DETAILED READING - Earth's size and shape

Date: 2015-10-07; view: 620.

Read the text and then complete the task. Fill in the gaps.

Instruments including artificial satellites have helped scientists work out the Earth's size, shape and other features. We know ours is one of the smallest, lightest planets of the solar system. Four others far exceed its mass and volume. But no planet has a greater density (5.5 times that of water).

Careful measurements prove the ball-like Earth is not in fact a sphere. It measures 24,901.5 mi. (40 074 km) around the equator, but only 24,859.7 mi. (40 008 km) around the poles. So our planet bugles slightly at the equator and is slightly flattened at the poles.

Centrifugal force created by the Earth's spin produced this shape, an oblate spheroid. Even that description oversimplifies, for the Earth is very slightly pear-shaped.

Scientists accordingly use the term “geoid“ (earth shaped) to describe the Earth's hypothetical, mean-sea-level surface-ignoring wrinkles formed by mountain chains and ocean floors. Geoid measurement involves taking sea-level gravity readings by gravimeter and studying “kinks” in orbits of artificial satellites. Both reveal so-called gravity anomalies reflecting local differences in mass in the Earth's crust and mantle. Such differences account for vast but slight dips and bumps in the geoid's surface.

Gravity anomalies also reinforce the theory of isostasy – a state of balance in the Earth's crust where continents of light material float on a denser substance into which deep continental “roots” project like the underwater mass of floating icebergs.

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

Fill in the gaps using the words under the line

Artificial satellites have helped scientists to work out the Earth's (1)__________ and shape. The Earth is the (2)__________ planet of the solar system. But it has a greater (3)__________ than all the other planets. Measurements prove that the Earth is not a (4)_________. Earth's (5)_________ produced this shape- oblate spheroid. Scientists use the term (6)_________ to describe the Earth's surface. It involves taking sea-level (7)_________ and studying (8) __________ in orbits. These reveal so-called (9)_________. They reflect the differences in the Earth's (10)________ and mantle. Gravity anomalies reinforce the theory of (11)________. This term means the state of balance in the Earth's (12)____________.

size density gravity lightest spin

kinks sphere geoid anomalies crust × (2)

isostasy

3.3 SCANNING- Earth's building blocks

You are looking for specific information. Don't read every word. Run your eye down the text and stop when you find the relevant (важный; существенный) words. You have 3-4 minutes.

1. building blocks-

2. abundance of elements-

3. elements-

4. silicate-

5. crystal-

6. mineral identification-

Earth's building blocks are elements and minerals of our planet. Of 92 naturally occurring elements, eight account for 98% of the weight of the Earth's crust – the rocky layer that scientists know best. Nearly 3/4 of its weight lies in two non-metals – oxygen and silicon; most of the rest consists of the six metals-aluminum, iron, calcium, sodium, potassium and magnesium.

Within the crust most elements occur as minerals- natural substances that differ chemically and have distinct atomic structures. The Earth's crust holds about 2000 kinds of minerals. A few (including gold) occur as just one element; most comprise two or more elements chemically joined as compounds.

Silicates (minerals containing silicon and oxygen) are the most abundant minerals in crust and mantle, which make up 4/5 of our planet's volume.

Most minerals formed from fluids that solidified – a process that arranged their atoms geometrically producing crystals. Scientists identify six crystal systems based on axes-imaginary lines passing through the middle of a crystal. Each system yields crystals with distinctive symmetry. Within each system, each mineral crystal grows in a special shape or habit, though this can be modified by temperature, pressure and impurities.

Traditional tests identify by hardness, color, streak (color of the powered mineral), luster, specific gravity, cleavage, fracture, form, tenacity (resistance to bending, breaking and other forces), odor, taste and feel.

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

Fig. 10 Relative abundance of elements in whole Earth