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Earth’s Crust Overhead Notes

Most of the land features that you see every day – rocks, rivers, lakes, hills – have changed over time and are still changing.

Some changes happen rapidly, while others occur over long periods of time.

  • ROCKSare all around you. Food, building material, soil, toothpaste, glass, pencils, and much more comes from rocks.
  • Rocks that have different properties, depending on how they are formed, and what different materials they are made from.

MINERALS

  • pure, naturally occurring substances that are found in Earth’s crust
  • all rocks are made of minerals – “building blocks” of rocks

GEOLOGY – study of rocks (classification and identification)

CLASSIFICATION OF ROCKS

Colour

  • Could be an important clue, but does not stand on its own as an identifier

Streak

  • The colour of the powdery mark that some minerals make when they are scratched across a hard surface
  • Use a streak plate (unpolished piece of porcelain tile)
  • Streak may be the same colour as the mineral, or a different colour – some minerals can be identified by their streak
  • Example: fool’s gold (pyrite) and gold are both coloured yellow, but gold has a yellow streak, and pyrite has a brown streak

Lustre

  • The degree of shininess
  • Includes, glassy, dull, and metallic

Hardness

  • Related to the hardness of other minerals
  • Mineral is harder if it can make a scratch on the other mineral
  • Mohs hardness scale uses ten standard minerals, ranging from very soft to very hard, to compare hardness

Mohs Hardness Scale / Hardness Scale of materials
you can easily find
1. talc SOFTEST / 1. soft pencil point
2. gypsum / 2. - 3. fingernail
3. calcite
4. flourite / 3. - 4. copper penny
5. apatite
6. feldspar / 5. - 6. nail or glass bottle
7. quartz / 6. - 7. steel file
8. topaz / 7. - 8. sandpaper
9. corundum / 9. emery paper
10. diamond HARDEST

Crystal Structure

  • All minerals are crystals
  • Crystals have regular shapes because they are made up of tiny particles that are connected in a repeating pattern
  • Size of crystals tells how quickly a mineral cooled from a liquid to a solid - large crystals = mineral cooled slowly (granite); small crystals = mineral cooled rapidly (basalt).
  • Most crystals too small to be seen without magnification

Cleavage

  • Describes the way minerals break or fracture into rough, uneven surfaces, or split or crack along parallel or flat surfaces
  • Can be tested by breaking with a hammer or splitting off into sheets with a table knife
  • E.g. mica splits into thin sheets, halite splits into cubes

Magnetism

  • The ability of a mineral to attract a magnet
  • Only minerals that contain iron are magnetic

Reaction with Certain Chemicals

  • Some minerals can be identified by their reaction with certain chemicals
  • E.g. the carbonate material in calcite, marble, and limestone react with acidic solutions (e.g. vinegar), creating fizzing or bubbling of carbon dioxide (CO2) on the surface.

FAMILIES OF ROCKS

There are many different minerals, but they are usually found mixed together in rocks. E.g. granite contains mica, quartz, and feldspar.

Three families based on how they are formed:

IGNEOUS ROCK

  • Hot molten rock beneath Earth’s surface is called MAGMA.
  • Rock that forms from the hardening of liquid magma is called igneous rock, e.g. granite
  • Most of Earth’s surface is composed of igneous rock.

If the magma cools underground, the rock that is formed is called INTRUSIVE igneous rock. It will appear only after the layers of rock over it have eroded.

  • If the magma if forced out onto Earth’s surface, it is called LAVA.
  • Rock that forms from the hardening of cooled lava is called EXTRUSIVE igneous rock. E.g. basalt, obsidian

BasaltObsidian

  • The rate at which the molten rock cools determines the size of the crystals in the rock.
  • Cool very slowly: crystals can be seen with the unaided eye
  • Cools very quickly: tiny crystals that cannot be seen without magnification.
  • Some igneous rocks (pumice) float due to gases trapped in frothy lava, which cools quickly.

