STEMulator_Nature

Clouds

The building blocks of clouds are water and particles of dust, dirt, or sea salt—known as cloud condensation nuclei. These nuclei are everywhere in the atmosphere. As the air rises and cools, the water vapour condenses to form liquid water or ice, which results in the formation of tiny globules called cloud droplets. Much smaller than raindrops, cloud droplets are incredibly light and amass while they float in the air, to form the fluffy formations we see in the sky.

Clouds generally form within the troposphere which is the layer of atmosphere closest to the earth. As they rise and fall, and the temperature of the air changes, they appear in various formation types.

There are three broad categories into which most clouds can be grouped:

High clouds: are found at the upper reaches of the troposphere, which, depending on geographic location, occur between roughly 3 000 and 20 000 metres above ground.

Mid-level clouds: generally occur between 2 000 and 8 000 metres above ground.

Low clouds: closest to the earth’s surface, hover at or below 2 000 metres above ground.

Clouds are good indicators of weather behaviour in upper air streams and can help to forecast future weather conditions.

A sac spider, Cheiracanthium species, that can be found on different crops. Photograph by Peter Webb.


Wikipedia	Natural Science Collections Facility	Agricultural Research Council

Spider

Spiders are one of the most common predator groups in South African agricultural ecosystems. This means they are abundant predators occurring on various crops, including orchards. They are considered to be natural control agents against pests and mites occurring on crops.

How are spiders able to help control pests and mites?
- Spiders are present on crops throughout the year. Their lifespan, that vary from species to species, have instars or juveniles stages that actively feed as predators. Food consumption decreases only before and during ecdysis. Ecdysis is the process where the spiders moult and shed their exoskeletons.

- The fact that spiders are usually polyphagous helps them to act as a natural control agents for pests and mites. Polyphagus means that spiders feed on a variety of available prey. They do not feed on adult prey only, but on different stages of the insect or mite. Research has shown that spiders are often some of the first predators to arrive and colonise a newly planted crop, which is important as many pests attack in the early stages of the crop during the growing season.

- Different species of spiders are found in different microhabitats. Wandering spiders live on the ground and they feed mainly on ground-living insects and larvae hibernating in the soil. The plant-dwelling spiders are present on the stems, foliage, bark or flowers of plants. Many of them are active hunters and move around in search of prey. A number of web-building spiders spin their webs between the leaves and branches, under the bark and even over the leaf surfaces. They will prey on insects that come into contact with the silk threads of their webs.

- As predators, spiders have a two-fold effect. Not only do they feed directly on their prey, but their presence also causes indirect mortality. For example, the presence of foraging spiders can disturb insect larvae, which then drop from the plant and die, or are exposed to predation by ground dwelling predators. The webs spun over the leaves also seem to make them less suitable for oviposition and feeding by pests.


Wikipedia	GSSA (Geological Society of South Africa)

Sedimentary Rocks

Sedimentary rocks are formed at or near the Earth’s surface by the accumulation and compaction of sediment or the precipitation from solution at normal surface temperatures. The lithification (literally “turning into stone”) process typically occurs slowly over time and depends on the chemistry of the sediments, the groundwater passing through the sediment, and how quick or deep the deposition process takes place.

Sedimentary rocks are produced by weathering (breaking down) of preexisting rocks, followed by the transportation and deposition of the weathering products. The weathered material is then eroded by either wind, ice, or water, and deposited in lakes and oceans. The erosion process grinds down rocks into smaller fragments. During transportation, fluids (water or wind) sort out the materials by separating heavier particles from lighter particles. The deposited materials undergo cementation, compaction and other solidification processes to form sedimentary rocks. Sedimentary rocks often have distinctive layering/bedding.

Sedimentary rocks can be classified as Clastic, Chemical or Biochemical:

Clastic sedimentary rocks are rocks composed of fragments (or detritus) derived from the mechanical breaking down of older rocks. Examples of clastic sediments include gravel, sand, silt and mud; these are lithified into conglomerate, sandstone, siltstone and mudstones.
Biochemical sedimentary rocks are those rocks derived from living organisms. When plant and animal organisms die, their remains can accumulate in particular environments to become sedimentary rocks. Examples include biochemical limestone, biochemical chert, dolomite, coal, etc.
Dissolved chemical particles, such as sodium and chlorine get into the groundwater and other bodies of water – rivers, lakes, pans, and oceans. When the concentration gets too high, the ions recombine by chemical precipitation to form minerals that can accumulate to form chemical sedimentary rocks. These include evaporites such as salt and gypsum (calcium sulphate) beds, travertine (calcium carbonate), and cherts (very fine-grained silica).


