The lower part of the mantle is its hottest part. It is made up entirely of molten magma due to this intense heat. As the magma becomes hot, it rises upwards towards the crust. As the magma rises, it begins to cool again before sinking to the lower mantle again. This is a very important process known as convection currents. We will look at this more closely in Section 1.4 on the theory of plate tectonics.
The Asthenosphere
The asthenosphere is the name given to the semi-molten part of the mantle. It consists of a thin layer of semi-molten rock roughly 100–200 km below the Earth’s surface. The semi-molten rock acts like a lubricant and allows the lithosphere to slide on top of it.
GEO DICTIONARY The Lithosphere
The lithosphere is the term given to the upper mantle and crust combined. It is between 50 and 100 km thick, with the crust making up only a small percentage on top. It is solid and rigid but also quite brittle. It sits on the semi-molten asthenosphere which allows it to slide. The boundary where the upper mantle meets the crust is called the Moho. The lithosphere is cracked in many places, forming segments called plates.
The crust can be divided into the continental crust (land) and oceanic crust (oceans and seas).
1.3 The Crust
The crust is the outermost layer of the Earth and makes up the top part of the lithosphere. It is made of solid rock.
Convection currents: when air or liquid is heated, it becomes buoyant, causing it to rise. When it is cooled, it becomes less buoyant and sinks
Moho: the boundary between the crust and the solid upper mantle (collectively known as the lithosphere)
Theory: A proposed idea or group of ideas to help explain events. A theory is widely held to be true but has not been proven
ACTIVE LEARNING Oceanic crust Continental crust Lithosphere Lithosphere
1. Describe the following: (a) The inner core (b) The outer core (c) The lower mantle (d) The asthenosphere
2. Go to the following website:
Asthenosphere
scienceline.ucsb.edu Explore the geography section where scientists and geographers answer commonly asked questions.
: Fig. 1.4 The continental crust is thicker and lighter than the oceanic crust. PATTERNS AND PROCESSES IN THE PHYSICAL ENVIRONMENT 7