SEDIMENTARY ROCK

  • Smaller pieces of rock, sand, clay, mud, gravel, and boulders carried downstream is called SEDIMENT.
  • Water approaches a wider body of water (lake, ocean), the sediment slows down and sinks to the bottom.
  • Sediment gradually piles up in layers.

Millions of years > weight of upper layers presses down on the lower layers > compacted and pushed together (higher density) > dissolved minerals fill in the gaps between the pieces and act as a natural cement that hardens the lower layers into rock.

EROSIONDEPOSITIONCoMPACTIOnlithification = sedimentary rock

  • May also contain plant and animal remains that were deposited along with the sediment.
  • Layers are compressed > form different kinds of rock, depending on the nature of the particles in the sediment.
  • Fast-moving streams or rivers on a mountain slope can move large rocks, while a slow-moving wide river can only carry fine clay particles.
  • Study of the sedimentary layers can give clues to the land formations of the past.
  • Most visible rock on Earth’s surface is sedimentary > being added to all the time by continuous weathering and erosion by wind and water.
  • example: sandstone (formed from layers of compressed sand);conglomerate (contains rounded pebbles and small stones);shale (tiny particles of clay or silt)

METAMORPHIC ROCK

  • Igneous or sedimentary rock becomes buried at a great depth, subjected to increased temperature and pressure.
  • Magma moving through Earth heats and squeezes these rocks

Parent Rock / Metamorphic Rock
shale (sedimentary) / slate
granite (igneous) / gneiss
limestone (sedimentary) / marble
sandstone (sedimentary) / quartzite
  • Rock may change in appearance and/or in the minerals it contains – some change so much that they no longer resemble the parent rock

GraniteGneiss

ShaleSlate

Sandstone Quartzite

LimestoneMarble

More heat and pressure on metamorphic rocks can change them into other types of metamorphic rocks.

More heat – slate turns to phyllite, which can turn into schist, one of the strongest rocks in the world.

PhylliteSchist

FOSSILS

  • Impressions or remains of organisms that were covered by sediment before they could decompose
  • Covered quickly by falling into mud or quicksand, or suddenly buried by a landslide of sediment or blowing volcanic ash
  • Covered by further layers of sediment that eventually becomes sedimentary rock
  • Minerals in the body parts are replaced by minerals that are in the wet sediment.
  • Final result = a fossil that looks exactly like the original organism but in a rock-like form

Fossil found near Cache Creek, BC

FOSSIL RECORD

  • Shows what kinds of plants and animals lived millions of years ago
  • Fossils buried in sediment in the order of their time on Earth, from bottom to top
  • time line of the changes of life on Earth: geologic time scale.

WEATHERING BREAKS DOWN ROCKS

  • The process that slowly breaks down natural materials, e.g. rocks and boulders, into smaller pieces
  • Also affects human-made structures (roads and buildings)
  • Caused by physical forces or chemical reactions
  • Includes changing temperature, wind, rainfall, and snowfall

THREE KINDS OF WEATHERING

Mechanical Weathering: caused by a physical force

Ice wedging:

  • Rainwater trapped in the cracks of rocks freezes in the winter, expands, forces the cracks to widen, causing pieces of the rocks to break off

Wind

  • Sand particles and small rocks carried by the wind wear down exposed surfaces into small pieces or particles of rock

Water

  • Fast-flowing water works in the same way as wind, wears away and smoothes the outer surfaces of the rocks
  • Pounding waves on a seashore can break large rocks into smaller fragments.

Glaciers

  • Rocks trapped under glaciers scrape the ground below and remove large chunks of rock
  • Long scratches in rock are called striations.

Chemical Weathering

  • Occurs when there is a chemical reaction between air, water, or another substance and the materials in the rocks.
  • Water can dissolve some rock materials, especially when it contains natural or man-made acids
  • Rainwater collects carbon dioxide gas to form a mild acid, carbonic acid (H2CO3)
  • Carbonic acid dissolves the mineral calcite in limestone on bedrock and on structures (remember the experiment with the eggshell and the vinegar? It’s similar to the effects of carbonic acid on limestone, because the shells have calcite as well)
  • Oxygen in the air can rust any iron in minerals found in rocks

Biological Weathering

  • Some living things cause mechanical or chemical weathering.
  • E.g. lichen grows on rocks and uses some of the materials in the rock as a source of nutrients. It produces an acid that dissolves and wears down the rocks. New lichens can grow in the weathered material that is left from the previous lichen.
  • E.g. the roots of plants that grow in the cracks in rocks can split the cracks even more. The plants grow in the weathered material blown into the cracks by wind and water

EROSION: The movement of weathered rock materials from place to place.