Wikipedia	GSSA (Geological Society of South Africa)

Rock Types

Rocks are all around us, making up the Earth’s crust, from the mountains we climb to the pebbles on the beach. In geology, rocks are grouped into three main types: igneous, sedimentary, and metamorphic. Each type forms in a unique way and tells a story about the Earth’s history. Let’s explore these rock types in a way that’s easy to understand!

Igneous rocks are born from fire—well, sort of! The word “igneous” comes from a Latin word meaning “fire.” These rocks form when molten rock, called magma or lava, cools and hardens. Magma is found deep inside the Earth, where it’s super hot. When a volcano erupts, the lava that flows out cools and becomes solid, forming igneous rocks like basalt. If the magma cools slowly underground, it forms rocks like granite, which has big crystals you can see with your eyes. Igneous rocks are tough and often used for building things like roads or statues because they’re so strong.

Sedimentary rocks are like nature’s scrapbooks. They form when bits of other rocks, minerals, or even remains of plants and animals pile up over time and get squashed together. Imagine sand on a beach or mud in a riverbed—over millions of years, these bits, called sediments, get pressed and cemented into solid rock. Examples include sandstone, made from sand, or limestone, which often has shells or coral in it. Sedimentary rocks are special because they can contain fossils, which are clues about life long ago, like dinosaur bones or ancient plants. They usually form in layers, so when you see a cliff with stripes, you’re looking at sedimentary rocks!

Metamorphic rocks are the transformers of the rock world. They start as one type of rock—igneous, sedimentary, or even another metamorphic rock—but change because of intense heat, pressure, or both, deep inside the Earth. This process is like baking a cake: the ingredients change to become something new. For example, limestone can turn into marble, a beautiful rock used for sculptures or fancy buildings. Another example is slate, which comes from shale and is used for roof tiles because it splits into thin sheets. Metamorphic rocks don’t melt during this change; they just get reshaped and recrystallised.

Each rock type has its own role in the Earth’s story. Igneous rocks show us how the Earth’s fiery insides work, sedimentary rocks give us a peek into the past with their layers and fossils, and metamorphic rocks reveal how the Earth’s forces can transform things. Next time you’re hiking in the Drakensberg or strolling along a Cape Town beach, have a look at the rocks around you. Can you guess which type they are? By studying rocks, we learn about the Earth’s history, from volcanoes to ancient oceans, and how our planet keeps changing.

In South Africa, we’re lucky to have all three rock types. The Karoo has lots of sedimentary rocks with dinosaur fossils, while the Witwatersrand’s metamorphic rocks hide gold! Igneous rocks, like those from ancient volcanoes, are also scattered across our land. Rocks are more than just stones—they’re pieces of the Earth’s puzzle, waiting for you to explore!


Wikipedia	GSSA (Geological Society of South Africa)

The Rock Cycle

Rocks might seem like they never change, but they’re part of a slow, amazing process called the rock cycle. This is how rocks transform from one type to another over millions of years, like a recycling system for the Earth’s crust. The rock cycle involves igneous, sedimentary, and metamorphic rocks, and it’s driven by forces like heat, pressure, and weathering. Let’s take a journey through the rock cycle to see how it works, using examples you might spot in South Africa!

The rock cycle starts with igneous rocks, which form when molten rock, called magma or lava, cools and hardens. Imagine a volcano erupting near Johannesburg long ago—its lava cools into basalt, an igneous rock. If this rock stays underground, it might cool slowly into granite, like the rocks around Paarl. These igneous rocks are the starting point, but they don’t stay the same forever.

Over time, igneous rocks get broken down by weathering. Rain, wind, and temperature changes chip away at the rock, turning it into tiny bits like sand or mud. For example, granite boulders in the Karoo might break into smaller pieces. These bits get carried away by erosion, like when a river sweeps sand downstream. Eventually, the bits settle in a new place through deposition, maybe at the bottom of a lake or the sea near Durban. Over millions of years, these layers pile up, get squashed, and stick together to form sedimentary rocks, like sandstone or shale. If you visit the Karoo, you might see these layered rocks, some with fossils of ancient creatures!

