Earth History and Fossils:

- Batholith, dyke, sill, volcano, igneous, plutonic, volcanic, sedimentary, shale, coal, conglomerate, sandstone, limestone, weathering, erosion, transport, depositon, cementation, compaction, passive, superposition, fold, fault, anticline, syncline, overfold, fossil, correlation.

Classifying rocks:

- Rock can be broadly classified into three groups – igneous, sedimentary or metamorphic- based on how they are formed. Igneous rocks are formed when a molten rock cools and solidifies. Sedimentary rocks are formed from the build up of pieces of other rock (sediment). Metamorphic rocks are formed when igneous, sedimentary or already existing metamorphic rocks are subjected to intense pressure and heat.

- A mineral is a substance of a particular chemical composition which has a particular geometric shape , cleavage, lustre, hardness and colour. However, most minerals (crystals) are not found in their exact geometric shape because they have been weathered, eroded or have formed alongside other minerals which have impeded its growth.

- A rock is a poly-crystalline aggregate

Igneous rocks are rocks that are formed by heat. They involve the cooling of molten rock to form a solid rock or glass. Molten rock under the ground is called magma. When an igneous rock forms, the size of its crystals depends on how fast the rock cools. Molten rock which cools quickly forms igneous rocks with fine grains and crystals. A long cooling time means the formation of igneous rocks with larger crystals. Lava which cools extremely quickly can form volcanic glass (pumice) or scoria.

There are a number of ways of classifying igneous rocks. The most basic way is by where they are formed. Volcanic igneous rocks (extrusive) are formed above the earth’s surface, whereas plutonic igneous rocks form underground. Igneous rocks can also be classified by chemical composition.

Where formed Chemical CompositionFelsic Intermediate Mafic Ultramafic

Intrusive (plutonic) Granite Diorite Gabbro PeridotiteExtrusive (volcanic) Ryolite Andesite Basalt Komatite

Felsic, on the other hand, is used for silicate minerals, magmas, and rocks which have a lower percentage of the heavier elements, and are correspondingly enriched in the lighter elements, such as silicon and oxygen, aluminum, and potassium.Greater than 65 percent silicon dioxide.

Rock Name Colour Grain size (mm)

Texture Situation formed in

Chemical composition

Minerals present

Classification

Granite Pink and gray

Less than 1 mm

Rough Deep in the Earth’s crust

Quartz, mica and feldsparRich in silicon and aluminium and low in magnesium

Plagioclase and alkali feldspars, quartz, orthoclase, biotite,

Felsic

Basalt Black and white

No grain Smooth Extrusive Quartz, feldspathoid, feldsparHigh in iron and magnesium – 42-55% silicons

plagioclase, pyroxene, olivine, biotite, apatite, magnetite;matrix homogeneous, sometimes phenocrysts (olivine, pyroxene)

Mafic

Gabbro Black, Grey, White

1mm Rough Same as basalt

Felsic

Rhyolite Grey, White

No grain Smooth Silicon Quartz, feldspar, mica and horroblende

Perodite Grey, White

1mm Rough Silicon, magnesium

plagioclase, spinel (commonly the mineral chromite), garnet (especially the mineral pyrope), amphibole, and phlogopite.

Pumice Cream No grain Aerated Aluminin, Silicate,

Pink Granite Pink, Brown, Beige

1 mm Rough Feldspar, iron etc

Plagioclase

Aim : To investigate the properties of igneous rokcs.

Equipment: rocks, magnifiying glass and ruler.

IGNEOUS STRUCTURES:

- Dyke:

A sheet like intrusion that cuts across existing rock and are usually fine grained.

Sill:

A Sheet like intrusion that is parallel to existing sedimentary beds. The magma pushes its way between the sedimentary beds and cools there.

Batholith:

Massive igneous intrusion, normally of intermediate or felsic composition.

They form very deep usually under mountain ranges at convergent plate boundaries e.g. under the Andes now

Dyke, sill, lapolith, lopoloth, batholith, vent, volcanic plug, lava, pyroclastic, ash, magma, lava

SEDIMENTARY ENVIRONMENTS:

- There are a number of different sedimentary environments. These are found in different locations and produce sedimentary rocks with different features.

- Lacustrine: Lakes characterised by very fine grains because there is a very low energy environment. Include shale and sometimes siltstone.

- Fluvial: not all river types produce the same types of sedimentary rock. Braided streams are relatively high energy environments and so the resulting sedimentary rocks can be relatively coarse grained. Whereas, meandering streams are lower energy environments and will often produce sandstone and shale.

- Shallow water marine: this is a saltwater environment; often on the continental shelf. They contain fossils of marine organisms.

- Volcanic: tuff and breccia are produced on and around volcanoes. - Desert: shaped by the winds and water courses; A common stone is sandstone.

Deposition, compation and cementation: Sediment – weathered to exposed to sunlight.

After being dispersed through the lake, I finally formed a sediment. Years went by and continuous deposition occurred. After a while the lake dried out and hardened. I was squeezed at the bottom of the lake bed. Millions of years went by

The law of Superposition Gneiss, Marble.

The law of superposition states that in an undisturbed sequence of sedimentary rocks, the rock at the bottom is the oldest and the rock at the top is the youngest. The law of cross cutting relationships is simply that any rock which cuts across other rocks is younger than those rocks. Contact metamorphism in the contact margin near the dyke.

Cross Bedding, Graded bedding, mud cracks, ripple marks

Geological Structures

There are a number of common geological structures which when understood, allow us to determine our geological history.

1. Faults:A fault is a crack in a rock or a sequence of rocks where displacement has occurred. There are three common types of fault. In a normal fault, the overall length of the rock has lengthened.They are caused by tensional forces. Reverse faults occur when rocks are experienced compressional forces and leads to the rock unit being shortened overall. Transform faults occur when rocks slide past each other but the overall length of the rock is unchanged. The most common example of transform faults occur at mid ocean ridges.

2. Fold:These are always caused in compressional situations as they always involve the shortening of units of rock. There are a number of different types of fold. Anticlines, synclines, monoclines. In lapstone. OVerfold

Unconformities are breaks in the continuity of a series of rocks which indicates that a period of time has elapsed where no rocks formation occurred but rather weathering and erosion of rocks has occurred. It is symbolised by a squiggly line. ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ How Fossils are Formed:There are a number of different ways fossils can form. Trace fossils, coprolite, mould, cast, replacement

Trace fossils are geological records of ecological activity.

What is a Mold Fossil ?A Mold Fossil is a fossilised impression made in the substrate, a negative image of the organism.

How Are Mold Fossils Formed ?Mold fossils form after hard parts have been buried in mud, clay, or other material that turns to stone. Later, water dissolves the buried hard part, leaving only an impression or mold of the original. The content can get compressed into a hard copy of what it was enclosed in.

THE PROCESS OF FOSSILISATION: