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Types of rocks in the Earth's crust When the Earth first cooled, Igneous Rocks were formed from the solidified molten crust. In the millions of years that followed, Sedimentary Rocks and Metamorphic Rocks were formed, by a variety of natural events. My sketch below shows a cross-section of a volcano, with the different kinds of rocks and their locations.
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* Gneiss - Def: Any coarse-grained metamorphic rock that is banded or foliated. Represents the last stage in the metamorphism of rocks before melting. ** Basalt - Def: A fine-grained dark basic ingeous rock consisting of plagioclase feldspar (a mixture of minerals).
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These rocks have been formed as a result of the cooling of the magma (molten rock) in the Earth's mantle. Igneous rocks are also created when volcanoes erupt, and the lava cools quickly. This produces rocks such as basalt. Igneous rocks such as quartz and granite are formed near the mantle, where the magma takes thousands of years to cool. This rock has a larger crystaline structure than basalt which has been subject to much quicker cooling. Granite
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Sediments such as sand and gravel are formed when rocks are subjected to weathering. Deposition is when these sediments are carried away by the currents of rivers or oceans, and then deposited in layers. Over millions of years these layers have built up to such depths, that they exert tremendous pressure on the materials below, creating soft rocks eg. coal, sandstone and chalk. This action is called consolidation. As the layers get buried deeper and deeper, the higher pressures they are subject to convert them into harder rocks eg. limestone. All such rocks are called sedimentary rocks: Sandstone from the weathering of other rocks, chalk and limestone from dead sea creatures, and coal from plant matter. Sandstone
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These harder rocks are the result of enormous pressures, or extremely high temperatures, both of which change the crystaline structure of the original rock. When clay and mud are subject to these extreme conditions, slate is produced, and when limestone is combined with hot igneous rock, marble is created. If the temperatures get too high, the metamorphic rock can melt and form magma. This can eventually cool and form igneous rock, thus beginning the rock cycle all over again. Marble
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The Rock Cycle The Earth's natural processes take place on different timescales, eg. flash floods, landslides and volcanoes can occur in minutes, hours or days, whereas the transport of materials by rivers and glaciers, are measured in weeks, years or decades. The creation of sedimentary rocks, the cooling of magma deep below the Earth's surface, and the effects of heat and pressure required to form metamorphic rocks, takes thousands/millions of years. These processes all working on different timescales come together to form the rock cycle, which has been in continual operation since the formation of the earth itself.
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"Six feet
of Earth makes us all equal"
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Office
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Quiz
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Mantle-
This is the molten rock or Magma,
that produces Igneous rocks.
Igneous
Rock - This layer, next to the mantle,
is made up of eg. quartz and granite.
Metamorphic
Rock- This layer, formed by tremendous
pressures, contains eg. slate and gneiss.*
Metamorphic
Rock- This layer is created by the
intense heat from the volcano, and
contains eg. marble.
Igneous
Rock- This layer, is comprised of eg.
basalt,** the result of the volcanoes
eruptions.
Sedimentary
Rock- This layer, as the name suggests,
is made up of the deposits of plants and sea animals, creating eg.
coal, chalk and limestone.
Igneous
Rock- This layer has been uncovered
by erosion, and is comprised of eg. quartz and granite.
Sea-
The seas, oceans and rivers, are the
source of much of the sediment contained in the sedimentary rock
layer.
Igneous
Rocks.