FORCES THAT AFFECT THE EARTH

my free school tannzania April 28, 2023

                 FORCES THAT AFFECT THE EARTH 

 

The earth can be affected by two forces which may result into various physical features. These two features are internal and external forces.

The earth can be affected by two forces which may result into various landforms. Forces that act on the earth can be grouped into internal and external forces.

INTERNAL FORCES

These are forces which operate within the earth’s crust. Internal forces include vulcanicity and earth movements, that is, horizontal (lateral) and vertical movements. These forces may result into formation of several landform features.

VULCANICITY (VOLCANIC ERUPTIONS)

This refers to all the various ways by which molten rocks (magma) and gases are forced from the earth’s crust and onto its surface. Vulcanicity therefore includes volcanic eruptions (the formation of volcanoes and lava plateaus and geysers, and the formation of volcanic features such as batholiths, sills and dykes, etc, in the earth’s crust.

TYPES OF VULCANICITY

There are two types of vulcanicity such as intrusive vulcanicity and extrusive vulcanicity.

(A) INTRUSIVE (INTERNAL) VULCANICITY

This occurs when magma cools, solidifies and forms features within the earth’s crust before it reaches the earth’s surface. The features (landforms) formed this way are sometimes termed as intrusive (internal) features.The following are the landforms formed through intrusive vulcanicity:

1. DYKE

This is a wall-like feature cutting across the bedding planes. It is formed when magma cools and solidifies vertically across the bedding planes. The dyke is termed as a small scale intrusive feature. Sometimes the dyke may form a water fall when exposed to denudation processes.

Examples of dykes are Mwadui dyke in Tanzania, Gabbro dyke in Lesotho, and Tyolo dyke in Malawi.

 2. SILL

This is an intrusive feature which lies horizontally along the bedding planes. It is formed when magma cools and solidifies horizontally along a bedding plane. Like the dyke, the sill is termed as a small scale intrusive feature. Or this is the sheet of rock formed when magma cools and solidifies horizontally along the bedding planes. It can be of any thickness and can extend for many kilometers. When exposed by erosion, a sill may from a ridge like escarpment or waterfalls eg South Africa along the railway line from Kimberley to Cape Town.

3. LACCOLITH

This is an intrusive feature which looks like a dome. It is formed when magma cools and solidifies in anticline bedding plane. Sometimes it can be exposed to the earth’s surface following denudation processes.

4. LAPOLITH

1.       This is an intrusive feature which looks like a saucer in shape. It is formed when magma (molten rocks) cools and solidifies in a syncline bedding plane. Or This is the saucer-shaped mass of rock formed in geosynclines,  it may be due to increased weight of the deposits and magma solidifies within the crust e.g. formed in Transvaal South Africa

 Examples of lapoliths are found in South Africa especially in Trans Vaal province.

5. BATHOLITH

This is a very large mass of magma which cools and solidifies in the earth’s crust. Sometimes it forms the root or core of a mountain. Batholiths are made of granite and they form surface features only after they have been exposed by denudation. Sometimes batholiths resist erosion and form uplands.


Examples of batholiths are found in Zimbabwe, Tanzania, Zambia and Gabon (The Chaillu Massif).

 

6. PHACOLITH

This is a lens-shaped mass of igneous rock. It is formed when magma cools and solidifies at anticline and syncline in folded rocks.

 (B) EXTRUSIVE VULCANICITY

This is the type of vulcanicity that occurs when molten rocks reach the surface of the earth. When magma emerges at the surface it is called lava. This forms features called extrusive features of vulcanicity. The following are landforms due to extrusive vulcanicity:

1.  ACIDIC LAVA CONE

This is the cone made of viscous lava. Normally lava cones have high heights and break into small fragments. The acidic lava always cools faster than basic lava because is it viscous.

Examples of acidic lava cone include:

Mount Kilimanjaro found in Tanzania (East Africa).

Mount Kenya found in Kenya (East Africa).

Mount Fuji found in Japan.

Mount Vesuvius found in Italy.

 2. BASIC LAVA CONE

This is a cone made up of basic (fluid) lava. Normally cones have gentle slopes and spread over a long distance.

Examples of basic lava cones are Mauna Loa cone of Hawaii and basaltic dome of Nyamlangir, near to Lake Kivu in DRC.

 3. ASH AND CINDER CONE

This is the cone made up of ashes and stones erupted from beneath (interior) the earth to form a concave cone. The slopes of a cone are usually concave due to the spreading tendency. Lava is blown to great heights when it is violently ejected, and it breaks into small fragments which fall back to the earth and build up a cone. Or this cone shaped mass of rock fragments around a vent. It is called ash and cinder cone because of the highest temperature which burns the molten materials to ash, it is also termed cinder for there are some rock fragments (cinders) associated with volcanism

 Several ash and cinder cones occur just south of Turkana, in Kenya. These are Likaiyu and Teleki (both cinder cones), and Nabuyatom (ash cone).

Other examples of cinder cones outside Africa are Volcano de Fuego, in Guatemala and Paricutin, in Mexico.

4. CRATER

The crater is the small depression on the volcanic cone or mountain. It is sometimes found filled with water to form a crater lake. It is formed when volcanic eruption ceases and leaves a hole on the basic lava cone. An example of a crater is Ngorongoro crater in Tanzania.

     5. VOLCANIC PUG

This is a big rock which plugs or blocks the top of the pipe. It is formed when lava solidifies quickly to block the pipe. Examples of volcanic plugs are Mount Palace in France and Hoggar mountains in Algeria.

6.  COMPOSITE CONE: It is the large cone with alternate layers of rock fragments; it also consists of ash and cinder as well as lava. It is so termed because it is composed of sub cones (parasitic cones). This is the most common type of volcano on the earth’s surface. Examples of this type of volcano are Mount Kilimanjaro, mount Meru, and mount Nyiragongo in Zaire. Or

This type of the cone is formed of alternate layers of ash and lava. The volcano begins each eruption with great violence forming a layer of ash. As the eruption proceeds, the violence ceases and lava pours out forming a layer on top of 

Examples of composite cones are Mount Kilimanjaro, in Tanzania and Mount Cameroon. Other examples outside Africa are Vesuvius, Etna and Stromboli, all of which are in Italy.

 7. CALDERA

This is a large depression on top of a volcanic cone. It is formed when a composite volcano explodes so violently that its top is blown off and disintegrates into a mass of rocks and ashes, leaving the crater greatly enlarged. This huge crater-like depression is what we call a caldera.  Sometimes a caldera can be filled with water to form a caldera lake. Lake Shala, in Ethiopia, is the largest caldera lake in the world. N: B. the term caldera comes from a Latin word caldaria which means boiling pot in Standard English Language

Examples of calderas are:

  • Ebogar caldera, in Cameroon; and
  • Longonot caldera, in Kenya, which lies in the Eastern Rift Valley, about 140 km south of Mount Kenya.