Rocks Preso

8/9/2019 Rocks Preso

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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.



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.



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.



Diagram: Rock Cycle



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.



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.



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.



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.



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.



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.



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.



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.



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/

Rocks Preso

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