Introduction to rocks

Dr P Allen

The Principles of Geologic Science

Fundamental to geology is the principle of Uniformitarianism

Definition?

The scientific law stating that the geological processes taking place in the present operated similarly in the past and can therefore be used to explain past geologic events

James Hutton (1726-1797)supernatural theories were not needed to explain the geologic history

Principle of Uniformitarianism

Uniformitarianism allows us to interpret rocks, however it is in the PRESENT that many geological processes are discovered, observed and analysied:

Application of modern observation to ancient rock = Actualism

The present is the key to the past

The Present is the Key to the Past

Great Phrase but not strictly correct – Why?

Because not all events of the past have been duplicated or observed within the Human time span

Did a asteroid cause the extinction of the Dinosaurs?Did the impact inject dust into the upper atmosphere to cause a nuclear winter?

Or was it something we have notConsidered?

Because some details remain Unclear actualism does not apply

James Hutton (1726-1797)Sir Charles Lyell (1797-1875)Principles of Geology

Fathers of Modern (Scientific) Geology

After a long debate they changed the viewOf the Scientific community and rejected a theory calledCatastrophism

Both thought the Earth was ever changing BUT retained its basic features

By applying THEIR principles we know this to be untrue!

Mothers of Geology

She sells sea-shells on the sea-shore,The shells she sells are sea-shells, I'm sureFor if she sells sea-shells on the sea-shoreThen I'm sure she sells sea-shore shells.

Mary Anning (1799-1847)

Mary Anning's first Plesiosaur

Materials and Processes of Geology

ROCK?

Rocks consist of interlocking or bonded grains of matterwhich are typically composed of minerals

Minerals

Naturally occurring inorganic solid element or compound with a particular chemical composition or range of compositions and a characteristic internal structure

Quartz (Silicon dioxide Si04)most common mineral on the face of the Earth. It is found in nearly every geological environment and is at least a component of almost every rock type

Nature and Origin of Rocks

Basic Rock types -How Many?Igneous, Sedimentary and Metamorphic

Intrusive formed below surface but eventually exposed at surface

Rock broken down by Weathering – weathered rock is transported to a siteand accumulates as sediment

Igneous rocks form when magma crystallizes and solidifies. The melt originates deep within the Earth and are divided into two groups, intrusive or extrusive (volcaneos), depending upon where the molten rock solidifies.

Sedimentary Rocks

Where does sediment accumulate? How does it become solid?

Deserts, rivers, slopes, beaches, seafloors, lakes,

Lithification – the process of sediment becoming solid rock

The grains of sediment become mutually attached by compression of the sediment after burial.

The sediment may be glued together by precipitation of mineral cement from watery solutions that flow through the sediment.1. Clastic –Sandstone - Conglomerates

Sedimentary Rocks (cont)

2. Biogenic

3. Chemical

Metamorphic Rock

Where does metamorphic rock come from? How does it develop?

Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot, mineral-rich fluids or, more commonly, some combination of these factors

Marble (non foliated)

Gneiss (foliated)

Igneous Rocks

• Extrusive igneous rocks– Hardens at the Earth’s surface

• Intrusive igneous rocks– Hardens within the Earth

Rock Cycle

Multiple cycle within the “rock cycle”

Intrusive relationships

Component relationships

Geologic Time Scale

• Developed using – Biostratigraphy (fossil

succession)– Radioactive decay

• Divided into– Phanerozoic– Precambrian/Archean

• Cambrian– Oldest rocks with

conspicuous fossils