Detailed reading: relative dating: using rocks

 Read the text and for questions 1-10, choose the best answer: A, B, C or D. Then explain the words in bold.

Earth’s history lies locked up in the rocks that form its crust. Sedimentary rock layers or strata were laid down on top of one another, like pages in a history book. Reading these pages is the study called stratigraphy. Much of this is based on studying the properties of rocks themselves.

⁵ Geologists identify individual strata largely by such properties as grain size, minerals and color. These features and distinctive fossils help experts to define rock units or formations into members and beds or lump them together in subgroups or super groups.

Many pages in Earth’s book are torn, turned upside down, displaced or lost.

¹⁰ Stratigraphy involves working out the true sequence in which rock strata formed in any given place, matching these with layers elsewhere, and noting local gaps where erosion has wiped strata from the record.

Various clues help rock detectives discover where earth movements or injected molten rock have tampered with the evidence. For instance, steeply

¹⁵ sloping strata never formed that way. Faults, folds and injections of molten rock are younger than the rocks they affect. Mud cracks, ripple marks and pillow lava create distinctive patterns on a layer’s upper surface, which becomes its base if the rock is overturned. A break between level rock layers above and crumpled layers below is an unconformity, hinting at a time gap

  ²⁰ when rock layers vanished by erosion.

Other clues help experts correlate the age of rocks formed at the same time in different places. Widely separated strata may share a unique set of characters, or facies. Migrating shorelines may mark a worldwide fall or rise in sea level. Widespread layers of volcanic ash could hint at an immense ²⁵ volcanic eruption. Lavas or sediments accumulating at the same time lock in particles aligned in the same direction by the Earth’s magnetic field, which has undergone a sequence of reversals. Matched alignments and matched fossils help geologists to correlate the relative ages of rock-cores sampled from around the world.

(David Lambert “The Field Guide to Geology” 1988, Cambridge University Press)

1. Where can one find the Earth’s history?

A. on the surface of the rocks

B. locked up in the rocks

C. under the Earth’s surface

D. inside the Earth’s crust

2. Sedimentary rock layers are like

A. a sandwich       B. a history film     C. a history book   D. a map

 

3. What science studies the Earth’s history?

A. sedimentology

B. stratigraphy

C. geology

D. mineralogy

4. What features help to define rock formations into groups and subgroups?

A. rock properties       B. fossils    C. grain size           D. minerals

5. Which lines show the objective of stratigraphy?

A.  10 – 12                 B. 5 – 8      C. 21 – 23              D. 25 – 27

6. How many clues help rock detectives to discover where earth movement has tampered with the evidence?

A. 4                               B. 5            C. 3                        D. 6

7. What helps geologists to correlate the relative ages of rock-cores?

A. matched fossils

B. matched alignments

C. matched alignments and fossils

D. aligned particles

8. What do migrating shorelines mark?

A. fall or rise in sea level

B. immense rise of sea floor

C. decrease of water level

D. shifting of continents

9. What is another word for set of characters?

A. features      B. facies     C. sequences D. rock strata

10. What does relative dating mean?

A. geological age of a rock or fossil

B. geological age of a rock or fossil, expressed in units of time.

C. age of a fossil organism or formation

D. chronological arrangement of fossils or rocks with respect to the geological time scale.

3.2. INFORMATIVE READING - Relative Dating: Fossils

 

Read the following text and fill in the chart with the necessary information from the text. (R.P. 2.2)

 

Definition	Formation	Evolution for fossil zone	Requirements for fossil zone	Significance

What we know of the relative ages of rocks owes much to paleontology- the study of fossils. Fossils are remains of prehistoric organisms, locked in sedimentary rocks being laid down when those organisms died.

Most dead organisms soon rot away. Fossils tend to be hard parts like wood or bone quickly buried and preserved by sediment below a sea or lake. Percolating minerals may reinforce a fossil. Or a corpse may dissolve to leave a hollow called a mold. Minerals that fill a mold create a cast. Plant leaves carbon films. Even burrows, tracks and droppings can be fossilized. In time Earth movements lift fossil beds above the sea. Rivers cut down through the topmost bed, exposing lower layers. So paleontologists find fossils formed in rocks laid down at different times.

These studies show that living things evolved through time. Minute one-celled organisms gave rise to many-celled plants and animals. Some soft-bodied sea creatures led on to animals with shells or inner skeletons. Fishes gave rise to amphibians; amphibians to reptiles; reptiles to the birds and mammals. Through many generations, tiny but accumulating changes in inherited characters altered organisms, better fitting them to feed, breed and survive their enemies. Most major groups are very old indeed. But there survives a mere fraction of the hundreds of millions of species evolving in the last 600 million years. New enemies or harsh environmental change destroyed the rest. Fossil records in the rocks show waves of mass extinction, and then explosive evolution as new organisms moved into habitats left empty by immense catastrophes.

Fossils offer valuable aids to relatively dating sedimentary rock strata and correlating these around the world. This process involves identifying faunal zones: rock strata containing unique assemblages of fossils. Geologists name each faunal zone after a distinctive species called a zone fossil. A good zone fossil meets four requirements. Its species was extremely plentiful; spread far and fast; left readily preserved remains; yet soon died out, so limiting its fossils to a few rock layers. They ranged from sizeable animals and plants to tiny forms. Here are four examples:

1.Trilobites were marine, segmented distant relatives of woodlice; zone macrofossils for rocks 590-250 million years old.

2.Ammonites were cephalopod mollusks with coiled, flat, wrinkled shells; zone macrofossils for rocks 370-65 million years old.

3.Bivalves are headless mollusks with hinged, two-part shells; zone macrofossils for rocks 370-65 million years.

4. Foraminifera are tiny one-celled protozoan organisms drifting in the seas and forming limy shells pierced by tiny holes; zone microfossils for rocks 570-0 million years old.

Besides providing guides to evolution and the ages of rocks, fossil individuals and groups reveal how prehistoric living things behaved and the kinds of place and climate they inhabited. There are limits to our knowledge. Most soft-bodied organisms left no fossil record. Relatively few land plants and animals were fossilized. Billions of fossils vanished when erosion wore away the rocks containing them, or these were baked or crushed by metamorphic change. Billions more are inaccessible. But kinds of fossil are discovered every year.