The Planet Oceanus

2-1The Earth’s Structure

Earth consists of a series of concentric layers or spheres which differ in chemistry and physical properties.The compositional layers of the Earth are the Crust, the Mantle, and the Core. The Core is subdivided into a molten outer core and solid inner core.

Note that Earth is density stratified, that is, each deeper layer is denser than the layer above.

Layered Earth - Chemical Properties

Chemical Properties of Earth’s LayersLayer Chemical PropertiesContinental Crust Composed primarily of granite

density = 2.7 g/cm3

Oceanic Crust Composed primarily of basaltdensity = 2.9 g/cm3

Mantle Composed of silicon, oxygen, iron and magnesiumdensity = 4.5 g/cm3

Core Composed mainly of ironDensity = 13 g/cm3

So, when we examine the chemical and physical properties of Earth’s layers, we see that a cool, rigid, less dense layer (the lithosphere) floats on a hot, slowly-flowing, more dense layer (the asthenosphere).

Layered Earth - Physical Properties

Physical Properties of Earth’s Layers Layer Physical Properties Lithosphere The cool, rigid outer layer Asthenosphere Hot, partially melted layer which flows slowly Mantle Denser and more slowly flowing than the

asthenosphere Outer Core Dense, viscous liquid layer, extremely hot Inner Core Solid, very dense and extremely hot

What evidence supports the idea that Earth has layers?

The behavior of seismic waves generated by earthquakes give scientists some of the best evidence about the structure of Earth.

(above-left) S waves cannot penetrate Earth’s liquid core.

(above-right) P waves are bent as they pass through the liquid outer core.

Layered Earth - Evidence Of Earth’s Layers

Physical state is determined by the combined effects of pressure and temperature.

• Increasing pressure raises the melting point of a material.

• Increasing temperature provides additional energy to the atoms and molecules of matter allowing them to move farther apart, causing the material to melt.

• Both pressure and temperature increase toward the center of the Earth, but at different rates.

• Divisions of the Earth based upon physical state are the Lithosphere, The Asthenosphere, the Mesosphere, the Outer core, and the Inner core

2-1The Earth’s Structure

Three fluid spheres surround the rocky portion of the Earth.

• Hydrosphere includes all of the “free” water of the Earth contained in the ocean, lakes, rivers, snow, ice, water vapor and groundwater.

• Atmosphere is the gaseous envelope that surrounds the Earth and is mainly a mixture of nitrogen and oxygen.

• Biosphere refers to all living and non-living organic matter.

2-1The Earth’s Structure

The air on earth is held by gravity, pressure, and density accordingly decreases with pressure. Log P = -0.06 (H), where P is pressure and H is altitude in km. At 700 km above sea level one encounters an almost perfect vacuum.By definition boundary of atmosphere set at 1000 kmExosphere-the sphere above the atmosphere - is where molecules escape into space and have to pass through the magnetosphere

Physiography and bathymetry (submarine landscape) allow the sea floor to be subdivided into three distinct provinces: continental margins, deep ocean basins and midoceanic ridges.

2-2The Physiography of the Ocean Floor

•Continental margins are the submerged edges of the continents and consist of massive wedges of sediment eroded from the land and deposited along the continental edge. The continental margin can be divided into three parts: the Continental shelf, the Continental slope, and the Continental rise.

•Deep Ocean Province is between the continental margins and the midoceanic ridge and includes a variety of features from mountainous to flat plains: Abyssal plains, Abyssal hills, Seamounts, and Deep sea trenches

Midoceanic Ridge Province consists of a continuous submarine mountain range that covers about one third of the ocean floor and extends for about 60,000 km around the Earth

Continents and ocean basins differ in composition, elevation and physiographic features.

• Elevation of Earth’s surface displays a bimodal distribution with about 29% above sea level and much of the remainder at a depth of 4 to 5 kilometers below sea level.

• Continental crust is mainly composed of granite, a light colored, lower density (2.7 gm/cm3) igneous rock rich in aluminum, silicon and oxygen.

• Oceanic crust is composed of basalt, a dark colored, higher density (2.9 gm/cm3) volcanic rock rich in silicon, oxygen and magnesium.

2-3Geologic Differences between Continents and Ocean Basins

•The moho is the boundary between rocks of the crust and the denser (3.3 gm/cm3) rocks of the mantle.

2-3Geologic Differences between Continents and Ocean Basins

Continents and ocean basins differ in composition, elevation and physiographic features.

2-3Geologic Differences between Continents and Ocean Basins

Isostacy refers to the balance of an object “floating” upon a fluid medium. Height of the mass above and below the surface of the medium is controlled by the thickness of the mass and its density (similar to ice floating in water).

• Greater the density of the mass, the lower it will sink in the medium.

• Greater the thickness of the mass, the higher a portion of it will rise above the medium.

• Continents are thick (30 to 40 km), have low density and rise high above the supporting mantle rocks.

• Sea floor is thin (4 to 10 km), has greater density and does not rise as high above the mantle.

2-3Geologic Differences between Continents and Ocean Basins