8/9/2019 Rocks Preso
http://slidepdf.com/reader/full/rocks-preso 1/14
8/9/2019 Rocks Preso
http://slidepdf.com/reader/full/rocks-preso 2/14
Sedimentary Rocks
For thousands, even millions of years, little
pieces of our earth have been eroded--
broken down and worn away
by wind and water. These little bits of ourearth are washed downstream where they
settle to the bottom of the rivers, lakes, and
oceans. Layer after layer of eroded earth is
deposited on top of each. These layers are
pressed down more and more through time,until the bottom layers slowly turn into rock.
Sedimentary rocks include sandstone,
limestone, shale, conglomerate and gypsum.
8/9/2019 Rocks Preso
http://slidepdf.com/reader/full/rocks-preso 3/14
Metamorphic Rocks
Metamorphic rocks are rocks that have
"morphed" into another kind of rock. These
rocks were once igneous or sedimentary
rocks. How do sedimentary and igneousrocks change? The rocks are under tons and
tons of pressure, which fosters heat build
up, and this causes them to change. If you
exam metamorphic rock samples closely,
you'll discover how flattened some of thegrains in the rock are.
Metamorphic rocks include Schist and
Gneiss.
8/9/2019 Rocks Preso
http://slidepdf.com/reader/full/rocks-preso 4/14
Igneous Rocks
Igneous rocks are called fire rocks and are formedeither underground or above ground.Underground, they are formed when the meltedrock, called magma, deep within the earth
becomes trapped in small pockets. As thesepockets of magma cool slowly underground, themagma becomes igneous rocks.
Igneous rocks are also formed when volcanoeserupt, causing the magma to rise above theearth's surface. When magma appears above the
earth, it is called lava. Igneous rocks are formedas the lava cools above ground.
Igneous rocks include Granite, Scoria, Pumice andObsidian.
8/9/2019 Rocks Preso
http://slidepdf.com/reader/full/rocks-preso 5/14
Diagram: Rock Cycle
8/9/2019 Rocks Preso
http://slidepdf.com/reader/full/rocks-preso 6/14
The Rock Cycle: Weathering
All rocks on the Earth's surface weather - though some weatherfaster than others. Three different forces work together to break uprocks into smaller pieces.
1. Physical weathering - cycles of hot and cold temperatures makerocks expand and contract, and rain may freeze and expand incracks in the rock. These processes eventually lead to rocks crackingand breaking up.
2. Chemical weathering - different chemicals can dissolve rock,helping to break it up; even water can dissolve some rocks. Polluted'acid rain' causes chemical weathering.
3. Biological weathering - tree roots can force themselves into small
cracks in rocks and eventually break the rocks apart.
8/9/2019 Rocks Preso
http://slidepdf.com/reader/full/rocks-preso 7/14
Erosion
As rocks weather, they are broken up into small,easily transportable pieces or particles.
The movement of these particles is called
erosion. There are four major ways erosion canoccur:
1. By gravity - broken pieces of rock fall to the ground, and roll
or slide down slopes.
2. By water - rivers and streams can transport all sizes of
particles.
3. By wind - small grains of sand can be picked up and moved
by the wind in dust storms.
4. By ice - ice rivers, called glaciers, can transport very large
pieces of stone.
8/9/2019 Rocks Preso
http://slidepdf.com/reader/full/rocks-preso 8/14
Deposition
Particles of rock cannot betransported forever. Rivers reachthe sea, the wind stops blowingand glaciers melt - they dump the
load of particles they werecarrying. This process is calleddeposition.
During deposition particles of rockare laid down in layers. Heavier
particles are normally dumped firstand then covered by finermaterial. Layers of sediment buildup over time. These layers form asedimentary sequence.
8/9/2019 Rocks Preso
http://slidepdf.com/reader/full/rocks-preso 9/14
Compaction/Cementation
As the layers of sediment build up, the pressure on the lower layersincreases. The layers are squeezed together and any water mixed inwith the sediments is forced out. This process is called compaction.
At the same time the particles of sediment begin to stick to eachother - they are cemented together by clay, or by minerals like silicaor calcite.
After compaction and cementation the sedimentary sequence haschanged into a sedimentary rock. Sedimentary rocks like sandstone,shale and limestone differ from other rocks in that they:
1. Are formed from layers of sediment built up over
many years.
2. Are grains of sediment cemented together by
various minerals.
3. May contain fossils - remains of plants and animals
that were caught up in the sediment.
8/9/2019 Rocks Preso
http://slidepdf.com/reader/full/rocks-preso 10/14
Metamorphism
Deep within the Earth's crust rocks can be put under hugepressures and temperatures are very high. These conditions cancause the minerals in the rock to change. This process is calledmetamorphism.
All rocks can be metamorphosed, and there are many differenttypes of metamorphic rock. Limestone can change into marble,shale and mudstones into slate, and igneous rocks like granitecan turn into gneiss. The extent to which the rocks are changeddepends on:
1. Whether they are exposed to heat, pressure or both.
2. Whether they are forced to change shape.
3. The time they are exposed to these conditions.
8/9/2019 Rocks Preso
http://slidepdf.com/reader/full/rocks-preso 11/14
Melting
It can get quite hot deep in the Earth's crust. In fact, itcan get so hot that the rocks that make up the crustcan actually begin to melt.
This molten material is called magma. It is less densethan the surrounding rock so it tends to moveupwards through the crust.
Magma also comes from material below the Earth'scrust - the mantle. This new material rises up fromthe mantle and adds to the magma produced from
the molten crust.
8/9/2019 Rocks Preso
http://slidepdf.com/reader/full/rocks-preso 12/14
Intrusive Crystallisation
Molten rock can sometimes form huge reservoirs called magmachambers within the Earth's crust. Left undisturbed over manyhundreds of thousands of years this magma will cool andcrystallise to form intrusive igneous rocks.
Intrusive igneous rocks like granite and gabbro have some thingsin common. They:
1. Are large grained - magma cools very slowly beneath the Earth'ssurface so the crystals in the rock have a long time to grow.
2. Are made up of angular interlocking crystals.
8/9/2019 Rocks Preso
http://slidepdf.com/reader/full/rocks-preso 13/14
Extrusion
Sometimes magma can force itself through a crack or faultin the rock at the Earth's surface. It pours out over theEarth's surface in a volcanic eruption. This process is calledextrusion.
The rocks that form from extruded magma are calledextrusive igneous rocks. Basalt and pumice are extrusiveigneous rocks. The type of rock that forms depends on themagma it came from, but generally extrusive igneous rocks:
1. Are very fine grained - magma cools very quickly when iterupts onto the Earth's surface and the crystals in the rockdon't have much time to grow.
2. May contain gas bubbles.
8/9/2019 Rocks Preso
http://slidepdf.com/reader/full/rocks-preso 14/14
For more information...
Discover How Rocks are Formed
http://www.fi.edu/fellows/fellow1/oct98/create
The Learning Zonehttp://www.oum.ox.ac.uk/thezone/rocks/cycle/
stage1/htm
Rocks for Kidshttp://www.rocksforkids.com/