Geology Notes
From Rock to Soil
1.1 Our Rocky Home
Understanding how rocks are made and learning the importance of rocks.
1.2 Three Rock Groups
Differentiate between the three types of rock: igneous, sedimentary, and metamorphic.
What is Rock?
A rock is a mixture of minerals, which are natural compounds.
Each mineral has a chemical name and formula.
Rock Samples
Granite: A mixture of minerals; pale blobs are crystals of silicon dioxide (quartz), other minerals are feldspars.
Sandstone: Mainly quartz, often mixed with feldspars; can be viewed using a magnifying glass.
Limestone: Mainly calcite or calcium carbonate (e.g., chalk).
Feldspar: A group of aluminosilicate minerals containing calcium, sodium, or potassium.
Uses of Rock
Building: Blocks cut from rock, concrete made from crushed rock and sand, cement made from limestone, crushed rock (aggregate) used for road building.
Metals: Metals come from metal ores in rocks; some metals occur naturally (gold, silver), but most occur within minerals.
Gemstones: Attractive minerals found in small quantities (e.g., diamond, ruby, jade, tanzanite); used in jewelry.
Types of Rocks
Igneous
Sedimentary
Metamorphic
Examples of Each Type:
Igneous: Granite, scoria, pumice, obsidian.
Sedimentary: Sandstone, limestone, shale, conglomerate, gypsum.
Metamorphic: Marble, slate, quartzite, gneiss.
Sedimentary Rock
Made up of tiny broken pieces of rocks, crystals, or minerals (sediments).
Different sediments yield different types of sedimentary rock.
Mud → mudstone
Sand → sandstone
Shells and coral → limestone
Examples:
Limestone
Sandstone
Chalk
Properties of Sedimentary Rocks
Often have layers showing deposition of sediment at different time periods.
Made of small grains weakly held together, making them porous, soft, and crumbly.
Often contain fossils.
Igneous Rock
Forms when rock melts below ground (magma), then cools and hardens.
Formation:
Magma rises to the Earth's surface, causing volcanic eruptions.
Igneous rocks form when magma cools and solidifies.
Cooling above the surface → extrusive igneous rocks.
Cooling below the surface → intrusive igneous rocks.
Properties of Igneous Rocks
Contain interlocking crystals held together strongly, making the rock hard.
Crystals have a disorderly arrangement.
Crystal size depends on the cooling rate.
Never contain fossils.
Metamorphic Rock
Rock changed underground without melting.
Formation:
Earth movements cause rocks to be buried deeply or squeezed.
Rocks are heated and put under great pressure.
Minerals change chemically, forming metamorphic rocks.
Heat and pressure compress grains, forming orderly layers or a mixture of grains in structure.
Sedimentary rocks transform into metamorphic rocks through extreme pressures and temperatures.
Location of Rock Groups
Earth's crust (up to 65 km thick) is the hard outer layer where we live.
Approximately 65% of the crust is igneous rock, and about 8% is sedimentary.
Most of the crust at the continents is igneous rock (mainly granite).
Most of the rock under the ocean is the igneous rock basalt.
A layer of sedimentary rock covers most of Earth's surface (up to 12 km thick on land, 1 km thick under the ocean).
Weathering and Rock Cycle
Weathering breaks down minerals and rocks through physical and chemical processes.
The rock cycle illustrates how rocks change over time due to weathering, erosion, pressure, and heat.
What is Weathering?
The breaking down or dissolving of rocks and minerals on the Earth's surface.
Caused by heat and cold, rain, gases from the air, and plants and animals.
Two types: physical and chemical.
Physical Weathering
Rock is broken into bits, but the minerals remain the same.
Ways Rocks Break Down:
Heating and cooling
Freeze-thaw weathering
Reduction in pressure (exfoliation)
Living things
Freeze-Thaw Weathering Process:
Water enters cracks in the rock.
Water freezes and expands, widening the crack.
Ice melts, and water goes deeper into the crack.
The process repeats until the rock splits.
