Grade 10 Geography Geomorphology Revision

Internal Structure of the Earth

The Earth is described as a ball composed of solid and molten minerals, structured into four distinct layers: the crust, the mantle, the outer core, and the inner core. The outermost layer, the crust, is further divided into two primary types based on composition and location. The Continental crust, referred to as sial, exists only beneath the continents and is composed mainly of granite. In contrast, the Oceanic crust, referred to as sima, is thinner and denser than the continental crust. It lies beneath the oceans as well as under the continental crust and consists primarily of basalt and gabbro. The continental crust is generally thicker than the oceanic crust.

Specific characteristics of these layers are identified through geological study. The mantle is approximately $2900\,\text{km}$ thick. The inner core is characterized as being solid rock and extremely dense, while the outer core is described as being dense but in a liquid state. This internal structure is fundamental to the geological processes observed on the Earth's surface.

Classification of Rocks and Intrusive Igneous Activity

Rocks forming the Earth's crust vary in size, color, and shape and are classified into three main categories: igneous rock, sedimentary rock, and metamorphic rock. Igneous rocks are further categorized by how they form, specifically through intrusive or extrusive activity. Intrusive igneous activity occurs when an enormous mass of magma fails to reach the Earth's surface and instead solidifies or hardens underground. These solidified masses are called intrusions.

Different types of intrusions are classified by their shape and size. A batholith is a massive intrusion that forms deep underground. A laccolith is mushroom-shaped, typically arching the overlying strata, and a lopolith is saucer-shaped. A prominent example of such a formation is Paarl Rock, which is specifically identified as a batholith composed of granite. Its formation involved the cooling of magma beneath the surface, which was later exposed through the erosion of overlying material. In topographical analysis, the slope at Paarl Rock is assessed as either steep or gentle based on contour lines and physical characteristics.

Continental Drift and Plate Tectonics

Continental drift refers to the movement of continents over geological time. Alfred Wegener was the first individual to notice that the continents appear to fit into each other like a jigsaw puzzle, providing the initial hypothesis for this theory. Evidence for the movement of continents includes the matching shapes of continental coastlines and the distribution of similar fossils and rock types across different continents.

Plate tectonics is the scientific theory that explains how the Earth is made up of giant, slowly moving tectonic plates. The interaction between these plates occurs at various boundaries, of which there are four primary types. A divergent plate boundary occurs where plates move away from each other. A subduction plate boundary involves one plate sinking beneath another into the mantle. A collision plate boundary occurs when two continental plates meet and push upward. Finally, a conservative plate boundary occurs when plates slide horizontally past one another, often leading to significant seismic activity.

Folding and Faulting in Earth's Crust

Folding is the process where rock layers are bent due to compressional forces within the Earth's crust. This results in landforms such as synclines, which are the downward-arching folds, and anticlines, which are the upward-arching folds. These structures are common in mountain-building regions.

Faulting occurs when there is a fracture or crack between two blocks of rock, allowing for relative movement between them. This movement is expressed through three main types of faults. A normal fault occurs when one block moves downward relative to the other. A reverse fault occurs when one block is pushed upward over the other. A strike-slip fault (or transform fault) is characterized by horizontal movement where blocks slide past each other. The San Andreas fault is a world-renowned example of a strike-slip fault.

Earthquakes: Causes, Measurement, and Impact

An earthquake is defined as the violent shaking of the Earth, caused by sudden movements where tectonic plates meet along a fault. A fault is a break in the Earth's outer layer. The specific point underground where the earthquake begins is called the focus. Directly above the focus on the Earth's surface is the epicentre. The energy released during an earthquake travels as invisible seismic waves.

To measure these events, scientists use a seismograph, which is an instrument that record and measures the vibrations of the Earth. The intensity or magnitude of an earthquake is measured on the Richter scale. The impacts of earthquakes on human populations can be devastating, including structural damage to buildings and infrastructure. Furthermore, underwater earthquakes can trigger a tsunami, which is a series of massive sea waves. To reduce the impact of these disasters, measures such as creating earthquake-resistant buildings and improving emergency response protocols are essential. A specific case study is the earthquake in San Francisco in $1989$ which occurred along the San Andreas fault.

Volcanic Activity and Stages

Volcanoes are classified into three stages based on their level of activity. An active volcano is one that is currently erupting or has erupted in recent history. A dormant volcano is currently quiet but is considered capable of erupting again in the future. An extinct volcano is one that is unlikely to erupt again. Volcanoes are also categorized by their shape and eruption style into three main types: cinder cones, shield volcanoes, and strato volcanoes (or composite volcanoes).

