Lesson 4

Internal structure of the Earth

Inside, the Earth is similar to the other terrestrial planets. It has an outer, solid rock layer called the crust. The crust has two different parts. One is the continental crust (under the land) and the other is oceanic crust (under the ocean). The continental crust is thicker, and the oceanic crust is thinner. On land the average thickness is between 30–50 kilometers. Under the oceans in some places the crust is only 6 kilometers thick. The crust is made up of different types of rocks; igneous, metamorphic and sedimentary rocks. About 75% of the Earth's crust is composed of two elements, oxygen and silicon. These usually occur in combination with common metals such as aluminium and iron. The Earth's crust is less than 1% of Earth's volume.

The temperature of the crust increases with depth. Where the crust meets the mantle, the temperatures can be between 200 °C (392 °F) to 400 °C (752 °F).

Below the crust there is a layer of thick, semi-liquid rock called the mantle. The upper part of the mantle is made up of peridotite, a rock denser than rocks common in the crust. The crust and the upper mantle make up the lithosphere. There are two types of lithosphere: 1. Oceanic lithosphere is associated with oceanic crust and exists in the ocean basins. Oceanic lithosphere is typically about 50–100 km thick. 2. Continental lithosphere, which is associated with continental crust. Continental lithosphere has a range in thickness from about 40 km to perhaps 200 km, of which about 40 km is crust. The lithosphere is divided into tectonic plates, which can move. They move gradually relative to one another.

Below the lithosphere is the asthenosphere. Although solid, the asthenosphere can flow like a liquid on long time scales. Large convection currents in the asthenosphere transfer heat to the surface, where plumes of less dense magma break apart the plates at the spreading centers. The deeper mantle below the asthenosphere is more rigid again. This is caused by extremely high pressure.

Schematic view of the interior of Earth. 1. continental crust; 2. oceanic crust; 3. upper mantle; 4. lower mantle; 5. outer core; 6. inner core; A: Mohorovičić discontinuity; B: core–mantle boundary (Gutenberg discontinuity); C: The outer-core–inner-core liquid–solid transition once called the Lehmann discontinuity.

Under the mantle lies a thin liquid layer called the outer core and then the solid iron inner core. The inside of the Earth is very hot, the temperature of the outer core may be as high as 7,000 °C (12,630 °F).

Memorize the words:

average – середній, середня величина igneous – магматичні sedimentary – осадові metamorphic -- метаморфічні oxygen – кисень silicon – кремній iron – залізо to increase – зростати, збільшувати(ся) semi-molten – напіврозплавлений mantle – мантія dense – густий, щільний associated – пов'язаний range – ряд, пасмо (гір) although – хоч, незважаючи на те, що to flow – текти, литися, струменіти scale – шкала, ступінь, рівень convection – конвекція current – течія to transfer – переносити, передавати plume – плюмаж, султан rigid – твердий, негнучкий discontinuity – переривчатість, розрив boundary – межа, границя, кордон core – ядро tectonic plates – тектонічні плити volume -- об’єм, кількість, маса

Answer the questions:

1. What layers does the inside of the earth consist of?

2. What is the earth’s crust?

3. What parts does the crust consist of?

4. What layers make up the lithosphere?

5. Where is the asthenosphere? What is its characteristic feature?

6. Why is the deeper mantle more rigid than the asthenosphere?

7. What layers does the core consist of?