How to make object move smoothly.

I have a script to make rocks float when I step onto a specific Collider.

The script i have works but it just changes the position in a flash. I want the rocks to flow smoothly :/

Here is the script.

 using UnityEngine;
 using System.Collections;
 
 public class Rock_Levitate : MonoBehaviour {
     public GameObject floatingRock;
     public GameObject collider;
     public GameObject startPosition;
     public GameObject endPosition;
     public float moveSpeed = 10.0f;
 
     // Use this for initialization
     void Start () {
     floatingRock.transform.position = new Vector3 (startPosition.transform.position.x, startPosition.transform.position.y, startPosition.transform.position.z);
     }
     
     // Update is called once per frame
     void Update () {
     
     }
 
 
         void OnTriggerEnter (Collider col){
             if (col.gameObject.tag == "Player"){
             print ("Player has touched Rock Float Trigger");
             float step = moveSpeed * Time.deltaTime;
             floatingRock.transform.position = Vector3.Lerp (startPosition.transform.position, endPosition.transform.position, moveSpeed);
             print ("Rock is Moving.");
         }
     }
 }