But the cycle doesn’t stop there. Sedimentary rocks can get buried deep in the Earth’s crust, where they face intense heat and pressure. This transforms them into metamorphic rocks. For example, limestone, a sedimentary rock, can turn into marble if it’s squeezed and heated deep underground. In the Witwatersrand, shale turned into slate, which helped hide gold deposits! Metamorphic rocks are like rocks that got a makeover, with new textures and minerals.

Metamorphic rocks can change even more. If they get pushed deeper into the Earth, they might melt into magma again. This magma can rise, cool, and form new igneous rocks, starting the cycle all over. For example, ancient metamorphic rocks in Namaqualand could melt and become new granite underground. The rock cycle is like a loop with no beginning or end—rocks keep changing from one type to another.

The rock cycle is powered by geoprocesses like plate tectonics, which moves the Earth’s crust, and weathering, which breaks rocks down. It’s super slow, taking millions of years, but it’s always happening. To picture it, imagine a rock in the Drakensberg: it might start as igneous, break into sediment, become sedimentary rock, get transformed into metamorphic rock, and maybe even melt back into magma one day. The rock cycle shows us that the Earth is always changing, recycling its materials to create new landscapes. Next time you’re exploring a rocky outcrop or a beach, think about the incredible journey those rocks have been on—they’re part of the Earth’s big recycling adventure!

Diagrams showing the formation of metamorphic rocks from both sedimentary and igneous types of rocks adapted from Earle (2019)

The types of metamorphic rock (right) adapted from Stephen and Stefan (2016).


Wikipedia	GSSA (Geological Society of South Africa)

Metamorphic Rocks

What are they? Metamorphic rocks are types of rocks which form because of transformation or change of pre-existing rocks in response to high pressure, heat, and/or hot mineral fluids. The parent rock undergoing transformation can originally have been sedimentary or igneous, and even metamorphic; this transformation process is termed metamorphism, from the Greek word ‘metamorphosis’ that means ‘to change form’. Metamorphism occurs deep in the Earth where temperatures range from ~250 ℃ to over 850 ℃ and pressures up to 300 MPa (megapascal). The setting where there are suitable conditions to form metamorphic rocks is where the tectonic plates that comprise the Earth’s crust meet. The tectonic plates collide to form mountain ranges, such as the Himalayas, Alps, and Rocky Mountains, where high pressures squeeze and deform the rocks.

Types of Metamorphism
There are two types of metamorphism, contact and regional metamorphism. Contact metamorphism occurs when a parent (original) rock encounters heat generated from a magma body that intrudes into it. The magma is formed deep in the Earth. The heat metamorphoses or bakes the rocks next to the intrusion, in the process forming new minerals in the metamorphic rock. With regional metamorphism, the entire original rock sequence, usually sedimentary and/or volcanic, is deeply buried by overlying rocks. The burial subjects the rocks to great pressures and high temperatures, causing metamorphism on a regional scale. This results in a suite of different metamorphic rocks that forms from the original ones.

Types of Metamorphic rocks
Metamorphic rocks can form from original sedimentary and igneous rocks, and sometimes even metamorphic ones. Sedimentary shales and sandstones give different types of schists and quartzites when metamorphosed, while volcanic basalts, rhyolites and tuffs will change to mafic (iron and magnesium-rich) and felsic (sodium and potassium-rich) schists and gneisses. These metamorphic rocks usually show a striped appearance, termed a foliation, that is caused by deformation due to high pressures. The platy minerals such as biotite and chlorite stack up to form the foliation. Quartzites are usually not foliated because they do not contain many platy minerals. Sedimentary limestones, made up of coral and shell fragments, metamorphose into different types of marble.

Metamorphic rocks have a variety of different uses, including building and paving stones, roof tiles, artworks, and garden decorations.

Figure 2: Cross section showing the formation of both volcanic (extrusive) and plutonic (intrusive) igneous rocks (Nelson, G.B.,2023).

Figure 1:
Left: Modern-day basaltic pahoehoe lava flow on Hawaii (scientiafantastica.wixsite.com).
Right: A 3-billion-year-old basaltic pahoehoe lava flow in Kwa-Zulu Natal, South Africa (David Russo, 2022).


