Landforms Associated With Different Rocks (Grade 10 NSC Matric Geography): Revision Notes

Landforms Associated With Different Rocks

What are landforms?

The type of rock present in an area plays a major role in determining what kinds of landforms will develop there.

Landforms associated with igneous rocks

Igneous rocks form from cooled magma or lava, and they create two main types of landforms depending on where they formed.

Extrusive igneous landforms

These landforms develop from volcanic activity that occurs on the Earth's surface:

• Lava flows: When thin lava pours out from volcanic vents, it spreads across large areas and hardens into rock layers. Over time, this creates stepped landscapes as different lava flows stack on top of each other.
• Volcanic calderas: These are large, collapsed volcanic craters that often contain lakes. They form when the underground magma chamber empties out, causing the volcano's peak to collapse inward.

Intrusive igneous landforms

These features form when magma cools underground and is later exposed by erosion:

• Tors: These appear as rounded columns of rock, usually about 20 metres tall. They are the weathered remains of large underground igneous intrusions that have been exposed as softer surrounding rocks eroded away.
• Volcanic necks: These stand out as tall columns of rock above the surrounding landscape. They formed inside old volcanic cones where magma cooled and hardened. After the rest of the volcano eroded away, only the hard core remains.
• Dykes: These are vertical sheets of hard igneous rock that cut through other rock layers. They appear as wall-like ridges in the landscape.

Landforms associated with sedimentary rocks

Sedimentary rocks form from layers of sediment deposited over time. The way these rock layers are positioned determines what landforms will develop.

Landforms from horizontal strata

When sedimentary rocks form in flat, horizontal layers, they create distinctive stepped landforms through erosion:

• Plateaus: These are large, flat-topped areas of land elevated above the surrounding terrain. They form when horizontal layers of hard and soft rock resist erosion.

• Mesas: These are smaller flat-topped hills with steep sides. They develop when streams cut into plateaus, creating isolated flat-topped features. The name comes from the Spanish word for "table."

• Buttes: These are even smaller versions of mesas. As erosion continues, mesas become smaller until they form these tower-like landforms.

  

Landforms from inclined strata

When sedimentary rock layers are tilted or folded, they create different types of landforms:

• Cuestas: These are tilted ridges with one steep slope and one gentle slope. The steep side forms where hard rock layers are exposed, while the gentle side follows the slope of the tilted rock layers. See image a below.

• Hog's backs: When rock layers are tilted at very steep angles, both sides of the ridge become steep, creating a narrow, sharp-topped ridge. See image b below.

  

Landforms associated with metamorphic rocks

Metamorphic rocks often become harder than the original rocks they formed from. This increased hardness makes them more resistant to erosion, creating prominent landscape features.

Differential erosion and hard rock features

Quartzite is a common metamorphic rock formed from sandstone. It is much harder than the original sandstone and highly resistant to erosion. When quartzite occurs alongside softer rocks, differential erosion creates distinctive features:

• The harder quartzite remains as ridges or elevated areas
• Softer surrounding rocks erode away more quickly
• This creates a landscape of alternating high and low areas

Waterfalls

Waterfalls often form where hard metamorphic rocks occur in river valleys.

Contour patterns

Different landforms create distinctive patterns when shown on topographic maps:

• Steep slopes: Shown by contour lines that are close together

• Gentle slopes: Shown by contour lines that are far apart

• Hills and mountains: Shown by circular or oval contour patterns

• Valleys: Shown by V-shaped contour patterns pointing uphill

  

South African examples

South Africa provides excellent examples of landforms associated with different rock types:

• Quartzite ridges in Johannesburg: These prominent ridges stand out in the urban landscape because quartzite is more resistant to erosion than surrounding softer rocks.
• Mesas and buttes in the Free State: These flat-topped landforms developed from horizontal sedimentary rock layers.
• Mountain ranges: Many South African mountains formed from sedimentary rocks that were folded and uplifted by tectonic forces.