During an eruption, volcanoes eject various materials besides lava, including gases, ash, and pyroclastic rock. Volcanic activity significantly affects human populations. For example, the number of people killed by volcanoes in the Philippines has historically been higher than in Japan, highlighting differences in population density, geographic vulnerability, and preparedness. Despite the risks, some people choose to live close to volcanoes because the volcanic soil is exceptionally fertile for agriculture and the heat can be harnessed for geothermal energy.

Geomorphology Concepts and Definitions

1. Crust: The outermost solid layer of the Earth.
2. Mantle: The layer below the crust, roughly $2900\,\text{km}$ thick.
3. Inner core: The solid, dense center of the Earth.
4. Outer core: The dense, liquid layer surrounding the inner core.
5. Continental crust (sial): Thicker crust mainly composed of granite that forms continents.
6. Oceanic crust (sima): Thinner, denser crust mainly composed of basalt and gabbro under the oceans.
7. Igneous rock: Rock formed through the cooling and solidification of magma or lava.
8. Sedimentary rock: Rock formed from the accumulation and cementation of sediment particles.
9. Metamorphic rock: Rock that has undergone transformation by heat and pressure.
10. Batholith: An enormous mass of intrusive igneous rock.
11. Laccolith: A mushroom-shaped igneous intrusion.
12. Lopolith: A saucer-shaped igneous intrusion.
13. Continental drift: The movement of the Earth's continents relative to each other.
14. Plate tectonics: The theory of the movement of large plates of the Earth's lithosphere.
15. Conservative plate boundary: A boundary where tectonic plates slide past each other.
16. Collision plate boundary: A boundary where plates collide, often forming mountains.
17. Divergent plate boundary: A boundary where plates move apart.
18. Subduction plate boundary: A boundary where one plate is forced under another.
19. Folding: The bending of rock layers due to stress.
20. Faulting: The cracking and shifting of rock blocks.
21. Syncline: A downward fold in rock strata.
22. Anticline: An upward fold in rock strata.
23. Normal fault: A fault where the rock above the fault plane moves down.
24. Reverse fault: A fault where the rock above the fault plane is pushed up.
25. Strike-slip fault: A fault where the movement is horizontal.
26. Focus: The underground origin point of an earthquake.
27. Epicentre: The surface point directly above the earthquake's focus.
28. Seismic waves: Energy waves released by the sudden break of rock.
29. Seismograph: An instrument used to detect and record earthquake waves.
30. Richter scale: A scale used to measure earthquake magnitude.
31. Active volcano: A volcano currently erupting or likely to erupt.
32. Dormant volcano: An inactive volcano that could erupt again.
33. Extinct volcano: A volcano that is no longer expected to erupt.
34. Cinder cone: A simple, cone-shaped volcano made of volcanic debris.
35. Shield volcano: A broad, flat volcano formed by fluid lava flows.
36. Strato volcano: A steep-sided volcano made of layers of lava and ash.

Questions and Discussion

1. Would you regard the slope at Paarl Rock as steep or gentle? Justify your answer based on map evidence.
2. What type of rock is Paarl rock and is it classified as a batholith, laccolith, or lopolith?
3. Explain the geological process behind the formation of Paarl Rock.
4. Describe the movement of the four different types of plate boundaries and provide an example for each.
5. Discussion Activity: Model the movement of tectonic plate boundaries using Oreo biscuits to represent the crust and the cream filling to represent the underlying layers.
6. What type of fault is the San Andreas fault, and what was the movement of the blocks during the $1989$ earthquake?
7. How many earthquakes have been documented along the San Andreas fault and what were the distance and direction of the $1989$ earthquake from San Francisco?
8. Discuss three likely impacts that the $1989$ earthquake had on the city of San Francisco based on photographic evidence.
9. Suggest and discuss measures that can be taken to protect people and property from the effects of earthquakes in an eight-line paragraph.
10. How many active volcanoes are currently on Earth according to the infographic?
11. What materials, other than lava, are ejected during a volcanic eruption?
12. Why is the death toll from volcanoes higher in the Philippines compared to Japan?
13. Identify the types of volcanoes labeled A, B, and C in the textbook infographic and list two characteristics of volcano B.
14. Explain in an eight-line paragraph why some communities continue to live in close proximity to active volcanoes.