Why ask nearby volcanic ash about a dinosaur fossil's age?
恐龙化石的年龄，为什么要问旁边的火山灰？
给一块恐龙化石标上年龄，答案未必来自直接测量那块骨头。
Assigning an age to a dinosaur fossil does not necessarily mean measuring the bone itself.
有时，关键证据藏在旁边的火山灰层里。
Sometimes the key evidence lies in nearby layers of volcanic ash.
但灰是灰，恐龙是恐龙，它们的时间怎样联系起来？
But ash is ash and a dinosaur is a dinosaur: how can their times be connected?
先看化石所在的岩层。
First, consider the rock layer containing the fossil.
泥沙一层层堆积时，新的层通常覆盖在旧层上。
As sediment accumulates layer by layer, new layers usually cover older ones.
在能确定原来先后关系的岩层中，研究者可以判断哪一层较老、哪一层较新。
Where the original sequence of rock layers can be established, researchers can judge which layers are older and which are younger.
这叫相对年代：它告诉我们顺序，却还没有告诉我们过去了多少年。
This is relative age: it tells us the order, but not yet how many years have passed.
火山灰中的某些矿物晶体，可以提供另一种时间信息。
Certain mineral crystals in volcanic ash can provide another kind of time information.
晶体里的某些原子会以已知速率发生放射性衰变，变成另一种原子。
Certain atoms in the crystals undergo radioactive decay at a known rate, becoming another kind of atom.
原来的叫“母体”，变化产生的叫“子体”。
The original type is called the parent, and the type produced by the change is called the daughter.
同一种放射性原子，数量减少到原来一半所需的时间，叫半衰期。
The time needed for the number of radioactive atoms of one type to fall to half its original value is called the half-life.
这不是每年减少相同数量，而是经过一个半衰期，剩余数量再减少一半。
This does not mean losing the same number each year; after each half-life, the remaining number is halved again.
研究者测量合适晶体中母体和子体的比例，用它估算晶体形成以来的时间。
Researchers measure the ratio of parent to daughter in suitable crystals and use it to estimate the time since the crystals formed.
这里测出的首先是晶体的年龄，并不是恐龙骨头的年龄。
What is measured here first is the age of the crystals, not the dinosaur bone.
接下来，才轮到岩层的位置发挥作用。
Next, the positions of the rock layers come into play.
假设一个含化石的岩层，位于两层已经测定年龄的火山灰之间。
Suppose a fossil-bearing rock layer lies between two volcanic ash layers whose ages have been determined.
如果可以确认这些层形成的先后关系，下面的灰层提供较早的时间界限，上面的提供较晚的界限。
If the sequence in which these layers formed can be confirmed, the lower ash layer provides an earlier time boundary and the upper one a later boundary.
这样，化石所在岩层的年龄就可以被限定在一个范围内。
The age of the fossil-bearing rock layer can thus be constrained to a range.
所以，研究者需要把数字和地质关系放在一起，而不是把附近一块石头的年龄直接贴到化石上。
Researchers therefore need to combine the numbers with geological relationships, rather than simply assign a nearby rock's age to the fossil.
不同方法适合不同材料和时间范围，选择合适的材料也是判断的一部分。
Different methods suit different materials and time ranges, so selecting suitable material is also part of the assessment.
一块化石没有写下自己的日期，但它周围的地层，可能保留了能把时间范围缩小的证据。
A fossil carries no written date, but the layers around it may preserve evidence that narrows down its time range.