  • Pieces range from grains of sand to giant boulders
  • Rapid erosion – landslide down a mountain
  • Slower – can also take hundreds of thousands of years
  • Distances range from a few centimetres to hundreds of kilometres away from the source (parent rock)
  • Gravity, wind, water, and ice all help to move weathered rock materials

DEPOSITION – when the eroded rock materials stop moving and settle on Earth’s surface

GRAVITY

  • Causes rock falls and avalanches
  • Can be triggered by earthquakes

WIND

  • Particles carried by
  • the wind are deposited when the wind stops
  • On beaches and deserts, the wind picks up loose sand and deposits it in regular piles called DUNES.
  • During times of drought, the wind may pick up and carry away rich layers of topsoil and deposit it several kilometres away

WATER

  • Rivers can move billions of tonnes of rock from the land it crosses
  • Water cuts into the land and makes deeper and deeper valleys (any low region of land between hills and mountains) – tend to be V-shaped
  • Rivers cross flat areas called plains near the coasts of oceans – the water slows down and deposits the heavier sediment on the riverbed of riverbanks
  • Rivers also slow down when they reach a lake or ocean – much of the sediment is deposited on the bottom of the lake or ocean, builds up over time and eventually causes the river to fan out over a large area, often the shape of a triangle – called a DELTA – breaks into a number of smaller channels, separated by islands of sediment

E.g. the Fraser River Delta was built in this manner – the cities of Richmond and Delta were formed from the deposition of sediment over thousands of years. The pictures below show Metro Vancouver 10 thousand years ago, and today (deposition in yellow). You can see that the mouth of the Fraser River used to be at New Westminster, but the river has deposited an additional 14 km of sediment at the mouth and some places inland. It continues to grow, in some spots, at a rate of 5m per year.

THE ROCK CYCLE

The ways that igneous, sedimentary, and metamorphic rocks relate to each other – each family of rock is linked to the others in this cycle

  • New rocks of each kind are constantly being formed and may eventually become exposed on Earth’s surface
  • Weathering wears them down > sediment forms layers > compression > forms sedimentary rock
  • Some rocks get pushed far into Earth > high pressures and temperatures > rocks melts and turns into magma
  • Magma can arise and erupt out of a volcano or cool gradually near the surface > igneous rock

EARTH’S CRUST IS MADE UP OF MOVING PLATES

CRUST (0 – 50 km):

  • Thin layer of solid rock that makes up Earth’s outermost layer
  • Materials in the crust tend to be lighter than the materials below
  • Earth’s crust floats on inner layers

MANTLE (2900 km)

  • Hot, thick layer of solid and partially melted rock
  • Increased pressure because of upper layers pushing down on you
  • Moves sluggishly, like syrup

OUTER CORE (2200 km):

  • Dense, hot region made up mostly of liquid iron and some nickel
  • Pressure is very high
  • Material flows more sluggishly

INNER CORE (1250 km):

  • Large ball of iron and nickel
  • Pressure from other layers compresses the inner core and keeps it solid
  • Temperature is nearly as hot as Sun’s surface

Sources of heat inside Earth – theories:

  • Decay of radioactive material within the mantle;
  • Heavy metals descending to Earth’s core
  • Heat may be generated by tidal force on the Earth as it rotates; since land cannot flow like water it compresses and distorts, generating heat.
  • Heat remaining from the impact of falling meteorites from Earth’s formation

Geothermal Gradient: Heat increases as you move towards the center of Earth: 25-30°C per km of depth in most of the world.

