments

Biogenous sediments (composed of microfossils) can reveal information from the past Ocean temperature at the time the microfossils were formed can be determined by looking at the ratios of magnesium to calcium or by different ratios of oxygen isotopes What Can We Learn from Sediments?

The margins of continents are boundaries between continental crust and oceanic crust They generally consist of: –Continental shelf (most landward) –Continental slope –Continental rise (most seaward) Continental Margins

Makes up about 8% of the ocean’s surface area However, it is biologically the richest area of the ocean The width varies from 1 km (0.6 mi) to 750 km (470 mi) Shelf ends at shelf break which occurs at a depth of 120 – 400 m ( ft) The Continental Shelf

Can be thought of as the “edge” of a continent Begins at the shelf break and continues to deep sea floor Much steeper than continental shelf and continental rise Continental Slope

Formed by sediments that have been pushed down from continental shelf and slope It can be thought of as an underwater river delta (the river in this case is formed of sediments) Continental Rise

Passive margins: –Relatively inactive geologically –Characterized by flat, wide coastal plains, wide continental shelves and gradually sloping continental slopes –Example: East Coast of US Types of Margins

 Active Margins:  Sites of more intense geologic activity including earthquakes, volcanoes and trenches  Characterized by steep, rocky shorelines, narrow continental shelves and steep continental slopes  Example: West Coast of US Types of Margins

Climate and Changes in Sea Level

Deep Ocean Basins

Mid-Ocean Ridge

Hydrothermal Vents