Sedimentary Rocks, Fossils & Relative Dating Techniques.

18
Sedimentary Rocks, Fossils & Relative Dating Techniques

Transcript of Sedimentary Rocks, Fossils & Relative Dating Techniques.

Page 1: Sedimentary Rocks, Fossils & Relative Dating Techniques.

Sedimentary Rocks, Fossils & Relative Dating Techniques

Page 2: Sedimentary Rocks, Fossils & Relative Dating Techniques.

How do geologists know about events like the extinction of the

dinosaurs and what the climate of earth was like in the past?

• By studying the rock record!• Fossils, types of rocks, sequence of rocks,

radioactive minerals in rocks all tell a history of earth

• Why do we care?

Page 3: Sedimentary Rocks, Fossils & Relative Dating Techniques.

What are the 3 major types of rocks?

• Sedimentary– Bits of rock, mineral or organic material deposited

by/in fluids like water, air or ice. Usually cemented together by mineral “glue”

• Igneous– Minerals that grew together out of hot molten rock

and cooled into a solid rock• Metamorphic– Rocks formed when heat and or pressure changed the

minerals over time into a new and different form

Page 4: Sedimentary Rocks, Fossils & Relative Dating Techniques.

How are sedimentary rocks formed?

• Depositioncompactioncementation– Sediment (bits of mimeral, rock, or oganic matter)

is deposited in layers – Over time, the layers of sediment get buried.– weight of the layers of sediment compacts it

together– Often, water flows through carrying dissolved

minerals and deposits mineral “glue” in the holes that cements it together

Page 5: Sedimentary Rocks, Fossils & Relative Dating Techniques.

• Conglomerate• Large roundish

peobbles in a smaller grained cement. You can always see the individual pebbles

• #4• Very fast moving rivers,

active wave coastlines

Clastic Sedimentary Rocks

Page 6: Sedimentary Rocks, Fossils & Relative Dating Techniques.

Clastic Sedimentary Rocks

• Sandstone• “sand”sized grains

“cemented” together. Sometimes hard to see individual grains but feels rough like sandpaper.

• #3• Forms in: medium fast

moving rivers, typical coastlines, deserts with sand dunes

Page 7: Sedimentary Rocks, Fossils & Relative Dating Techniques.

Other Sedimentary Rocks

• If not made from from broken up bits of rock and mineral they are called NON-clastic

• Made from remains of living things (leaves, shells etc.) or precipitated minerals from water (like salt)

Page 8: Sedimentary Rocks, Fossils & Relative Dating Techniques.

Clastic Sedimentary Rocks

• Shale• “mud” sized particles that you

can not see with your naked eye. Feels smooth to the touch. Great for storing fossils

• #5• Forms in slow moving

streams, just off-shore of the waves of the ocean, at the bottom of swamps and lakes on land

Page 9: Sedimentary Rocks, Fossils & Relative Dating Techniques.

What are fossils• Preserved remains of living things– Shells, footprints, bone shaped rocks, whole insect

• Can be formed in many ways, typically:– Organism dies near/under water – Body does not decompose fully before being covered by

sediment (or leaves hard parts)– Water flows through and changes the body parts into

mineral material by replacing each cell, turning it to rock.

Page 10: Sedimentary Rocks, Fossils & Relative Dating Techniques.

What type of rocks have fossils?• Sedimentary rocks are the best

preservers of fossils, sometimes they will survive the heat and pressure and be in metamorphic rocks.

• Ice, hardened sap (amber), ash

Page 11: Sedimentary Rocks, Fossils & Relative Dating Techniques.

What conditions favor fossil formation?

• Presence of hard parts• Rapid burial – beneath sediment or in some other material like

sap or ice or ash that preserves the organism from decay due to exposure to air and microbes

– Ocean, swamp, lake are ideal

Page 12: Sedimentary Rocks, Fossils & Relative Dating Techniques.

Echinodermata: Crinoid

• A filter feeding, shallow sea-living, plant-like animal!

• Paleozoic

Page 13: Sedimentary Rocks, Fossils & Relative Dating Techniques.

Anthozoa: Heterophrentis

• “Horn coral”• Bottom of shallow ocean

living plant-like animal…like a sea anemony

• Soft tenticals that organism used to sweep food into its mouth not preserved

• Each wrinkle represents 1 day!

Page 14: Sedimentary Rocks, Fossils & Relative Dating Techniques.

Trilobita: Phacops Rana

• “trilobite”• Shallow sea dwelling

arthropod with a hard exoskeletan (like a lobster)

• From 1 mm-1m lots of types

• Some swam, some crawled

Page 15: Sedimentary Rocks, Fossils & Relative Dating Techniques.

Brachiopoda: Mucrospirifer mucronatus

• shallow sea floor dwelling

• Fed on tiny organic “snow” floating in ocean water

• paleozoic

Page 16: Sedimentary Rocks, Fossils & Relative Dating Techniques.

Sedimentary rocks are useful to geologists trying to figure out earth

history because of 2 things…

• They contain fossils! – Can tell about life and environment on earth

• They are laid down in layers in a predictable way with the oldest at the bottom– Can help to sequence events on earth

Page 17: Sedimentary Rocks, Fossils & Relative Dating Techniques.

Relative Dating Principles

• Sequencing events…older than, younger than• Principle of superposition– Oldest rocks/events are on the bottom

• Principle of original horizontality– Sedimentary rocks are always flat layers when formed,

if not flat now…plate shifting has changed them• Principle of cross-cutting relationships– If igneous rocks or faults go across sedimentary rocks

the igneous rocks or faults are younger than the rocks they cut across.

Page 18: Sedimentary Rocks, Fossils & Relative Dating Techniques.

Note: H=cooled magma/igneous rockblack linebetween I and B=fault1. I deposited, compacted cemented2. B3. F4. Tectonic activity: faulting pushes

rock layers on left up and on right down

5. Erosion of rocks at surface6. M7. R8. H is formed as magma melts its

way through cracks or weak spots in rock layers.