SEDIMENTARY ROCK (Read pages 85 & 88)
Sedimentary rock is made from sand and mud carried by water and then deposited as sediment. Over
millions of years, the lower layers of sediment are compressed into rock by the weight of the upper layers.
Anecdote: Squeezing communion bread
Table Mountain is made of sandstone that was deposited in layers under the sea! Thousands of years
later, the ground was pushed up high above the sea by earthquake movement. Over thousands more
years, wind and water have eroded all the surrounding softer rock down to the present level of the Cape
Flats where Cape Town is. The mountain is now ??? metres high but deep gorges and ravines have been
carved in it by rivers. Well known ones are Nursery Ravine, Skeleton Gorge.
People have prevented the mountain being carved in half by building dams across gorges.
Two dams on top of Table Mountain are: ???
Some sandstone has a red or brown colour because there is rusted iron in it.
Shale is a sedimentary rock made from mud.
Limestone is a sedimentary rock made from shells of sea animals.
Drawings of rocks: Put labels around them that describes what they look like
Colour: creamy, shiny specks, layered, variegated, coffee brown, dirty white, dark grey
Shape: round, flat sides, sharp edges
Texture: rough, sandy, scratchy, grainy, smooth, shiny, glassy, full of holes, bubbly, nuggety, fibres, flaky
Mass: heavy, light
4 Sedimentary Rocks
IGNEOUS ROCK (Read page 86)
Igneous rock is made from hot, molten rock (called magma) from the mantle below the crust. It pushes up
through cracks in the crust. As it cools, it hardens and becomes part of the crust.
Magma that is shot into the air is very light and
ashy. It is called pumice.
Magma that pushes up close to the Earth’s
surface cools and hardens quickly.
It forms a dark black rock called dolerite.
If there is gas in the magma, it is lighter and
called basalt. You can see basalt that has pushed
up through a fault (crack) at Sea Point Beach.
Magma that rises but stays deeper in the
Earth’s crust will cool and harden slowly.
If there is water present, crystals may have time to
form in the magma.
Granite is a very hard rock it contains crystals. Granite has glassy quartz crystals, black mica crystals and
white feldspar crystals. The crystals in granite tell us it formed slowly deeper down in the crust.
Granite rock formed below the sandstone of Table Mountain. The TMS has eroded away so much that
granite outcrops in various places around our Peninsula. Exposed granite erodes into large round boulders.
You can see granite boulders at Clifton, Camps Bay, Boulders Penguin Beach.
Why are fossils not found in igneous rocks? Igneous rock comes from deep down but fossils start at the
surface. Igneous rock is so hot when forming that carcasses would burn to ash.
3 Igneous rocks
There are 2 other categories of rock which come from sedimentary or igneous rocks:
Metamorphic Rocks (Read pg 90)
Metamorphic rock is made from igneous and sedimentary rocks that are changed by heat and pressure
inside the Earth’s crust.
Anecdote: licking the dough – so tasty! The cooked cake is nice but different!
Everyone knows that some mothers make much better cakes than others! Some cakes are spongy and
taste delicious. Others don’t rise or are tasteless or over cooked and dry.
The word metamorphic means ‘changed’. Metamorphic rock is like a cake that has been baked in an
underground oven long ago. The raw ingredients were the various types of sedimentary and igneous rocks.
Like in a cooking recipe, they were mixed in different combinations and cooked at different temperatures
and squeezed under different pressures to make them what they are. This explains why there are so many
different types.
Igneous rock cools from hot, liquid magma pushing into the crust. Metamorphic rock is formed from resqueezing and re-heating. It softens but never quite melts into liquid.
Make sure you know where the following metamorphic rocks come from:
Sandstone can change to quartzite.
Shale can change to slate or schist or mica
Limestone can change to marble.
Trees can change to coal
Coal can change to diamonds
Crystalline Rock (glassy with smooth facets and sharp edges)
Underground rock usually contains plenty of cracks. When it rains, water will seep down and lie in the
cracks and begin dissolving parts of the rock. This water is sometimes called hydrothermal fluid because it
changes temperature. The water will enlarge the cracks into cavities (little spaces) and caves. When the
water slowly evaporates in dry seasons, dissolved silicates and minerals form into crystalline rock.
Summary: Underground water dissolves holes in rocks. The water becomes full of dissolved minerals. If the
water evaporates, the dissolved minerals grow into solid crystals.
Sometimes the crystals form in cavities right inside a rock. A hollow rock with crystals inside is called a
geode.
Sometimes, sedimentary rock is heated and folded by earthquake movement. The layers break apart and
then crystalline rocks form in the spaces between the layers.
Quartz is a smooth glassy crystalline rock formed from dissolved silica.
Rose quartz has a pink colour.
Amethyst is quartz with tungsten which gives it its purple colour.
Agates are quartz rocks with patterns and layers of different colours from various minerals.
When there is asbestos present in the water, the crystal formation looks like glassy strands of hair. Tigers
Eye is formed when silica has dissolved into and taken the place of the asbestos. Rather like when silica
takes the place of carbon in the process of fossilization.
3 Crystalline Rocks