Biological Weathering:
Roots enter small cracks in the rock.
As roots grow, the crack gets larger.
Rock breaks away.
Chemical Weathering
Minerals in the rock undergo chemical change, weakening the rock and breaking it up.
Process:
Rain is slightly acidic, dissolving from the air.
Acidic rainwater reacts with calcium carbonate in limestone, causing it to dissolve.
Water reacts with feldspars to form clay.
Quartz resists chemical weathering; when rock containing quartz breaks down, quartz crystals are set free as sand.
End Result of Weathering
Physical and chemical weathering break rock down into stones, sand, clay, and dissolved compounds.
Sand and clay mix with rotting vegetation to form soil.
Rate of Weathering
Depends on climate and rock type.
Rock Cycle
Weathering and erosion break down surface rocks.
Sediments are transported and deposited, often underwater.
Compaction and cementation turn sediments into sedimentary rock.
Burial deep underground subjects rocks to heat and pressure, forming metamorphic rock.
Melting forms magma, which crystallizes into igneous rock.
Driving Forces of the Rock Cycle
Rock pushed down into Earth’s crust.
Earth’s outer part cracked into huge slabs (plates).
Plate movements drive the rock cycle, pushing against each other, moving away, or sliding past each other.
Benefits of the Rock Cycle
Forms useful materials like building materials, gemstones, diamonds.
Provides access to limestone and fertile soil.
Continental Drift
550 mya: Precambrian eon – sponges and soft-bodied animals; no organisms with shells or bones, no life on land.
450 mya: Land plants appeared.
200 mya: Pangaea existed with dinosaurs, reptiles, and lush forests.
65 mya: Continents resembled their current form.
Rock and Relief in the UK
Relief: Plate movements and rock types influence the landscape.
Bedrock is the hard, solid rock beneath surface materials such as soil and gravel, also underlies sand and other sediments on the ocean floor. Different types of sedimentary rock form because the land gathered sediment in a different environment.
Sometimes land lay underwater, sediments of dead organisms create limestone or chalk.
Rivers carry tonnes of sand or mud to create sandstone or mudstone.
Sometimes the land was a desert and wind build sand dunes to create sandstone.
The age of a rock can also be determined via radiometric dating.
Higher land indicates resistant rock to weathering, while low land indicates easily weathered rock.
Rock and Landscape
Patterns in rock formations and fossils demonstrate changes over time.
Granite:
Resistant to weathering, breaks down slowly to sand, clay, and stones.
Impermeable.
Higher land
Thin soil
Boggy areas
Sheep farming
Limestone:
Weathered by acidic rain that dissolves limestone.
Rain soaks between blocks, widening gaps.
Potholes and underground caves form.
Streams may appear down sinkholes and emerge elsewhere.
Thin, stony soil; sheep farming.
Mudstone:
Weathers quickly to produce clay.
Low flat land and thick soil.
Clay swells when wet, preventing water soaking through, creating ponds and streams.
Nutrient-rich soil suitable for crops.
Link to Soil
Soil is a mixture of tiny particles of rock, dead plants and animals, air, and water.
Soil Types:
Sandy Soil: Pale colored, large particles, lots of air gaps, drains water easily, feels dry.
Clay Soil: Sticky, small particles, few air gaps, water doesn't drain easily.
Chalky Soil: Light brown, drains water quickly.
Peat: No rock particles, made from decayed plants, dark, crumbly, and rich in nutrients.
Soil Layers:
Humus: Thin layer of rotting vegetables; provides nutrients.
Topsoil: Rich in humus and minerals, good for growing crops; plants take in nutrients from rock, bacteria fix nitrogen from the air; sand makes it crumbly.
Subsoil: Little humus but rich in minerals; tree roots reach this layer.
Weathered Rock: Broken into chunks; few signs of life.
Bedrock: Solid rock, not yet weathered.
Life in Soil
Biodiverse environment.
Earthworms: Digest leaves and clay, waste is rich in nutrients, tunnels allow air and water to circulate.