Earth s Structure and Surface
Structure of the Earth The earth is thought have originated about 4.5 billion years ago from a cloud or clouds of dust. The dust was the remains of a huge cosmic explosion which is estimated to have occurred about 15 billion years ago.
As the particles collided they continually formed larger and larger rocks until eventually forming the earth and other planets.
Density On a young earth, most matter was molten. This allowed for denser objects to sink and less dense objects to settle on the surface. The light material cooled to make a thin crust. Eventually the atmosphere and oceans began to form.
Earth s Surface More than 70% is covered by water. Water distribution: Pacific Ocean = 52% Atlantic Ocean = 25% Indian Ocean = 20% Ice = 2% Lakes, Rivers, Atmosphere = 1%
Earth s Surface Average Depth of the oceans = 3800m *deepest is Mariana Trench = 11035m Average Elevation of land = 840m *highest area is Mt. Everest = 8848m
Internal Structures
The innermost layer of the earth, the core, is comprised of mostly alloys of iron. Core: Estimated to be over 4,000 C (7,200 F)
Mantle The layer outside of the core is called the mantle. Though most is thought to be solid it is very hot, near the melting point of the rocks. Because of this the mantle flows like a liquid but much slower.
Crust The outermost layer, and most familiar, the crust is relatively very thin like a rigid floating on the mantle. The composition of the crusts greatly differ greatly between the oceans and the continents.
Continental & Oceanic Crusts Differences between the two crusts are caused by the physical and chemical differences in the rocks themselves. Oceanic crust consists of minerals called basalt while the continental rocks consist of granite. This results in a several characteristic differences
Continental vs. Oceanic Crusts Continental Oceanic Density: 2.7 g/cm3 Density: 3 g/cm3 20 to 50 km thick 0-10 km thick Can be very old Geologically young Light in color Dark in color Rich in Na, Ca, & Al Rich in Fe and Mg
Theory of Plate Tectonics about 55% of the mass of the continents is submerged into the upper mantle. however, because the crust cannot react to changes in its weight, when forces of uplift or downbending exceed mechanical strength, the rock fractures along a plane of weakness (fault).
Origin and Structure of the Ocean Basins As early as 1620 Sir Francis Bacon noted that the coasts of continents on opposite sides of the Atlantic fit together like a puzzle.
Pangaea (225 MYA) Evidence mounted to support this observation. Coal deposits matched and fossils could be collected and matched from each side. In 1912, Alfred Wegener suggested that all the continents had once been joined in a single supercontinent.
Theory of Plate Tectonics Wegener s theory was not widely accepted because he could not supply a mechanism that accounted for the motion of the continents. In the 1950/60s the evidence came together and scientists concluded that the continents did drift. This process is called plate tectonics.
Theory of Plate Tectonics much material is turned aside under the crust and moves laterally, causing seafloor spreading which provides the force to cause continental drift.
proof for sea-floor spreading and 3 sources: plate tectonics 1) steadily increasing age of the sedimentary rocks which rest on the crust with increasing distance from the ridges (radiometric dating) most rocks have small amounts of radioactive elements (uranium, thorium, potassium) which decay into atoms of other elements
proof for sea-floor spreading and plate tectonics 2) paleomagnetization patterns in the basaltic rocks - crust retains the magnetic pattern (like a tape recording) on either side of the ridge. - can date these patterns to the timescale of geomagnetic reversals, which occur at irregular intervals of every 300,000 to 500,000 years
proof for sea-floor spreading and plate tectonics 3) polar wandering: if the direction of magnetic north is plotted for each continent and it is assumed that the continents remained stationary throughout time, then it appears that the earth's magnetic poles have wandered from the present locations relative to the earth's north-south axis (path plotted is called the polar wandering curve)
proof for sea-floor spreading and plate tectonics However, because rocks of the same age point to two different locations for magnetic north and because all evidence points to the magnetic poles remaining close to the geographic poles, then it is the continents that have moved.
Provinces of the Ocean The sea floor is divided into two main categories: The deep-sea floor and the continental margins. Continental margins are the boundaries between continental crusts and oceanic crusts.
Continental Margin
Continental Margin Generally consists of a gently sloping shelf, a shelf break, a stepper continental slope, another gently sloping region called the continental rise at the base of the continental slope.
Margins Passive Active
Passive Margins Relatively inactive margins that typically have flat coastal plains, wide shelves, and gradual continental slopes.
Active Margins Active Margins are regions where the continental margins are highly active with volcano and earthquakes. They are identified by little to no shelf and a steep slope ending in a trench, no rise.
Mid-Ocean Ridge In the years after World War II, sonar surveys of the sea floor resulted in the discovery of the mid-ocean ridge system. This system is a continuous chain of volcanic submarine mountains that encircles the earth like the seams on a baseball.
When these under water volcanoes occur in the middle of a oceanic plate they are called a hot spot and are responsible for island chains like Hawaii.
Comprehension Check 1) The three main divisions of the Earth s interior are,, and. 2) A long chain of submarine volcanic mountains that runs throughout the world ocean is called the system. 3) A single super continent called began to break up onto the present-day continents about years ago. 4) The continental margins consists of,,, and. 5) The average depth of the ocean is m.