Wikipedia	GSSA (Geological Society of South Africa)

Igneous Rocks

Igneous rocks are rocks which form as the result of cooling and solidification of molten rock or magma. The term “igneous” comes from the Latin word ‘ignis’ which means fire. Igneous rocks can be divided into two main groups depending on where they form. If magma reaches the surface of the earth and erupts from a volcano, the erupting molten rock known as lava then flows onto the surface of the Earth (Figure 1) where it cools rapidly to form rocks that are called extrusive or volcanic igneous rocks (Figure 2). Due to the rapid cooling, there is no time for large mineral crystals to grow in these rocks. Examples of such rocks are basalt and rhyolite.

If magma is injected and cools within the surface of the Earth, the resulting igneous rocks that form are known as intrusive or plutonic igneous rocks (Figure 2). These rocks form over longer times and cool slower than their extrusive counterparts and therefore have enough time to grow larger mineral crystals. Examples of such rocks are gabbro and granite.

Igneous rocks play a vital role in a country’s economy as they contain many types of mineral deposits needed to build the modern world.

The 90 000 km2 Bushveld Complex in South Africa hosts the world’s largest deposit of platinum group metals (PGMs), chromium and vanadium as well as large amounts of titanium.

A rare volcanic rock known as kimberlite (named after the town of Kimberley) in South Africa is a major source of diamonds. When the magma which formed these rocks travelled to the surface, it carried diamonds that formed deep within the earth along with it.

Igneous rocks also are responsible for creating some of South Africa’s famous tourist destinations. The Drakensberg Mountains were formed as a result of massive eruptions of lava onto the Earth’s surface which cooled to form basaltic rock.

The Pilanesberg National Park is found within the centre of what was once a massive volcano where both the intrusive and some of the extrusive rocks of that volcano can be seen today.


Wikipedia	GSSA (Geological Society of South Africa)

Geological Processes

Have you ever wondered why mountains are so tall, why rivers carve valleys, or why earthquakes shake the ground? These are all caused by geological processes, which are the natural forces and actions that shape the Earth’s surface and what’s beneath it. Geological processes are like the Earth’s way of constantly redecorating itself, and they happen both above and below the ground. Let’s dive into some of these amazing processes that make our planet so dynamic!

One major geoprocess is weathering. This is when rocks break down into smaller pieces because of things like wind, rain, or temperature changes. Imagine a big rock on a hill in the Free State. When it gets hot, the rock expands, and when it cools at night, it shrinks. Over time, this makes cracks, and bits break off. Water can also seep into these cracks, freeze, and split the rock further. There’s also chemical weathering, where rainwater, which is slightly acidic, can dissolve rocks like limestone, creating caves like the Cango Caves in the Western Cape.

Another important geoprocess is erosion. This happens when wind, water, or ice carry away the broken bits of rock from weathering. For example, rivers like the Orange River flow fast and pick up sand and pebbles, carving out valleys or canyons over time. Waves crashing on Durban’s beaches also erode cliffs, shaping the coastline. Erosion moves material from one place to another, like a delivery truck for the Earth’s surface.

Deposition is what happens after erosion. It’s when the bits of rock, sand, or mud carried by wind, water, or ice settle in a new place. Think of a river slowing down as it reaches the sea, dropping sand and mud to form a delta. The Tugela River sometimes does this, creating new land where it meets the ocean. Deposition builds up layers that can eventually turn into sedimentary rocks.

Then there’s plate tectonics, a massive geoprocess that shapes the Earth on a huge scale. The Earth’s crust is made of giant plates that float on a hot, semi-liquid layer below. These plates move slowly—about as fast as your fingernails grow! When they crash into each other, they can form mountains like the Drakensberg. When they pull apart, they create valleys or even new ocean floors. Earthquakes, like the ones felt sometimes in the Western Cape, happen when plates grind past each other. Volcanoes, though rare in South Africa now, are also part of this process, as magma escapes where plates meet or split.

Finally, soil formation is a geoprocess that happens slowly but is super important. Soil forms when rocks break down and mix with organic stuff like dead plants and animals. This creates the fertile ground where crops grow, like the maize fields in Mpumalanga. Geoprocesses work together to shape the Earth we see today, from the flat Highveld to the rugged Cape Fold Mountains. They’re always happening, even if we don’t notice, changing the land bit by bit. Next time you’re outside, look for signs of these processes—maybe a cracked rock, a riverbank, or a sandy beach—and you’ll see the Earth at work!