EARTH’S CRUST IS MADE UP OF MOVING PLATES

Alfred Wegener (1880-1930) first came up with idea that Earth’s continents were once joined as one supercontinent called Pangaea; continents separated and moved to their current locations in a process called continental drift.

Wegener observed that South America and Africa fit together like pieces of a jigsaw puzzle.

Support for Continental Drift

  • Fossil Record: Scientists have found fossils of identical plants and animals on different sides of the ocean; they could not have travelled across the ocean, therefore they must have lived on the same continent at some time in the past.
  • Landforms: landforms on the continents matched, e.g. mountain ranges and unusual rock formations between South Africa and Brazil, Eastern USA and Scotland.
  • Ancient Ice Age: Similar striation patterns left by glaciers along the coasts of South America and Africa, indicating that they were once situated around the South Pole

Criticism of Continental Drift

  • No explanation of how the continents drifted

OCEAN FLOOR

Not flat, as originally thought: many long, deep trenches running parallel and near the edges of oceans

Mid-ocean ridge: a 50 000 km long mountain range that almost circles the Earth and runs through the middle of the oceans

Age of Rocks on the Ocean Floor

  • The layer of sediment on the ocean floor is quite thin: the ocean floor is not as old as they thought – if it was unchanged for millions of years, then it would have been much thicker
  • Young rocks found at the top of mid-ocean ridges – the further away from the ridge, the older the rocks

Conclusion:

  • the ridge is where the crust is splitting apart
  • Magma is rising at the ridge to form new crust
  • The sea floor at the mid-ocean ridge is increasing in size as new crust if formed
  • Process is called Sea-Floor Spreading

Magnetic Reversals

  • Magnetic Field has reversed several times over millions of years
  • Grains of magnetite displaying the magnetic field at different times was locked into the rock at the time it was formed
  • Stripes of rock parallel to the mid-ocean ridge alternate normal and reversed magnetic fields, indicating that new rock was formed at the ridge.

Theory of Plate Tectonics

  • The surface of the Earth consists of about a dozen large plates that are continually moving
  • The parts of Earth’s crust that have continents on them are called continental crust
  • Plates that have ocean floor on them are called oceanic crust
  • Plates can be made up of either crust, or both

Plates move at different rates: fastest is 15 cm per year, slowest is 2.5.

DIVERGENT BOUNDARIES

  • Boundaries between plates that are moving apart
  • Plates separate > hot magma rises to Earth’s surface to form new crust
  • Separation and production of new crust is called SEA-FLOOR SPREADING
  • Magma cools and hardens, forming ridges of new rock up to a kilometre from the ocean floor
  • Entire length of the Atlantic Ocean has the MID-ATLANTIC RIDGE spreading at a rate of 2.5 cm per year

Divergent plate boundaries on land are called RIFTS – this is a picture of a rift between the North American Plate and the Eurasian Plate visible on land in Iceland.

CONVERGENT BOUNDARIES

  • Located where plates are moving against each other
  • Recycling of old crust while new crust is formed at divergent boundaries
  • Collisions that occur can last millions of years
  • One plate sinks below the other in the SUBDUCTION ZONE
  • The new landform depends on whether the converging plates are ocean-ocean, ocean-continental, or continental-continental.

Oceanic Plate converging with Continental Plate

When an oceanic plate collides with a continental plate, the oceanic plate is subducted under the continental plate because its rock is denser. This creates deep ocean trenches along the edge of a continent. In BC, this is seen where the Juan de Fuca Plate is being subducted under the North American Plate. In the cutaway diagram, you will see the trench located at the subduction zone (note the location of Mt. St. Helens in relation to the rising magma). This type of convergence can also cause landmasses to rise and create mountains, either by the crumpling of the plates, or by the scraping of the subducting plate: this is called continental accretion.

Oceanic plate converging with Oceanic Plate

When two oceanic plates converge, one plate subducts below the other. The deepest trench in the world is the Mariana Trench, located in the Pacific Ocean on the east side of the Philippine Plate, just east of The Philippines. It has been estimated to be nearly 11 km deep (deeper than Mount Everest is tall).