Earth’s Interior Natural Disasters: Part B

Earth’s Spheres & Systems

What is the solid Earth made of? Planet Earth is a rocky planet. It is made up of rocks and minerals. This sphere is called the “geosphere”. The Earth is made up of a number of elements. The interior of the Earth is quite different than the exterior (continental crust).

The Earth can be compared to a hard-boiled egg. Its major layers (crust, mantle, core) are like the shell, egg-white and yolk of an egg. The deepest man has dug into the Earth is 5 km in the South African gold mines. (This is within the crust.) Here the temperature increases by 10 to 15 ˚C for every kilometer down. We have not actually been to the center of the Earth. So how can we infer what the composition of the Earth’s interior is like?

Earthquakes: Seismic Waves If Earth were made of the same materials throughout, seismic waves (energy released from earthquakes) would travel in straight lines at a constant speed. However, seismic waves speed up and refract (bend) when they reach the mantle as it is more dense (due to increased pressure). Scientists can infer the composition of the Earth’s interior based on the behavior of seismic waves.

Uncertainties: This is our “best guess” as we have never actually seen the mantle or core. Our “best guess” scenario cannot explain the location of “hot spots” and why they occur where they do. We think rock within the asthenosphere (mid to lower mantle) melts due to a decrease in pressure of surrounding rock. However, our “best guess” cannot explain what causes this change in pressure.

Earth’s Interior based on Seismic Wave Behavior: The Earth is divided into 3 concentric layers: The crust (upper Lithosphere) The mantle (lower Lithosphere and upper Asthenosphere) The core Each layer has different properties.

The Crust The crust is a very thin skin which surrounds the planet (like the shell on an egg). There are two types of crust: Oceanic crust: which is thin (≈7 km) and is made up of dense volcanic rock (basaltic). Oceanic crust is “young” (180 million years old). Continental crust: which is thick (≈ 35 km) and is less dense than oceanic crust. It is rich in quartz and feldspar (granitic) and much older (4 billions years old). It is thicker under mountains.

The Lithosphere The crust and the upper part of mantle form the Lithosphere. It is fragmented into a series of segments called plates. These plates move with respect to each other and are the cause of most earthquakes and volcanoes. The lithosphere is usually 100 km thick.

The Mantle The mantle is located between the crust and the core. It is mainly solid, but acts as a “fluid-like” solid (putty). The upper part of the mantle is cooler. This part of the mantle is joined together with the crust to form the lithosphere. Its temperature reaches about 1300˚C. The mantle, directly beneath the lithosphere, is much hotter. This is called the asthenosphere. Its temperature reaches about 1700 ˚C. The lithosphere slides over the asthenosphere as it is less dense. The area below this is called the lower mantle

Convection currents in the mantle We think the mantle acts like hot, sticky tar (putty-like). It is heated by the core below it (decay of radioactive elements and enormous pressure create intense heat) and cooled by the crust above. When you heat a fluid, it’s density drops and it expands. If you cool it, it will contract and become more dense. Therefore, the hotter material at the base rises toward the top while the cooler material at the top sinks. This generates slow flowing convection currents in the mantle.

The Core The core is divided into two parts: outer core which is liquid inner core which is solid The core is made up of mainly iron and nickel which makes it very dense. The movement of the liquid outer core over the solid inner core generates the Earth’s magnetic field. (?)

Earth’s Magnetic Field Like a bar magnet, the Earth has two magnetic poles. The lines of force of the Earth’s magnetic field extend between the North Magnetic Pole and the South Magnetic Pole. The source may be the liquid iron in the Earth’s outer core. Because iron is a good conductor, scientists hypothesize that motions within the core produce electrical currents that in turn create the Earth’s magnetic field.

Uncertainties: Research has shown that both the sun and moon have magnetic fields as well. The sun contains no iron and the moon does not have a liquid core. Discovering the sources of the magnetic fields of the sun and moon may help scientists identify the source of the Earth’s magnetic field.

Magnetic Reversal

The interior of the Earth has a major impact on other Earth systems. Convection currents in the mantle cause the movement of the plates. The internal heat is responsible for volcanoes. The outer core generates the Earth’s magnetic field. (?) Earth’s early atmosphere and water was probably formed from volcanic gases derived from inside the Earth. The position of the continents effects ocean currents which impact the Earth’s climates. Plates also effect the geographic distribution of organisms as well as their evolution.

Citations Continents Adrift: An Introduction to Continental Drift and Plate Tectonics. Rainbow Educational Media unitedstreaming. 23 January 2008